Table of contents

COMPASS is an on-axis 2.6 m telescope coupled to a correlation polarimeter operating at a wavelength of 1 cm. The entire instrument was built specifically for cosmic microwave background (CMB) polarization studies. We report here on observations of 2001 February-April using this system. We set an upper limit on E-mode polarized anisotropies of 1036 μK² (95% confidence limit) in the l range 93-555.

We report initial results for spectroscopic observations of candidates of Lyman break galaxies (LBGs) at z ~ 5 in a region centered on the Hubble Deep Field-North by using the Faint Object Camera and Spectrograph attached to the Subaru Telescope. Eight objects with I_C ≤ 25.0 mag, including one active galactic nucleus, are confirmed to be at 4.5 < z < 5.2. The rest-frame UV spectra of seven LBGs commonly show no or weak Lyα emission lines (rest-frame equivalent width of 0-10 Å) and relatively strong low-ionization interstellar metal absorption lines of Si II λ1260, O I+Si II λ1303, and C II λ1334 (mean rest-frame equivalent widths of them are -1.2 to -5.1 Å). These properties are significantly different from those of the mean rest-frame UV spectrum of LBGs at z ~ 3 but are quite similar to those of subgroups of LBGs at z ~ 3 with no or weak Lyα emission. The weakness of Lyα emission and strong low-ionization interstellar metal absorption lines may indicate that these LBGs at z ~ 5 are chemically evolved to some degree and have a dusty environment. Since the fraction of such LBGs at z ~ 5 in our sample is larger than that at z ~ 3, we may witness some sign of evolution of LBGs from z ~ 5 to z ~ 3, although the present sample size is very small. It is also possible, however, that the brighter LBGs tend to show no or weak Lyα emission, because our spectroscopic sample is bright (brighter than L*) among LBGs at z ~ 5. More observations are required to establish spectroscopic nature of LBGs at z ~ 5.

We investigate the large-scale inhomogeneities of the hydrogen-ionizing radiation field in the universe at redshift z = 3. Using a ray-tracing algorithm, we simulate a model in which quasars are the dominant sources of radiation. We make use of large-scale N-body simulations of a Λ cold dark matter universe and include such effects as finite quasar lifetimes and output on the light cone, which affects the shape of quasar light echoes. We create Lyα forest spectra that would be generated in the presence of such a fluctuating radiation field, finding that the power spectrum of the Lyα forest can be suppressed by as much as 15% for modes with k = 0.05-1 h Mpc^-1. This relatively small effect may have consequences for high-precision measurements of the Lyα power spectrum on larger scales than have yet been published. We also investigate a second probe of the ionizing radiation fluctuations, the cross-correlation of quasar positions and the Lyα forest. For both quasar lifetimes that we simulate (10⁷ and 10⁸ yr), we expect to see a strong decrease in the Lyα absorption close to other quasars (the "foreground" proximity effect). We then use data from the Sloan Digital Sky Survey First Data Release to make an observational determination of this statistic. We find no sign of our predicted lack of absorption, but instead increased absorption close to quasars. If the bursts of radiation from quasars last on average less than 10⁶ yr, then we would not expect to be able to see the foreground effect. However, the strength of the absorption itself seems to be indicative of rare objects and hence much longer total times of emission per quasar. Variability of quasars in bursts with timescales of between 10⁴ and 10⁶ yr could reconcile these two facts.

We study the statistics of large-separation gravitational lens systems produced by nonspherical halos in the cold dark matter (CDM) model. Specifically, we examine how the triaxiality of CDM halos affects the overall lensing probabilities and the relative numbers of different image configurations (double, quadruple, and naked cusp lenses). We find that triaxiality significantly enhances lensing probabilities by a factor of ~2-4, so it cannot be ignored. If CDM halos have central density slopes α ≲ 1.5, we predict that a significant fraction (≳20%) of large-separation lenses should have naked cusp image configurations; this contrasts with lensing by isothermal (α ≈ 2) galaxies where naked cusp configurations are rare. The image multiplicities depend strongly on the inner density slope α: for α = 1, the naked cusp fraction is ≳60%, while for α = 1.5, quadruple lenses are actually the most probable. Thus, the image multiplicities in large-separation lenses offer a simple new probe of the inner density profiles of dark matter halos. We also compute the expected probabilities and image multiplicities for lensed quasars in the Sloan Digital Sky Survey and argue that the recent discovery of the large-separation quadruple lens SDSS J1004+4112 is consistent with expectations for CDM.

We study the possibility of magnetic mass detection using the gravitational microlensing technique. Recently, the theoretical effect of magnetic mass in NUT space on the microlensing light curve has been studied. It has been shown that in the low photometric signal-to-noise ratio and sampling rate of MACHO experiment light curves, no signature of the NUT factor has been found. In order to increase the sensitivity of magnetic mass detection, we propose a systematic search for microlensing events, using the currently running alert systems and complementary telescopes for monitoring Large Magellanic Clouds stars. This observation strategy provides the lowest observable limit of the NUT factor, and we calculate the magnetic mass detection efficiency. This survey method for gravitational microlensing detection can also be used as a tool for searching other exotic spacetimes.

We present seven epochs of spectroscopy on the quadruply imaged quasar SDSS J1004+4112, spanning observed-frame time delays from 1 to 322 days. The spectra reveal differences in the emission lines between the lensed images. Specifically, component A showed a strong enhancement in the blue wings of several high-ionization lines relative to component B, which lasted at least 28 days (observed frame) and then faded. Since the predicted time delay between A and B is ≲30 days, our time coverage suggests that the event was not intrinsic to the quasar. We attribute these variations to microlensing of part of the broad emission line region of the quasar, apparently resolving structure in the source plane on a scale of ~10¹⁶ cm at z = 1.734. In addition, we observed smaller differences in the emission-line profiles between components A and B that persisted throughout the time span, which may also be due to microlensing or millilensing. Further spectroscopic monitoring of this system holds considerable promise for resolving the structure of the broad emission line region in quasars.

Gravitational lenses that produce multiple images of background quasars can be an invaluable cosmological tool. Deriving cosmological parameters, however, requires modeling the potential of the lens itself. It has been estimated that up to a quarter of lensing galaxies are associated with a group or cluster that perturbs the gravitational potential. Detection of X-ray emission from the group or cluster can be used to better model the lens. We report on the first detection in X-rays of the group associated with the lensing system PG 1115+080 and the first X-ray image of the group associated with the system B1422+231. We find a temperature and rest-frame luminosity of 0.8 keV and 7 × 10⁴² ergs s^-1, respectively, for PG 1115+080 and 1.0 keV and 8 × 10⁴² ergs s^-1, respectively, for B1422+231. We compare the spatial and spectral characteristics of the X-ray emission with the properties of the group galaxies, with lens models, and with the general properties of groups at lower redshift.

The position of the giant elliptical galaxy M87 at the center of the Virgo Cluster means that the inferred column density of dark matter associated with both the cluster halo and the galaxy halo is quite large. This system is thus an important laboratory for studying massive dark objects in elliptical galaxies and galaxy clusters by gravitational microlensing, strongly complementing the studies of spiral galaxy halos performed in the Local Group. We have performed a microlensing survey of M87 with the WFPC2 instrument on the Hubble Space Telescope. Over a period of 30 days, with images taken once daily, we discover seven variable sources. Four are variable stars of some sort, two are consistent with classical novae, and one exhibits an excellent microlensing light curve, although with a very blue color, implying the somewhat disfavored possibility of a horizontal-branch source being lensed. On the basis of sensitivity calculations from artificial stars and from artificial light curves, we estimate the expected microlensing rate. We find that the detection of one event is consistent with a dark halo with a 20% contribution of microlensing objects for both M87 and the Virgo Cluster, similar to the value found from observations in the Local Group. Further work is required to test the hypothesized microlensing component to the cluster.

We have modeled the time-variable profiles of the Hα emission line from the nonaxisymmetric disk and debris tail created in the tidal disruption of a solar-type star by a 10⁶M_☉ black hole. Two tidal disruption events were simulated using a three-dimensional relativistic smoothed particle hydrodynamics code to describe the early evolution of the debris during the first 50-90 days. We have calculated the physical conditions and radiative processes in the debris using the photoionization code CLOUDY. We model the emission-line profiles in the period immediately after the accretion rate onto the black hole became significant. We find that the line profiles at these very early stages of the evolution of the postdisruption debris do not resemble the double-peaked profiles expected from a rotating disk, since the debris has not yet settled into such a stable structure. As a result of the uneven distribution of the debris and the existence of a "tidal tail" (the stream of returning debris), the line profiles depend sensitively on the orientation of the tail relative to the line of sight. Moreover, the predicted line profiles vary on fairly short timescales (of the order of hours to days). Given the accretion rate onto the black hole, we also model the Hα light curve from the debris and the evolution of the Hα line profiles in time.

We present an initial sample of 19 intermediate-mass black hole candidates in active galactic nuclei culled from the first data release of the Sloan Digital Sky Survey. Using the line width-luminosity mass scaling relation established for broad-line active nuclei, we estimate black hole masses in the range of M_BH ≈ 8 × M_☉, a regime in which only two objects are currently known. The absolute magnitudes are faint for active galactic nuclei, ranging from M_g ≈ -15 to -18 mag, while the bolometric luminosities are all close to the Eddington limit. The entire sample formally satisfies the line width criterion for so-called narrow-line Seyfert 1 galaxies; however, they display a wider range of Fe II and [O III] λ5007 line strengths than is typically observed in this class of objects. Although the available imaging data are of insufficient quality to ascertain the detailed morphologies of the host galaxies, it is likely that the majority of the hosts are relatively late-type systems. The host galaxies have estimated g-band luminosities ~1 mag fainter than M* for the general galaxy population at z ≈ 0.1. Beyond simply extending the known mass range of central black holes in galactic nuclei, these objects provide unique observational constraints on the progenitors of supermassive black holes. They are also expected to contribute significantly to the integrated signal for future gravitational wave experiments.

We present Chandra observations of 17 optically selected, X-ray-weak narrow-line Seyfert 1 (NLS1) galaxies. These objects were optically identified by Williams et al. in the Sloan Digital Sky Survey Early Data Release but were not found in the ROSAT All-Sky Survey (RASS) despite having optical properties similar to RASS-detected NLS1s. All objects in this sample were detected by Chandra and exhibit a range of 0.5-2 keV photon indices Γ = 1.1-3.4. One object was not detected in the soft band but has a best-fit Γ = 0.25 over the full 0.5-8 keV range. These photon indices extend to values far below those normally observed in NLS1s. A composite X-ray spectrum of the hardest objects in this sample does not show any signs of absorption, although the data do not prohibit one or two of the objects from being highly absorbed. We also find a strong correlation between Γ and L_{1 keV}; this may be due to differences in L_bol/L_Edd among the NLS1s in this sample. Such variations are seemingly in conflict with the current paradigm that NLS1s accrete near the Eddington limit. Most importantly, this sample shows that strong, ultrasoft X-ray emission is not a universal characteristic of NLS1s; in fact, a substantial number may exhibit weak and/or low-Γ X-ray emission.

We explore the near-infrared (NIR) K-band properties of galaxies within 93 galaxy clusters and groups using data from the Two Micron All Sky Survey. We use X-ray properties of these clusters to pinpoint cluster centers and estimate cluster masses. By stacking all these systems, we study the shape of the cluster luminosity function and the galaxy distribution within the clusters. We find that the galaxy profile is well described by the Navarro, Frenk, & White (NFW) profile with a concentration parameter c ~ 3, with no evidence for cluster mass dependence of the concentration. Using this sample, whose masses span the range from 3 × 10¹³ to 2 × 10¹⁵M_☉, we confirm the existence of a tight correlation between total galaxy NIR luminosity and cluster binding mass, which indicates that NIR light can serve as a cluster mass indicator. From the observed galaxy profile, together with cluster mass profile measurements from the literature, we find that the mass-to-light ratio is a weakly decreasing function of cluster radius and that it increases with cluster mass. We also derive the mean number of galaxies within halos of a given mass, the halo occupation number. We find that the mean number scales as N ∝ M^0.84±0.04 for galaxies brighter than M_K = -21, indicating that high-mass clusters have fewer galaxies per unit mass than low-mass clusters. Using published observations at high redshift, we show that higher redshift clusters have higher mean occupation numbers than nearby systems of the same mass. By comparing the luminosity function and radial distribution of galaxies in low-mass and high-mass clusters, we show that there is a marked decrease in the number density of galaxies fainter than M_* as one moves to higher mass clusters; in addition, extremely luminous galaxies are more probable in high-mass clusters. We explore several processes, including tidal interactions and merging, as a way of explaining the variation in galaxy population with cluster mass.

We have analyzed the Chandra, BeppoSAX, ASCA, and ROSAT PSPC observations of A754 and report evidence of a soft, diffuse X-ray component. A radial analysis shows that it is detected within a region that extends out to 8' from the X-ray center and that the emission is higher in the central region of the cluster. Fitting a thermal model to the combined BeppoSAX and PSPC spectra show excess emission below 1 keV in the PSPC and above 100 keV in the BeppoSAX PDS. The source, 26W20, is in the field of view of the PDS. The addition of a power law, with the spectral parameters measured by Silverman et al. in 1998 for 26W20, successfully models the hard component in the PDS. The excess soft emission can be attributed to a low-temperature, 0.77-1.21 keV, component. The soft excess is also modeled with a power law, although the 90% uncertainty for the normalization of the power law is consistent with zero. Either component added to a hot thermal component provides a statistically significant improvement over a single hot thermal component. The Chandra temperature map provides a detailed description of the thermal state of the gas on a scale of 100 kpc and larger and does not show any region cooler than 5.9 keV (90% confidence) within the region where the cool component was detected. Calculations of the expected emission from one or more groups randomly embedded in a hot gas component were performed that demonstrate that groups are a plausible source of ~1 keV emission, in that they can match the measured cool-component luminosity without violating the spatial temperature constraints provided by the temperature map. The cool component is centrally peaked in the cluster, and the gas density and temperature are relatively high, arguing against the warm hot intergalactic medium as the source of the X-ray emission. Furthermore, because the cool component is centrally peaked, the groups are likely embedded in the intracluster gas, rather than in the intercluster gas. The typical X-ray emission from early-type galaxies is not high enough to provide the total cool-component luminosity, 2.1 × 10⁴³ ergs s^-1. The peak of the cool component is located between the low-frequency radio halos, thus arguing against a nonthermal interpretation for the emission based on the synchrotron inverse Compton model, which requires that the nonthermal X-ray and radio emission be cospatial. Thus, we conclude that emission from embedded groups is the most likely origin of the cool component in A754.

We present new multifrequency radio continuum imaging of the dwarf starburst galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm reveal global continuum emission dominated by free-free emission, with only mild synchrotron components. Each of the major H II regions is detected; the individual spectral indices are thermal for the youngest regions (showing largest Hα emission) and nonthermal for the oldest. We do not detect any sources that appear to be associated with deeply embedded, dense, young clusters, although we have discovered one low-luminosity, obscured source that has no luminous optical counterpart and resides in the region of highest optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent star formation, these radio properties suggest that the youngest star formation complexes have not yet evolved to the point where their thermal spectra are significantly contaminated by synchrotron emission. The nonthermal components are associated with regions of older star formation that have smaller ionized gas components. These results imply a range of ages for the H II regions and radio components that agrees with our previous resolved stellar population analysis, where an extended burst of star formation has pervaded the disk of NGC 625 over the last ~50 Myr. We compare the nature of radio continuum emission in selected nearby dwarf starburst and Wolf-Rayet galaxies, demonstrating that thermal radio continuum emission appears to be more common in these systems than in typical H II galaxies with less recent star formation and more evolved stellar clusters.

This is the second in a series of papers on the effects of dust on multifluid, magnetohydrodynamic shock waves in weakly ionized molecular gas. We investigate the influence of dust on the critical shock speed, v_crit, above which C shocks cease to exist. Chernoff showed that v_crit cannot exceed the grain magnetosound speed, V_gms, if dust grains are dynamically well coupled to the magnetic field. Since V_gms ≃ 5 km s^-1 in a dense cloud or core, the potential implications for models of shock emission are profound. We present numerical simulations of steady shocks where the grains may be well or poorly coupled to the field. We use a time-dependent, multifluid MHD code that models the plasma as a system of interacting fluids: neutral particles, ions, electrons, and various "dust fluids" comprised of grains with different sizes and charges. Our simulations include grain inertia and grain charge fluctuations, but to highlight the essential physics we assume adiabatic flow and single-size grains and neglect the effects of chemistry. We show that the existence of a phase speed v_ϕ does not necessarily mean that C shocks will form for all shock speeds v_s less than v_ϕ. When the grains are weakly coupled to the field, steady, adiabatic shocks resemble shocks with no dust: the transition to J-type flow occurs at v_crit ≈ 2.76V_nA, where V_nA is the neutral Alfvén speed, and steady shocks with v_s > 2.76V_nA are J shocks with magnetic precursors in the ion-electron fluid. When the grains are strongly coupled to the field, v_crit = min (2.76V_nA,V_gms). Shocks with v_crit < v_s < V_gms have magnetic precursors in the ion-electron-dust fluid. Shocks with v_s > V_gms have no magnetic precursor in any fluid. We present time-dependent calculations to study the formation of steady, multifluid shocks. The dynamics differ qualitatively, depending on whether or not the grains and field are well coupled.

We have calculated the radiation field, dust grain temperatures, and far-infrared emissivity of numerical models of turbulent molecular clouds. When compared to a uniform cloud of the same mean optical depth, most of the volume inside the turbulent cloud is brighter, but most of the mass is darker. There is little mean attenuation from center to edge, and clumping causes the radiation field to be somewhat bluer. There is also a large dispersion, typically by a few orders of magnitude, of all quantities relative to their means. However, despite the scatter, the 850 μm emission maps are well correlated with surface density. The fraction of mass as a function of intensity can be reproduced by a simple hierarchical model of density structure.

The most accurate way to measure the energy levels for the O II 2p³ ground configuration has been from the forbidden lines in planetary nebulae. We present an analysis of modern planetary nebula data that nicely constrain the splitting within the ²D term and the separation of this term from the ground ⁴S_3/2 level. We extend this method to H II regions using high-resolution spectroscopy of the Orion Nebula, covering all six visible transitions within the ground configuration. These data confirm the splitting of the ²D term while additionally constraining the splitting of the ²P term. The energies of the ²P and ²D terms relative to the ground (⁴S) term are constrained by requiring that all six lines give the same radial velocity, consistent with independent limits placed on the motion of the O⁺ gas and the planetary nebula data.

Interstellar magnetic fields exist over a broad range of spatial scales, extending from large Galactic scales (~10 kpc) down to very small dissipative scales (≪1 pc). In this paper, we use a set of 490 pulsars distributed over roughly one-third of the Galactic disk out to a radius R ≃ 10 kpc (assuming R_☉ = 8.5 kpc) and combine their observed rotation and dispersion measures with their estimated distances to derive the spatial energy spectrum of the Galactic interstellar magnetic field over the scale range 0.5-15 kpc. We obtain a nearly flat spectrum, with a one-dimensional power-law index α = -0.37 ± 0.10 for E_B(k) = Ck^α and an rms field strength of approximately 6 μG over the relevant scales. Our study complements the derivation of the magnetic energy spectrum over the scale range 0.03-100 pc by Minter & Spangler, showing that the magnetic spectrum becomes flatter at larger scales. This observational result is discussed in the framework of current theoretical and numerical models.

MWC 349A is the brightest radio continuum star in the centimeter domain. The thermal radio continuum emission is believed to originate in an ionized bipolar flow that photoevaporates from the surfaces of a neutral Keplerian disk. In this work we present high angular resolution observations taken with the Very Large Array (VLA) at wavelengths from 7 mm to 90 cm that allow the study of structures over 2 orders of magnitude in size in this object. The 7 mm image shows an intermediate equatorial region ~004 wide, with no free-free emission, that could be the neutral photoevaporating disk around MWC 349A. Combining these data with archival observations at 1.3, 2, 3.6, 6, and 20 cm, we estimate that the flux increases with frequency as ν^0.67±0.03 and the angular size decreases with frequency as ν^-0.74±0.03, confirming the presence of a biconical thermal wind that expands at constant velocity. We also report the marginal detection of MWC 349A at 90 cm. At the wavelengths of 3.6, 6, and 20 cm, we image and model the interaction zone between the winds of MWC 349A and MWC 349B, supporting the physical association of these components. Finally, by comparing 6 cm data taken in 1982 and 1996 we find evidence of variability in MWC 349A that is interpreted as a decrease of ~2% in the mass-loss rate over the time interval of the observations.

We present a search for water maser emission at 22 GHz associated with young low-mass protostars in six H II regions—M16, M20, NGC 2264, NGC 6357, S125, and S140. The survey was conducted with the NRAO Very Large Array from 2000 to 2002. For several of these H II regions, ours are the first high-resolution observations of water masers. We detected 16 water masers: eight in M16, four in M20, three in S140, and one in NGC 2264. All but one of these were previously undetected. No maser emission was detected from NGC 6357 or S125. There are two principle results to our study. (1) The distribution of water masers in M16 and M20 does not appear to be random but instead is concentrated in a layer of compressed gas within a few tenths of a parsec of the ionization front. (2) Significantly fewer masers are seen in the observed fields than expected based on other indications of ongoing star formation, indicating that the maser-exciting lifetime of protostars is much shorter in H II regions than in regions of isolated star formation. Both of these results confirm predictions of a scenario in which star formation is first triggered by shocks driven in advance of ionization fronts and then truncated ~10⁵ yr later when the region is overrun by the ionization front.

The process of cosmic-ray first-order Fermi acceleration at relativistic shock waves is studied with the method of Monte Carlo simulations. The simulations are based on numerical integration of particle equations of motion in a turbulent magnetic field near the shock. In comparison to earlier studies, a few "realistic" features of the magnetic field structure are included. The upstream field consists of a mean field component inclined at some angle to the shock normal with finite-amplitude sinusoidal perturbations imposed upon it. The perturbations are assumed to be static in the local plasma rest frame. Their flat or Kolmogorov spectra are constructed with randomly drawn wavevectors from a wide range (k_min, k_max). The downstream field structure is derived from the upstream one as compressed at the shock. We present and discuss particle spectra and angular distributions obtained at mildly relativistic sub- and superluminal shocks. We show that particle spectra diverge from a simple power law; the exact shape of the spectrum depends on both the amplitude of the magnetic field perturbations and the wave power spectrum considered. Features such as spectrum hardening before the cutoff at oblique subluminal shocks and formation of power-law tails at superluminal ones are presented and discussed. The simulations have also been performed for parallel shock waves. The presence of finite-amplitude magnetic field perturbations leads to the formation of locally oblique field configurations at the shock and the respective magnetic field compressions. This results in the modification of the particle acceleration process, introducing some features present in oblique shocks, e.g., particle reflections from the shock. For the first time, we demonstrate for parallel shocks a (nonmonotonic) variation of the accelerated particle spectral index with the turbulence amplitude. At the end, a few astrophysical consequences of the results we obtained are mentioned.

Gamma-ray spectra from cosmic-ray proton and electron interactions with dense gas clouds have been calculated using a Monte Carlo event simulation code, GEANT4. Such clouds are postulated as a possible form of baryonic dark matter in the universe. The simulation fully tracks the cascade and transport processes that are important in a dense medium, and the resulting gamma-ray spectra are computed as a function of cloud column density. These calculations are used for predicting the Galactic diffuse gamma-ray spectrum that may be contributed by baryonic dark matter; the results are compared with data from the EGRET instrument and used to constrain the fraction of Galactic dark matter that may be in the form of dense gas clouds. In agreement with previous authors, we find useful constraints on the fraction of Galactic dark matter that may be in the form of low column density clouds (Σ ≲ 10 g cm^-2). However, this fraction rises steeply in the region of Σ ~ 10² g cm^-2, and for Σ ≳ 200 g cm^-2 we find that baryonic dark matter models are virtually unconstrained by the existing gamma-ray data.

In the favored progenitor scenario, Type Ia supernovae (SNe Ia) arise from a white dwarf accreting material from a nondegenerate companion star. Soon after the white dwarf explodes, the ejected supernova material engulfs the companion star; two-dimensional hydrodynamic simulations by Marietta et al. (2000) show that in the interaction, the companion star carves out a conical hole of opening angle 30°-40° in the supernova ejecta. In this paper we use multidimensional Monte Carlo radiative transfer calculations to explore the observable consequences of an ejecta-hole asymmetry. We calculate the variation of the spectrum, luminosity, and polarization with viewing angle for the aspherical supernova near maximum light. We find that the supernova looks normal from almost all viewing angles except when one looks almost directly down the hole. In the latter case, one sees into the deeper, hotter layers of ejecta. The supernova is relatively brighter and has a peculiar spectrum characterized by more highly ionized species, weaker absorption features, and lower absorption velocities. The spectrum viewed down the hole is comparable to those of the class of SN 1991T-like supernovae. We consider how the ejecta-hole asymmetry may explain the current spectropolarimetric observations of SNe Ia and suggest a few observational signatures of the geometry. Finally, we discuss the variety currently seen in observed SNe Ia and how an ejecta-hole asymmetry may fit in as one of several possible sources of diversity.

Explosions of Type Ic supernovae (SNe Ic) are investigated using a relativistic hydrodynamic code to study the role of their outermost layers of the ejecta in light-element nucleosynthesis through spallation reactions as a possible mechanism of the "primary" process. We have confirmed that the energy distribution of the outermost layers with a mass fraction of only 0.001% follows the empirical formula proposed by previous work when the explosion is furious. In such explosions, a significant fraction of the ejecta (>0.1% in mass) have energy greater than the threshold energy for spallation reactions. On the other hand, we find that the outermost layers of ejecta become more energetic than the empirical formula would predict when the explosion energy per unit ejecta mass is smaller than ~1.3 × 10⁵¹ ergs M. As a consequence, it is necessary to numerically calculate explosions to estimate light-element yields from SNe Ic. The usage of the empirical formula would overestimate the yields by a factor of ≳3 for energetic explosions, such as SN 1998bw, and underestimate the yields by a similar factor for less energetic explosions, such as SN 1994I. The yields of the light elements Li, Be, and B (LiBeB) from SNe Ic are estimated by solving the transfer equation of cosmic rays originated from ejecta of SNe Ic and compared with observations. The abundance ratios Be/O and B/O produced by each of our SNe Ic models are consistent with those of metal-poor stars. The total amounts of these elements estimated from observations indicate that energetic SNe Ic, such as SN 1998bw, could be candidates for a production site of Be and B in the Galactic halo only when the fraction of this type out of all the SNe was more than a factor of 100 higher than the value estimated from current observational data. This primary mechanism would predict that there are stars significantly deficient in light elements that were formed from the interstellar medium not affected by SNe Ic. Since this has no support from current observations, other primary mechanisms, such as the light-element formation in superbubbles, are needed for other types of SNe. The observed abundance pattern of all elements including heavy elements in metal-poor stars suggests that these two mechanisms should have supplied similar amounts of Be and B. Our calculations show that SNe Ic can not produce an appreciable amount of Li.

We performed 1.5-dimensional general relativistic hydrodynamic simulations with a Kerr metric to construct a model for high-frequency quasi-periodic oscillations (QPOs) in microquasars. The simulations were performed assuming an initial accretion disk without viscosity rotating around a Kerr black hole at sub-Keplerian velocity (sub-Keplerian case), which induces various wave modes everywhere in the disk. We found that quasi-periodic inward shock waves propagate from the accretion disk toward the black hole. The frequency of the shock formation is about the maximum epicyclic frequency in the disk (κ_max), which depends on the rotation of the black hole. In order to understand the mechanism of the shock formation, we also performed a simulation assuming an initial linear perturbation injected at one point in the Keplerian disk (linear perturbation case) and found an oscillation with frequency ~κ at the point where the perturbation injection occurred. To explain the simulation result, we derived an analytic solution for the time evolution of the linear perturbation of physical variables near the point of the perturbation injection and found that the time evolution of the oscillation can be described well. From comparison of the result in the sub-Keplerian case with that of the linear perturbation case, we found that the periodicity of the quasi-periodic shock formation in the sub-Keplerian case is due to a filtering effect by the epicyclic frequency distribution in the disk, which acts on the wave propagation toward the black hole. The only necessary condition for quasi-periodic shock formation is having a nonsteady character for the disks, which can be a source of acoustic waves. The frequency of the shock formation (~κ_max) is on the order of the frequency of the high-frequency QPOs in microquasars and depends on the rotation of the black hole. Hence, we can estimate the spin parameter (a) of a black hole candidate (BHC) in a microquasar by comparing the frequency of the high-frequency QPO with κ_max. The spin parameters of the BHCs in microquasars are roughly estimated to be a = 0.345 ± 0.345 for GRS 1915+105 and a = 0.895 ± 0.105 for GRO J1655-40.

Simulations in general relativity show that the outcome of collapse of a marginally unstable, uniformly rotating star spinning at the mass-shedding limit depends critically on the equation of state. For a very stiff equation of state, which is likely to characterize a neutron star, essentially all of the mass and angular momentum of the progenitor are swallowed by the Kerr black hole formed during the collapse, leaving nearly no residual gas to form a disk. For a soft equation of state with an adiabatic index Γ - 4/3 ≪ 1, which characterizes a very massive or supermassive star supported predominantly by thermal radiation pressure, as much as 10% of the mass of the progenitor avoids capture and goes into a disk about the central hole. We present a semianalytic calculation that corroborates these numerical findings and shows how the final outcome of such a collapse may be determined from simple physical considerations. In particular, we employ a simple energy variational principle with an approximate, post-Newtonian energy functional to determine the structure of a uniformly rotating, polytropic star at the onset of collapse as a function of polytropic index n, where Γ = 1 + 1/n. We then use this data to calculate the mass and spin of the final black hole and ambient disk. We show that the fraction of the total mass that remains in the disk falls off sharply as 3 - n (equivalently, Γ - 4/3) increases.

The physics of the hot spots on stellar surfaces and the associated variability of accreting magnetized rotating stars is investigated for the first time using fully three-dimensional magnetohydrodynamic simulations. The magnetic moment of the star, μ, is inclined relative to its rotation axis, Ω, by an angle Θ (we call this angle the "misalignment angle"), while the disk's rotation axis is parallel to Ω. A sequence of misalignment angles between Θ = 0° and 90° was investigated. The hot spots arise on the stellar surface because of the impact on the surface of magnetically channeled accretion streams. The distribution of different parameters in the hot spots reflects those in the funnel streams near the surface of the star. Typically, at small Θ the spots as observed are shaped like a bow curved around the magnetic axis, while at the largest values of Θ the spots are shaped like a bar crossing the magnetic pole. The physical parameters (density, velocity, temperature, matter, energy fluxes, etc.) increase toward the central regions of the spots; thus, the size of the spots is different at different values of these parameters. At relatively low density and temperature, the spots occupy approximately 10%-20% of the stellar surface, while at the highest values of these parameters this area may be less than 1% of the area of the star. The size of the spots increases with the accretion rate. Rotation of the star leads to the observed variability of brightness. The light curves were calculated for different values of Θ and inclination angles of the disk, i. They show a range of variability patterns, including curves with one maximum per period (at most angles Θ and i) and curves with two maxima per period (at large Θ and i). At small Θ, the funnel streams may rotate faster or slower than the star, and this may lead to quasi-periodic variability of the star. The results are of interest for understanding the variability and quasi variability of classical T Tauri stars, millisecond pulsars, and cataclysmic variables.

We report on the first X-ray observations of the neutron star soft X-ray transient (SXT) XTE J2123-058 in quiescence made by the Chandra X-Ray Observatory and BeppoSAX, as well as contemporaneous optical observations. In 2002, the Chandra spectrum of XTE J2123-058 is consistent with a power-law model or the combination of a blackbody plus a power law, but it is not well described by a pure blackbody. Using the interstellar value of N_H, the power-law fit gives Γ = 3.1 and indicates an 0.3-8 keV unabsorbed luminosity of 9 × 10³¹(d/8.5 kpc)² ergs s^-1 (90% confidence errors). Fits with models consisting of thermal plus power-law components indicate that the upper limit on the temperature of a 1.4 M_☉, 10 km radius neutron star with a hydrogen atmosphere is kT_eff < 66 eV, and the upper limit on the unabsorbed, bolometric luminosity is L_∞ < 1.4 × 10³² ergs s^-1, assuming d = 8.5 kpc. Of the neutron star SXTs that exhibit short (<1 yr) outbursts, including Aql X-1, 4U 1608-522, Cen X-4, and SAX J1810.8-2609, the lowest temperatures and luminosities are found for XTE J2123-058 and SAX J1810.8-2609. From the BeppoSAX observation of XTE J2123-058 in 2000, we obtained an upper limit on the 1-10 keV unabsorbed luminosity of 9 × 10³² ergs s^-1. Although this upper limit allows for the X-ray luminosity to have decreased between 2000 and 2002, that possibility is not supported by our contemporaneous R-band observations, which indicate that the optical flux increased significantly. Motivated by the theory of deep crustal heating by Brown and coworkers, we characterize the outburst histories of the five SXTs. The low quiescent luminosity for XTE J2123-058 is consistent with the theory of deep crustal heating without requiring enhanced neutron star cooling if the outburst recurrence time is ≳70 yr.

The merger of binary neutron stars is likely to lead to differentially rotating remnants. In this paper, we survey several cold nuclear equations of state (EOSs) and numerically construct models of differentially rotating neutron stars in general relativity. For each EOS we tabulate maximum allowed masses as a function of the degree of differential rotation. We also determine effective polytropic indices and compare the maximum allowed masses with those for the corresponding polytropes. We consistently find larger mass increases for the polytropes, but even for the nuclear EOSs we typically find maximum masses 50% higher than the corresponding values for nonrotating (Tolman-Oppenheimer-Volkoff) stars. We evaluate our findings for the six observed binary neutron star (pulsar) systems, including the recently discovered binary pulsar J0737-3039. For each EOS we determine whether their merger could automatically lead to prompt collapse to a black hole, or whether the remnant can be supported against collapse by uniform rotation (possibly as a supramassive star) or differential rotation (possibly as a hypermassive star). For hypermassive stars, delayed collapse to a black hole is likely. For the most recent EOSs we survey the merger remnants can all be supported by rotation against prompt collapse, but their actual fate will depend on the nonequilibrium dynamics of the coalescence event. Gravitational wave observations of coalescing binary neutron stars may be able to distinguish these outcomes—no, delayed, or prompt collapse—and thereby constrain possible EOSs.

Giant pulses and giant micropulses from pulsars are distinguished from normal pulsed emission by their large fluxes, rarity, approximately power-law distribution of fluxes, and, typically, occurrence in restricted phase windows. Here existing observations of flux distributions are manipulated into a common format and interpreted in terms of theories for wave growth in inhomogeneous media, with the aim of constraining the emission mechanism and source physics for giant pulses and micropulses. Giant micropulses near 2 GHz (PSRs B8033-45 and B1706-44) and 0.4 GHz (PSR B0950+08) have indices α = 6.5 ± 0.7 for the probability distribution P(E) of the electric field E, with P(E) ∝ E^-α. Giant pulses (PSRs B0531+24, B1937+214, and B1821-24) have α ranging from 4.6 ± 0.2 to 9 ± 2, possibly increasing with frequency. These are similar enough to regard giant micropulses and pulses as a single phenomenon with a common physical explanation. The power-law functional form and values of α observed are consistent with predictions for nonlinear wave collapse but inconsistent with known self-organized critical systems, nonlinear decay processes, and elementary burst theory. While relativistic beaming may be important, its statistics are yet to be predicted theoretically and collapse is currently the favored interpretation. Other possibilities remain, including stochastic growth theory (consistent with normal pulse emission) and, less plausibly, refractive lensing. Unresolved issues remain for all four interpretations, and suggestions for further work are given. The differences between normal and giant pulse emission suggest that they have distinct source regions and emission processes.

Properties of the grain-scattered X-ray halo of the eclipsing X-ray binary pulsar 4U 1538-52 are derived from a 25 ks observation by the Chandra X-Ray Observatory extending from just before its eclipse immersion to near mideclipse. Profiles of the observed halo compiled in two energy ranges, 2-4 and 4-6 keV, and three time intervals before and after the eclipse exhibit a three-peak shape indicative of a concentration of the interstellar dust grains in three discrete clouds along the line of sight. The observed profiles are fitted by the profiles of a simulated halo generated by a Monte Carlo ray-tracing code operating on a model of three discrete clouds and a spectrum of the photons emitted by the source over a period of time extending from 270 ks before the observation began until it ended. In the model, the spectrum before the observation began is expressed as a function of the orbital phase of the pulsar and is derived as an average over 6.3 yr of data accumulated by the All-Sky Monitor of the Rossi X-Ray Timing Explorer. The distances of the two nearer dust clouds are fixed at the distances of the peaks of atomic hydrogen derived from the 21 cm spectrum in the direction of the X-ray source, namely at 1.30 and 2.56 kpc. With these constraints, a good fit is achieved with the source at a distance 4.5 kpc, the distance of the third cloud at 4.05 kpc, the total scattering optical depth of the three clouds equal to 0.159 at 3 keV, and the column density of hydrogen set to 4.6 × 10²² cm^-2. With A_V = 6.5 ± 0.3 mag for the binary companion star QV Nor, the ratio of the scattering optical depth at 3 keV to the visual extinction is 0.0234 ± 0.0010 mag^-1. The column density of hydrogen in the model is much greater than the column density of atomic hydrogen derived from the 21 cm spectrum, which indicates that most of the hydrogen is in molecules, probably concentrated in the three dust clouds.

When a white dwarf (WD) is weakly magnetized and its accretion disk is thin, accreted material first reaches the WD's surface at its equator. This matter slows its orbit as it comes into corotation with the WD, dissipating kinetic energy into thermal energy and creating a hot band of freshly accreted material around the equator. Radiating in the extreme-ultraviolet and soft X-ray, this material moves toward the pole as new material piles up behind it, eventually becoming part of the WD once it has a temperature and rotational velocity comparable to those at the surface. We present a set of solutions that describe the properties of this "spreading layer" in the steady state limit on the basis of the conservation equations recently derived by Inogamov and Sunyaev for accreting neutron stars. Our analysis and subsequent solutions show that the case of WDs is qualitatively different. We investigate example solutions of the spreading layer for a WD of mass M = 0.6 M_☉ and radius R = 9 × 10⁸ cm. These solutions show that the spreading layer typically extends to an angle of θ_SL ≈ 0.01-0.1 (with respect to the equator), depending on accretion rate and the magnitude of the viscosity. At low accretion rates, ≲ 10¹⁸ g s^-1, the amount of spreading is negligible, and most of the dissipated energy is radiated back into the accretion disk. When the accretion rate is high, such as in dwarf novae, the material may spread to latitudes high enough to be visible above the accretion disk. The effective temperature of the spreading layer is ~(2-5) × 10⁵ K with approximately T_eff ∝ ^9/80. This power-law dependence on is weaker than for a fixed radiating area and may help explain extreme-ultraviolet observations during dwarf novae. We speculate about other high accretion rate systems ( ≳ 10¹⁸ g s^-1) that may show evidence for a spreading layer, including symbiotic binaries and supersoft sources.

We report the results of a 45 ks Chandra observation of the cataclysmic variable (CV) V426 Ophiuchus. The high-resolution spectrum from the high-energy transmission grating spectrometer is most consistent with a cooling flow model, placing V426 Oph among the group of CVs including U Gem and EX Hya. An uninterrupted light curve was also constructed, in which we detect a significant 4.2 hr modulation together with its first harmonic at 2.1 hr. Reanalysis of archival Ginga and ROSAT X-ray light curves also reveals modulations at periods consistent with 4.2 and/or 2.1 hr. Furthermore, optical photometry in V, simultaneous with the Chandra observation, indicates a modulation anticorrelated with the X-ray, and later more extensive R-band photometry finds a signal at ~2.1 hr. The earlier reported X-ray periods at ~0.5 and 1 hr appear to be only transient and quasi-periodic in nature. In contrast, the 4.2 hr period or its harmonic is stable and persistent in X-ray/optical data from 1988 to 2003. This periodicity is clearly distinct from the 6.85 hr orbit and could be due to the spin of the white dwarf. If this is the case, V426 Oph would be the first long-period intermediate polar with a ratio P_spin/P_orb of 0.6. However, this interpretation requires unreasonable values of magnetic field strength and mass accretion rate.

The pulsating white dwarf star PY Vul (G185-32) exhibits pulsation modes with peculiar properties that set it apart from other variable stars in the ZZ Ceti (variable DA white dwarf [DAV]) class. These peculiarities include a low total pulsation amplitude, a mode with bizarre amplitudes in the ultraviolet, and a mode harmonic that exceeds the amplitude of its fundamental. Here we present optical time-series spectroscopy of PY Vul acquired with the Keck II Low Resolution Imaging Spectrograph. Our analysis has revealed that the mode with unusual UV amplitudes also has distinguishing characteristics in the optical. Comparison of its line profile variations to models suggests that this mode has a spherical degree of 4. We show that all the other peculiarities in this star are accounted for by a dominant pulsation mode of l = 4 and propose this hypothesis as a solution to the mysteries of PY Vul.

The discovery of a low-luminosity common proper-motion companion to the white dwarf GD 392 at a wide separation of 46'' is reported. BVRI photometry suggests a low temperature (T_eff ~ 4000 K), while JHK data strongly indicate suppressed flux at all near-infrared wavelengths. Thus, GD 392B is one of the few white dwarfs to show significant collision-induced absorption due to the presence of photospheric H₂ and the first ultracool white dwarf detected as a companion to another star. Models fail to explain GD 392B as a normal-mass white dwarf. If correct, the cool companion may be explained as a low-mass white dwarf or unresolved double degenerate. The similarities of GD 392B to known ultracool degenerates are discussed, including some possible implications for the faint end of the white dwarf luminosity function.

We present quantitative studies of eight late O- and early B-type supergiants in the Magellanic Clouds using far-ultraviolet Far Ultraviolet Spectroscopic Explorer, ultraviolet International Ultraviolet Explorer/Hubble Space Telescope, and optical VLT-UVES spectroscopy. Temperatures, mass-loss rates, and CNO abundances are obtained using the non-LTE, spherical, line-blanketed model atmosphere code of Hillier & Miller. We support recent results for lower temperatures of OB-type supergiants as a result of stellar winds and blanketing, which amounts to ~2000 K at B0 Ia. In general, Hα-derived mass-loss rates are consistent with UV and far-UV spectroscopy, although from consideration of the S IV λλ1063, 1073 doublet, clumped winds are preferred over homogenous models. AV 235 (B0 Iaw) is a notable exception, which has an unusually strong Hα profile that is inconsistent with the other Balmer lines and UV wind diagnostics. We also derive CNO abundances for our sample, revealing substantial nitrogen enrichment, with carbon and oxygen depletion. Our results are supported by comparison with the Galactic supergiant HD 2905 (BC0.7 Ia) for which near-solar CNO abundances are obtained. This bolsters previous suggestions that "normal" OB-type supergiants exhibit atmospheric compositions indicative of partial CNO processing.

We search for microvariability in a sample of 485 Mira variables with high-quality I-band light curves from the second-generation Optical Gravitational Lensing Experiment (OGLE-II). Rapid variations with amplitudes in the ~0.2-1.1 mag range lasting hours to days were discovered in Hipparcos data by de Laverny et al. Our search is primarily sensitive to events with timescales of ~1 day but retains a few percent efficiency (per object) for detecting unresolved microvariability events as short as 2 hr. We do not detect any candidate events. Assuming that the distribution of the event time profiles is identical to that from the Hipparcos light curves, we derive a 95% confidence level upper limit of 0.038 yr^-1 star^-1 for the rate of such events (1 per 26 yr per average object of the ensemble). The high event rates of the order of ~1 yr^-1 star^-1 implied by the Hipparcos study in the H_P band are excluded with high confidence by the OGLE-II data in the I band. Our nondetection could still be explained by much redder spectral response of the I filter compared to the H_P band or by population differences between the bulge and the solar neighborhood. In any case, the OGLE-II I-band data provide the first limit on the rate of the postulated microvariability events in Mira stars and offer new quantitative constraints on their properties. Similar limits are obtained for other pulse shapes and a range of the assumed timescales and size-frequency distributions.

We present a spectroscopic study of candidate brown dwarf members of the Orion Nebula cluster (ONC). We obtained new J- and/or K-band spectra of ~100 objects within the ONC that are expected to be substellar on the basis of their K magnitudes and H-K colors. Spectral classification in the near-infrared of young low-mass objects is described, including the effects of surface gravity, veiling due to circumstellar material, and reddening. From our derived spectral types and existing near-infrared photometry, we construct an H-R diagram for the cluster. Masses are inferred for each object and used to derive the brown dwarf fraction and assess the mass function for the inner 51 × 51 of the ONC, down to ~0.02 M_☉. The logarithmic mass function rises to a peak at ~0.2 M_☉, similar to previous initial mass function determinations derived from purely photometric methods but falls off more sharply at the hydrogen-burning limit before leveling through the substellar regime. We compare the mass function derived here for the inner ONC with those presented in recent literature for the sparsely populated Taurus cloud members and the rich cluster IC 348. We find good agreement between the shapes and peak values of the ONC and IC 348 mass distributions but little similarity between the ONC and Taurus results.

By collecting optical and infrared photometry and low-resolution spectroscopy, we have identified a large number of low-mass stars and brown dwarf candidates belonging to the young cluster (∼5 Myr) associated with the binary star λ Orionis. The lowest mass object found is an M8.5 with an estimated mass of 0.02 M_⊙ (∼0.01 M_⊙ for objects without spectroscopic confirmation). For those objects with spectroscopy, the measured strength of the Hα emission line follows a distribution similar to other clusters with the same age range, with larger equivalent widths for cooler spectral types. Three of the brown dwarfs have Hα emission equivalent widths of order 100 Å, suggesting that they may have accretion disks and thus are the substellar equivalent of classical T Tauri stars. We have derived the initial mass function for the cluster. For the substellar regime, the index of the mass spectrum is α = 0.60 ± 0.06, very similar to other young associations.

Radial velocity data sets hold information about the direct observability (e.g., separation and flux) of inferred companions. They also contain information about the types of Keplerian solution compatible with the data. "Monte Carlo projection" and "χ² portrayal" are two techniques for discovering and pursuing the implications of this information. The first (projection) involves random solutions consistent with the data set, from which we can estimate (1) the probability distribution of the true solution in the six-dimensional space of the Keplerian parameters and (2) the probability distribution of the companion's position in space at future times, in order to predict observability. The second technique (portrayal) involves the distribution in parameter space of values of the numerical χ² function, from which we can estimate the regions that contain the true solution at various levels of confidence. We study the case of HD 72659, a Sun-like star at 51 pc with a radial velocity companion inferred from 16 data points. We find at least two types of Keplerian solutions present in the data set: (1) periods 2500-25,000 days and eccentricity 0-0.8 (type A1), (2) periods 25,000-250,000 days and eccentricity 0.8-0.95 (type A2), and (3) periods 2000-2500 days and eccentricity 0-0.5 (type B). (Types A1 and A2 may not be distinct.) Pursuing direct observability, we randomize the inclination angle and compute the apparent separation, true separation, and phase angle of the companion. We compute a minimum flux ratio to the star assuming no self-luminosity and that the companion is Jupiter sized and has Jupiter's albedo and the phase function of a Lambert sphere. We plot the probability distribution of direct observability at specific epochs.

A new extrasolar system consisting of two planets orbiting HD 169830 has been discovered by the Geneva Extrasolar Planet Search team. We study its orbital dynamics in the framework of the N-body problem. The analysis of the orbital stability is performed using long-term integrations and fast indicators, the mean exponential growth factor of nearby orbits, and a modified Fourier transform. The HD 169830 system appears to be located in a wide stable region of the phase space. The ratio of the mean motions of the planets, HD 169830 b and c, is between the low-order mean motion resonances, 9 : 1 and 10 : 1. The long-term integration of the coplanar configurations, conducted over 1 Gyr, reveals that the eccentricities of the companions reach large values, ≃0.4-0.5, but there is no sign of instability. Highly inclined configurations appear to be stable as well. The planetary system around HD 169830 belongs to the class of hierarchical planetary systems. In particular, the orbital parameters of the planets around HD 169830 resemble those of another two-planet system, around HD 12661. We investigate whether these two planetary systems are dynamically similar.

The Kelvin-Helmholtz (KH) and tearing instabilities are likely to be important for the process of fast magnetic reconnection that is believed to explain the observed explosive energy release in solar flares. Theoretical studies of the instabilities, however, typically invoke simplified initial magnetic and velocity fields that are not solutions of the governing magnetohydrodynamic (MHD) equations. In the present study, the stability of a reconnecting current sheet is examined using a class of exact global MHD solutions for steady state incompressible magnetic reconnection, discovered by Craig & Henton. Numerical simulation indicates that the outflow solutions where the current sheet is formed by strong shearing flows are subject to the KH instability. The inflow solutions where the current sheet is formed by a fast and weakly sheared inflow are shown to be tearing unstable. Although the observed instability of the solutions can be interpreted qualitatively by applying standard linear results for the KH and tearing instabilities, the magnetic field and plasma flow, specified by the Craig-Henton solution, lead to the stabilization of the current sheet in some cases. The sensitivity of the instability growth rate to the global geometry of magnetic reconnection may help in solving the trigger problem in solar flare research.

In order to understand the nature of magnetic reconnection in "free space," which is free from any influence of external circumstances, I have studied the structure of spontaneous reconnection outflow using a shock tube approximation. The reconnection system of this case continues to expand self-similarly. This work aims to (1) solve the structure of reconnection outflow and (2) clarify the determination mechanism of the reconnection rate of the "self-similar evolution model" of fast reconnection. Many cases of reconnection in astrophysical phenomena are characterized by the huge dynamic range of expansion of the size (~10⁷ for typical solar flares). Although such reconnection is intrinsically time dependent, the specialized model underlying the situation has not been established yet. The theoretical contribution of this paper is in obtaining a solution for outflow structure that is absent in our previous papers proposing the above new model. The outflow has a shock tube-like structure, i.e., forward slow shock, reverse fast shock, and contact discontinuity between them. By solving the structure in a sufficiently wide range of plasma-β, 0.001 ≤ β ≤ 100, we obtain an almost constant reconnection rate (~0.05: this value is the maximum for spontaneous reconnection and is consistent with previous models) and boundary value along the edge of the outflow (good agreement with our simulation result), which is important to solve the inflow region. Note that everything, including the reconnection rate, is spontaneously determined by the reconnection system itself in our model.

We have studied Mees Solar Observatory Hα spectroheliogram and Doppler velocity movies of surges in 11 active regions. We used these movies to observe the surges' rotating motion, direction of propagation, and the implied handedness of that motion. We were able to coregister movies containing 47 surges with Haleakala Stokes Polarimeter vector magnetograms. We could hence determine the direction of twist stored in the magnetic field at the point of origin of each surge. We found (with a 99% confidence) that the direction of observed spin of these surges is consistent with the relaxation of the stored twist in the magnetic field. Magnetic reconnection of twisted flux tubes with their less twisted surroundings can account for the production and rotating motion of these surges.

A surge event with multi-instrument and multiwavelength observations is presented. The event occurred in a surge-productive region on 2001 August 30. TRACE white-light images, Huairou vector magnetograms, and Hβ filtergrams show that this surge was closely associated with new bipoles that emerged in 1 hr. Evidence of enhanced magnetic cancellation is revealed at the footpoints of the surge where a simultaneous surge flare is shown in the Hβ line. In particular, the interesting observational results for this surge are that (1) the surge was associated with pronounced photospheric bright points near the emerging spots; (2) in vector magnetograms, during the surge ascending period, reliable transverse fields appeared between the emerging flux and the ambient fields cancelled at the base of the surge, and these fresh transverse fields disappeared after 20 minutes; (3) the preceding spots of the emerged bipoles almost disappeared in white-light images when the surge activity stopped; and (4) the TRACE UV (1550 Å) channel showed a bright surge (~10⁵ K) well correlated with the dark Hβ surge (~10³-10⁴ K). The UV surge had a bright spike shape and spouted out along the outer edges of the Hβ surge. Similarly, the bright components in TRACE EUV (~10⁶ K) were also located at the edges of the Hβ surge. Both the SOHO/EIT and Yohkoh/SXT observations demonstrate that the jet plasma was ejected from one footpoint of a flaring coronal loop identified during the surge. We estimate the magnetic energy released from the site of the magnetic cancellation, the kinetic energy of the surge, and the thermal energy for the loop brightening in SXR, finding that the magnetic reconnection could supply enough energy for the surge activities and the coronal loop heating. For this surge, all the correlated phenomena in multiwavelengths (Hβ, white light, UV, EUV, and SXR) are in good temporal and spatial relationship. These facts support a magnetic reconnection model in which surges originate in the low atmosphere. Moreover, low-altitude magnetic reconnections can result in the magnetic cancellation observed on the photosphere.

We present three methods for deriving the velocity field in magnetized regions of the Sun's photosphere. As a preliminary step, we introduce a Fourier-based local correlation tracking (LCT) routine that we term "FLCT." By explicitly employing the observation made by Démoulin & Berger, that results determined by LCT applied to magnetograms involve a combination of all components of the velocity and magnetic fields, we show that a three-component velocity field can be derived, in a method we term algebraic decomposition, or ADC. Finally, we introduce ILCT, a method that enforces consistency between the normal component of the induction equation and results obtained from LCT. When used with photospheric vector magnetograms, ILCT determines a three-component photospheric velocity field suitable for use with time sequences of such magnetograms to drive boundary conditions for MHD simulations of the solar corona. We present results from these methods applied to vector magnetograms of NOAA AR 8210 on 1998 May 1.

We measure differences in structure between active and quiet regions of the Sun using the frequencies of high-degree modes determined from ring-diagram analyses. We find that both the speed of sound and the adiabatic index Γ₁ differ in active regions as compared with quiet regions. In the immediate subsurface layers, the sound speed is lower in active regions, but below a depth of about 7 Mm the opposite is true. A comparison of sound-speed inversion results with those for Γ₁ indicates that at least a part of the differences between active and quiet regions is likely to be due to the structural and thermal perturbations caused by magnetic fields in the active region.

This work extends our previous two-dimensional self-consistent model of the cosmic rays interacting with the solar wind to include anomalous cosmic rays. As before, energetic particles are described kinetically using a Parker equation. The model includes diffusion, convection, and drift effects, as well as shock and compression acceleration and expansion cooling by nonuniform solar wind flow. A new numerical model has been developed featuring an adaptive-mesh refinement algorithm to accommodate small diffusive length scales of low-energy shock-accelerated particles. We show that anomalous cosmic rays have only a minor effect on the termination shock during the time near solar minima. Specifically, cosmic-ray gradients cause the subshock to move away from the Sun by about 1 AU with its compression ratio decreasing by about 5% compared to the reference case without cosmic-ray effects. We also study the effect of solar wind slowdown by charge exchange downstream of the termination shock, producing compressive flow in this region and resulting in additional acceleration of anomalous cosmic rays in the heliosheath. For the first time, spectra calculated with our self-consistent model show a good agreement with the cosmic-ray data from the two Voyager spacecraft, giving more confidence in the model predictions than the previous parametric studies.

We present an XMM-Newton spectrum of diffuse X-ray emission from within the solar system. The spectrum is dominated by probable C VI lines at 0.37 and 0.46 keV, an O VII line at 0.56 keV, O VIII lines at 0.65 and ~0.8 keV, Ne IX lines at ~0.92 keV, and Mg XI lines at ~1.35 keV. This spectrum is consistent with that expected from charge exchange emission between the highly ionized solar wind and either interstellar neutrals in the heliosphere or material from Earth's exosphere. The emission is clearly seen as a low-energy (E < 1.5 keV) spectral enhancement in one of a series of four observations of the Hubble Deep Field-North. The X-ray enhancement is concurrent with an enhancement in the solar wind measured by Advanced Composition Explorer, Wind, and Solar and Heliospheric Observatory spacecraft. The solar wind enhancement reaches a flux level an order of magnitude more intense than typical fluxes at 1 AU and has a significantly enhanced O⁺⁷/O⁺⁶ ratio. Besides being of interest in its own right for studies of the solar system, this emission can have significant consequences for observations of cosmological objects. It can provide emission lines at zero redshift, which are of particular interest in studies of diffuse thermal emission (e.g., O VII and O VIII), and which can therefore act as contamination in the spectra of objects that cover the entire detector field of view. We propose the use of solar wind monitoring data as a diagnostic to screen for such possibilities.

We report the detection of atomic chlorine emissions in the atmosphere of Io using Hubble Space Telescope observations with the Goddard High Resolution Spectrograph (GHRS). The Cl I λ1349 dipole allowed and Cl I] λ1386 forbidden transition multiplets are detected at a signal-to-noise ratio of 6 and 10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996. Oxygen and sulfur emissions are simultaneously detected with the chlorine, which allows for self-consistent abundance ratios of chlorine to these other atmospheric species. The disk-averaged ratios are Cl/O = 0.017 ± 0.008, Cl/S = 0.10 ± 0.05, and S/O = 0.18 ± 0.08. We also derive a geometric albedo of 1.0% ± 0.4% for Io at 1335 Å, assuming an SO₂ atmospheric column density of 1 × 10¹⁶ cm^-2.

The proposed field of view of the Kepler mission is at an ecliptic latitude of ~55°, where the surface density of scattered Kuiper Belt objects (KBOs) is a few percent that in the ecliptic plane. The rate of occultations of Kepler target stars by scattered KBOs with radii r ≳ 10 km is ~10^-6 to 10^-4 star^-1 yr^-1, where the uncertainty reflects the current ignorance of the thickness of the scattered KBO disk and the faint-end slope of their magnitude distribution. These occultation events will last only ~0.1% of the planned t_exp = 15 minute integration time and thus will appear as single data points that deviate by tiny amounts. However, given the target photometric accuracy of Kepler, these deviations will nevertheless be highly significant, with typical signal-to-noise ratios (S/Ns) of ~10. I estimate that ~1-20 of the 10⁵ main-sequence stars in Kepler's field of view will exhibit detectable occultations during its 4 yr mission. For unresolved events, the S/N of individual occultations scales as t, and the minimum detectable radius could be decreased by an order of magnitude to ~1 km by searching the individual 3 s readouts for occultations. I propose a number of methods by which occultation events may be differentiated from systematic effects. Kepler should measure or significantly constrain the frequency of highly inclined, ~10 km-sized KBOs.

We further investigate subpixel event repositioning (SER) algorithms in application to Chandra X-Ray Observatory (Chandra) CCD imaging. SER algorithms have been applied to backside-illuminated (BI) Advanced CCD Imaging Spectrometer (ACIS) devices and demonstrate spatial resolution improvements in Chandra ACIS observations. Here a new SER algorithm that is charge split-dependent is added to the SER family. We describe the application of SER algorithms to frontside-illuminated (FI) ACIS devices. The results of SER for FI CCDs are compared with those obtained from SER techniques applied to BI CCD event data. Both simulated data and Chandra ACIS observations of the Orion Nebular Cluster were used to test and evaluate the achievement of the various SER techniques.

Image restoration including deconvolution techniques offers a powerful tool to improve resolution in images and to extract information on the multiscale structure stored in astronomical observations. We present a new method for statistical deconvolution, which we call expectation through Markov Chain Monte Carlo (EMC2). This method is designed to remedy several shortfalls of currently used deconvolution and restoration techniques for Poisson data. We use a wavelet-like multiscale representation of the true image to achieve smoothing at all scales of resolution simultaneously, thus capturing detailed features in the image at the same time as larger scale extended features. Thus, this method smooths the image, while maintaining the ability to effectively reconstruct point sources and sharp features in the image. We use a principled, fully Bayesian model-based analysis, which produces extensive information about the uncertainty in the fitted smooth image, allowing assessment of the errors in the resulting reconstruction. Our method also includes automatic fitting of the multiscale smoothing parameters. We show several examples of application of EMC2 to both simulated data and a real astronomical X-ray source.

Branching ratios and absolute cross sections have been measured for the dissociative recombination of S¹⁸O using the CRYRING ion storage ring. The branching ratio of the S¹⁸O + e^- → S¹⁸O + ¹⁸O channel amounts to 61%, while the three-body breakup S¹⁸O + e^- → S + 2¹⁸O accounts for the remaining 39% of the total reaction. The cross section of the reaction could be fitted by the expression σ = × 10^-15E^-0.96±0.02 cm², which leads to a thermal reaction rate of k(T) = × 10^-7(T/300 K)^-0.52±0.02 cm³ mol^-1 s^-1.

The High Resolution Fly's Eye (HiRes) experiment is an air fluorescence detector which, operating in stereo mode, has a typical angular resolution of 06 and is sensitive to cosmic rays with energies above 10¹⁸ eV. The HiRes cosmic-ray detector is thus an excellent instrument for the study of the arrival directions of ultra-high-energy cosmic rays. We present the results of a search for anisotropies in the distribution of arrival directions on small scales (<5°) and at the highest energies (>10¹⁹ eV). The search is based on data recorded between 1999 December and 2004 January, with a total of 271 events above 10¹⁹ eV. No small-scale anisotropy is found, and the strongest clustering found in the HiRes stereo data is consistent at the 52% level with the null hypothesis of isotropically distributed arrival directions.

We study the rest-frame (U - V) color-magnitude relation in four clusters at redshifts 0.7-0.8, drawn from the ESO Distant Cluster Survey (EDisCS). We confirm that the red-sequence galaxies in these clusters can be described as an old, passively evolving population, and we demonstrate that, by comparison with the Coma Cluster, there has been significant evolution in the stellar mass distribution of red-sequence galaxies since z ∼ 0.75. The EDisCS clusters exhibit a deficiency of low-luminosity passive red galaxies. Defining as "faint" all galaxies in the passive evolution-corrected range 0.4 ≳ L/L_* ≳ 0.1, the luminous-to-faint ratio of red-sequence galaxies varies from 0.34 ± 0.06 for the Coma Cluster to 0.81 ± 0.18 for the high-redshift clusters. These results exclude a synchronous formation of all red-sequence galaxies and suggest that a large fraction of the faint red galaxies in current clusters moved on to the red sequence relatively recently. Their star formation activity presumably came to an end at z ≲ 0.8.

We present Chandra observations of the cool cluster A168, for which previous X-ray imaging and optical studies indicated a merger of two subclusters nearly in the plane of the sky. We derive a temperature map for A168, which shows that the merger has proceeded beyond the core passage and is near subcluster turnaround. It also reveals an unusual feature: the gas core of one of the subclusters forms a tonguelike structure extending ahead (in the direction of motion) of the subcluster center. The coolest cluster gas is found in a crescent-shaped region at the tip of this tongue and forms a cold front in pressure equilibrium with the external gas. In contrast with this feature's forward location, previously observed merger cold fronts (e.g., A3667, 1E 0657-56) lagged behind their host subclusters, as expected in the presence of ram pressure. We propose that A168 illustrates a much later stage in the evolution of a cold front, when its host subcluster approaches the apocenter of the merger orbit where the ram pressure on its gas drops sharply. As a result, a large chunk of the subcluster gas "slingshots" past the dark matter center, becomes unbound from the subcluster, and expands adiabatically, as seen in some recent hydrodynamic simulations.

We present the two-point correlation function (2PCF) of narrow-line active galactic nuclei (AGNs) selected within the First Data Release of the Sloan Digital Sky Survey. Using a sample of 13,605 AGNs in the redshift range 0.055 < z < 0.2, we find that the AGN autocorrelation function is consistent with the observed galaxy autocorrelation function on scales from 0.2 to greater than 100 h^-1 Mpc. The AGN hosts trace an intermediate population of galaxies and are not detected in either the bluest (youngest) disk-dominated galaxies or many of the reddest (oldest) galaxies. We show that the AGN 2PCF is dependent on the luminosity of the narrow [O III] emission line (L), with low L AGNs having a higher clustering amplitude than high L AGNs. This is consistent with lower activity AGNs residing in more massive galaxies than higher activity AGNs, and L providing a good indicator of the fueling rate. Using a model relating halo mass to black hole mass in cosmological simulations, we show that AGNs hosted by ~10¹²M_☉ dark matter halos have a 2PCF that matches that of the observed sample. This mass scale implies a mean black hole mass for the sample of M_BH ~ 10⁸M_☉.

Cold dark matter models for galaxy formation predict that low-mass systems will be the first sites of star formation. As these objects have shallow gravitational potential wells, the subsequent growth of their stellar populations may be halted by heating and gas loss due to reionization. This effect has been suggested to have profoundly influenced properties of present-day dwarf galaxies, including their stellar populations and even survival as visible galaxies. In this Letter we draw on results from quantitative studies of Local Group dwarf galaxy star formation histories, especially for Milky Way satellites, to show that no clear signature exists for a widespread evolutionary impact from reionization. All nearby dwarf galaxies studied in sufficient detail contain ancient populations indistinguishable in age from the oldest Galactic globular clusters. Ancient star formation activity proceeded over several gigayears, and some dwarf spheroidal galaxies even experienced fairly continuous star formation until just a few gigayears ago. Despite their uniformly low masses, their star formation histories differ considerably. The evolutionary histories of nearby dwarf galaxies appear to reflect influences from a variety of local processes rather than a dominant effect from reionization.

The Large Magellanic Cloud (LMC) has a unique cluster formation history in that nearly all of its globular clusters were formed either ~13 Gyr ago or less than ~3 Gyr ago. It is not clear what physical mechanism is responsible for the most recent cluster formation episode and thus the mysterious age gap between the LMC clusters. We first present results of gasdynamical N-body simulations of the evolution of the LMC in the context of its Galactic orbit and interactions with the Small Magellanic Cloud (SMC), paying special attention to the effect of tidal forces. We find that the first close encounter between the LMC and the SMC about 4 Gyr ago was the beginning of a period of strong tidal interaction that likely induced dramatic gas cloud collisions, leading to an enhancement of the formation of globular clusters that has been sustained by strong tidal interactions to the present day. The tidal interaction results in the formation of a barred, elliptical thick disk in the LMC. The model also predicts the presence of a large diffuse stellar stream circling the Galaxy, which originated from the LMC.

Recently, radial velocities have been measured for a large sample of M giants from the Two Micron All Sky Survey catalog, selected to be part of the Sgr dwarf leading and trailing streams. Here we present a comparison of their kinematics to models of the Sgr dwarf debris orbiting Galactic potentials, with halo components of varying degrees of flattening and elongation. This comparison shows that the portion of the trailing stream mapped so far is dynamically young and hence does not provide very stringent constraints on the shape of the Galactic dark matter halo. The leading stream, however, contains slightly older debris, and its kinematics provide for the first time direct evidence that the dark matter halo of our Galaxy has a prolate shape with an average density axis ratio within the orbit of Sgr close to 5/3.

We report the detection, in archival ROSAT and ASCA observations, of X-ray emission from the direction of DA 495 (G65.7+1.2), a likely supernova remnant of uncertain classification but with similarities to the Crab Nebula. An unusual feature of the radio nebula is its annular morphology, with a flux minimum at the geometrical center. In the soft X-ray band, the ROSAT data reveal a compact source near the edge of the central radio "hole"; the hard X-ray morphology, at the limit of ASCA's angular resolution, is suggestive of extended emission coincident with the ROSAT source. The X-ray flux is roughly constant with time, and its spectrum is well described by a power law with photon index ~1.7. Taken together, this evidence suggests identification of the X-ray source with a magnetospherically active neutron star and its associated wind nebula. Timing analysis of the ASCA data yields only a weak upper bound on pulsations with periods ≳30 ms. These results reveal for the first time the high-energy engine that powers the DA 495 radio nebula and strengthen its classification as a plerionic supernova remnant, one that may represent the poorly explored late evolutionary stages of Crab-like nebulae.

Asymmetric variability in ultraviolet images of the Homunculus obtained with the Advanced Camera for Surveys/High Resolution Camera on the Hubble Space Telescope suggests that η Carinae is indeed a binary system. Images obtained before, during, and after the recent "spectroscopic event" in 2003.5 show alternating patterns of bright spots and shadows on opposite sides of the star before and after the event, providing a strong geometric argument for an azimuthally evolving, asymmetric UV radiation field as one might predict in some binary models. The simplest interpretation of these UV images, where excess UV escapes from the secondary star in the direction away from the primary, places the major axis of the eccentric orbit roughly perpendicular to our line of sight, sharing the same equatorial plane as the Homunculus, and with apastron for the hot secondary star oriented toward the southwest of the primary. However, other orbital orientations may be allowed with more complicated geometries. Selective UV illumination of the wind and ejecta may be partly responsible for line profile variations seen in spectra. The brightness asymmetries cannot be explained plausibly with delays due to light-travel time alone, so a single-star model would require a seriously asymmetric shell ejection.

Despite much theoretical and observational progress, there is no known firm upper limit to the masses of stars. Our understanding of the interplay between the immense radiation pressure produced by massive stars in formation and the opacity of infalling material is subject to theoretical uncertainties, and many observational claims of "the most massive star" have failed the singularity test. LBV 1806-20 is a particularly luminous object, L ∼ 10⁶L_⊙, for which some have claimed very high mass estimates (M_initial > 200 M_⊙), based in part on its similarity to the Pistol star. We present high-resolution near-infrared spectroscopy of LBV 1806-20, showing that it is possibly a binary system with components separated in velocity by ∼70 km s^-1. If correct, then this system is not the most massive star known, yet it is a massive binary system. We argue that a binary, or merged, system is more consistent with the ages of nearby stars in the LBV 1806-20 cluster. In addition, we find that the velocity of V_LSR = 36 km s^-1 is consistent with a distance of 11.8 kpc, a luminosity of 10^6.3L_⊙, and a system mass of ∼130 M_⊙.

We used high-resolution infrared spectra of the heavily embedded T Tauri star HL Tau to search for evidence of absorption due to the R0, R1, and R2 gas-phase CH₄ ν₃ lines near 3.3 μm. From this, we report a 3 σ upper limit of 1.3 × 10¹⁵ cm^-2 for the CH₄ gas column density toward HL Tau. Our results are compared to those found for CO gas toward this source and to the recent model for chemistry in the inner (10 AU) disks around T Tauri stars by Markwick et al. We find that the upper limit of methane ice+gas column density toward HL Tau, when compared to CO, is somewhat lower than but consistent with that measured toward other interstellar sources (~1%) but that it is much lower than that predicted in the Markwick et al. model and much less than the CH₄/CO ratio (10%-80%) found in cometary volatiles. This has important implications for the processing of interstellar material and its incorporation into planetary bodies.

We present the first thermal-infrared imaging photometry for several embedded sources in the OMC-1 South cloud core in the Orion Nebula, and we propose that some of these drive the optical Herbig-Haro jets emerging from the region. Thermal-infrared images at 8.8 and 11.7 μm obtained at Gemini South show a handful of sources in OMC-1 South with no visual-wavelength counterparts, although a few can be seen in recent near-infrared data. For the three brightest mid-infrared sources, we also present 18.75 μm photometry obtained with the Keck telescope. The most prominent blueshifted outflows in the Orion Nebula at visual wavelengths, such as HH 202, HH 203/204, HH 529, and HH 269, all originate from OMC-1S. The brightest infrared source in OMC-1S at 11.7 μm is located at the base of the prominent jet that powers HH 202 and is likely to be the sought-after driver of this outflow. The second brightest infrared source is located at the base of the HH 529 jet. We consider the possibility that HH 203/204 and HH 269 trace parts of a single bent outflow from the third brightest infrared source. While there may be some lingering ambiguity about which infrared stars drive specific jets, there is now a sufficient number of embedded sources to plausibly account for the multiple outflows from OMC-1S.

We present sensitive 1.3 cm radio continuum observations of the region OMC-1 South (OMC-1S) in Orion using the Very Large Array in its B configuration. We detect 11 radio sources clustered in a 30^'' × 30^'' region, of which only three had been detected previously at radio wavelengths in deep 3.6 cm observations. The eight new radio sources are compact (θ_s ≤ 01), and we set lower limits to their spectral indices, α > 0.8 ± 0.3 (with S_ν ∝ ν^α), that suggest that they may be optically thick H II regions. However, one of the new sources exhibits significant circular polarization, indicating that gyrosynchrotron emission with large positive spectral indices may be an alternative explanation. Furthermore, we find that four other sources are associated with infrared sources of low bolometric luminosity that cannot drive an H II region. Finally, two of the sources previously detected at 3.6 cm are angularly resolved in the 1.3 cm image, and their major axes have position angles that align well with large-scale outflows emanating from OMC-1S. The radio source 143-353 has a major axis with a position angle consistent with those of the HH 202 and HH 528 flows, while the radio source 134-411 has a major axis with a position angle consistent with that of the low-velocity molecular outflow associated with the far-infrared source FIR 4.

We present the results of mid-infrared nulling interferometric observations of the main-sequence star α Lyr (Vega) using the 6.5 m MMT with its adaptive secondary mirror. From the observations at 10.6 μm, we find that there is no resolved emission from the circumstellar environment (at separations greater than 0.8 AU) above 2.1% (3 σ limit) of the level of the stellar photospheric emission. Thus, we are able to place an upper limit on the density of dust in the inner system of 650 times that of our own solar system's zodiacal cloud. This limit is roughly 2.8 times better than those determined with photometric excess observations such as those by IRAS. Comparison with far-infrared observations by IRAS shows that the density of warm dust in the inner system (<30 AU) is significantly lower than cold dust at larger separations. We consider two scenarios for grain removal, the sublimation of ice grains and the presence of a planetary mass "sweeper." We find that if sublimation of ice grains is the only removal process, a large fraction (>80%) of the material in the outer system is ice.

Recent analysis of relatively cool (∼1 MK) active region loops observed with TRACE has suggested that these loops have been heated impulsively and are cooling through the TRACE bandpasses. In this Letter we explore the evolution of cooling loops to determine if the TRACE EUV observations can be used to determine the magnitude, duration, and location of the energy release. We find that the evolution of the apex density and temperature in an impulsively heated cooling loop depends only on the total energy deposited (not the magnitude, duration, or location of the energy deposition) after the loop cools past an "equilibrium point," where the conductive and radiative cooling times are comparable. Hence, observations must be made early in the evolution of a loop to determine the heating parameters. Typical TRACE observations of cooling loops do not provide adequate information to discriminate between different heating scenarios.

A technique for investigating the temporal relationship between Hα filament eruptions and soft X-ray flux is presented. The method is fast, simple, based on statistical measures, and requires no a priori knowledge regarding the location of the filaments on the solar disk. The method is used to study the filament eruption associated with a coronal mass ejection and the M-class flare that originated from NOAA Active Region 9163 on 2000 September 12. The technique provides a quantitative determination of the temporal relationship between changes in the Hα filament and soft X-ray flux. We conclude that the filament eruption begins 2 hr prior to the first detectable enhancement in soft X-ray flux.

We present the first center-to-limb G-band images synthesized from high-resolution simulations of solar magnetoconvection. Toward the limb the simulations show "hilly" granulation with dark bands on the far side, bright granulation walls, and striated faculae, similar to observations. At disk center G-band bright points are flanked by dark lanes. The increased brightness in magnetic elements is due to their lower density compared with the surrounding intergranular medium. One thus sees deeper layers where the temperature is higher. At a given geometric height, the magnetic elements are cooler than the surrounding medium. In the G band, the contrast is further increased by the destruction of CH in the low-density magnetic elements. The optical depth unity surface is very corrugated. Bright granules have their continuum optical depth unity 80 km above the mean surface, the magnetic elements 200-300 km below. The horizontal temperature gradient is especially large next to flux concentrations. When viewed at an angle, the deep magnetic elements' optical surface is hidden by the granules and the bright points are no longer visible, except where the "magnetic valleys" are aligned with the line of sight. Toward the limb, the low density in the strong magnetic elements causes unit line-of-sight optical depth to occur deeper in the granule walls behind than for rays not going through magnetic elements, and variations in the field strength produce a striated appearance in the bright granule walls.