2000ApJ...538L.141R

We have detected a bright X-ray flare from the nearby (d=5.0 pc) brown dwarf LP 944-20 with the Chandra Advanced CCD Imaging Spectrometer-S. This is an old (500 Myr), rapidly rotating, lithium-bearing M9 object, with a bolometric luminosity of ~6x10²⁹ ergs.s^–1. It was detected only by Chandra during an X-ray flare of duration 1-2 hr near the end of a 12.1 hr observation. The peak X-ray luminosity was 1.2^+0.5_–0.3x10²⁶ ergs.s^–1 in the brightest ~550 s, corresponding to L_X/L_bol~2x10^–4. A total of 2x10²⁹ ergs was released during the 43,773 s observation, giving a time-averaged L_X/L_bol~7x10^–6. LP 944-20 was not detected before the flare, with a 3 σ upper limit on the emission at L_X/L_bol<2x10^–6 (L_X<1x10²⁴ ergs.s^–1). This is faint for a rapidly rotating late-type star and establishes a record lower limit to the quiescent flux about an order of magnitude below the flux limit (and a factor of 5 below the L_X/L_bol limit) placed on quiescent X-ray emission from the M8 dwarf VB 10. The inferred flaring duty cycle is comparable to that measured via variable Hα emission for other late M-type, fully convective stars.