Chandra detection of an X-ray flare from the brown dwarf LP 944-20

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 approximate to 6 x 10(29) 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(-0.3)(+0.5) x 10(26) ergs s(-1) in the brightest approximate to 550 s, corresponding to L-X/L-bol < 2 x 10(-4). A total of 2 x 10(29) ergs was released during the 43,773 s observation, giving a time-averaged L-X/L-bol approximate to 7 x 10(-6). LP 944-20 was not detected before the flare, with a 3 sigma upper limit on the emission at L-X/L-bol < 2 x 10(-6) (L-X < 1 x 10(24) ergs s(-1)). This is faint for a rapidly rotating late-type star and establishes a record lower limit to the quiescent Bur about an order of magnitude below the flux limit land 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 Hot emission for other late M-type, fully convective stars.