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 ≈6 × 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^(+0.5)_(-0.3) × 10^(26) ergs s^(-1) in the brightest ≈550 s, corresponding to L_X/L_(bol) ≈ 2 × 10^(-4). A total of 2 × 10^(29) ergs was released during the 43,773 s observation, giving a time-averaged L_X/L_(bol) ≈ 7 × 10^(-6). LP 944-20 was not detected before the flare, with a 3 σ upper limit on the emission at L_X/L_(bol) < 2 × 10^(-6) (L_X < 1 × 10^(24) 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.