[ITAL]Chandra[/ITAL] 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 × 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.