We have detected a bright X-ray flare from the nearby ( d = 5.0 pc ) brown dwarf LP 944-20 with the Chandra /ACIS-S .
This is an old ( 500 Myr ) , rapidly rotating , lithium-bearing M9 object , with a bolometric luminosity of \approx 6 \times 10 ^ { 29 } erg sec ^ { -1 } .
It was only detected by Chandra during an X-ray flare of duration 1-2 hours near the end of a 12.1 hour observation .
The peak X-ray luminosity was 1.2 ^ { +0.5 } _ { -0.3 } \times 10 ^ { 26 } erg sec ^ { -1 } in the brightest \approx 550 seconds , corresponding to L _ { X } / L _ { bol } \approx 2 \times 10 ^ { -4 } .
A total of 2 \times 10 ^ { 29 } ergs was released during the 43,773 sec observation , giving a time-averaged L _ { X } / L _ { bol } \approx 7 \times 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 \times 10 ^ { -6 } ( L _ { X } < 1 \times 10 ^ { 24 } erg sec ^ { -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 H \alpha emission for other late M-type , fully convective stars .