XMM-Newton observations of the nearby brown dwarf LP 944-20

The nearby (d=5.0 pc) brown dwarf LP 944-20 was observed with the XMM-Newton satellite on 07 January 2001. The target was detected with the Optical Monitor (V=16.736+/-0.081), but it was not detected during the approximate to48 ks observation with the X-ray telescopes. We determine a 3sigma upper limit for the X-ray emission from this object of L-X<3.1x10(23) ergs.s(-1), equivalent to a luminosity ratio upper limit of log(L-X/L-bol)≤ -6.28. This measurement improves by a factor of three the previous Chandra limit on the quiescent X-ray flux. This is the most sensitive limit ever obtained on the quiescent X-ray emission of a brown dwarf. Combining the XMM-Newton data with previous ROSAT and Chandra data, we derive flare duty cycles as a function of their luminosities. We find that very strong flares [Log(L-X/L-bol)= -2.5] are very rare (less than 0.7% of the time). Flares like the one detected by Chandra [Log(LX/L-bol)= -4.1] have a duty cycle of about 6%, which is lower than the radio flare duty cycle (∼13%). When compared with other M dwarfs, LP 944-20 appears to be rather inactive in X-rays despite of its relative youth, fast rotation and its moderately strong activity at radio wavelengths. (C) 2002 Elsevier Science B.V. All rights reserved.