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

Abstract 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 ≈48 ks observation with the X-ray telescopes. We determine a 3σ upper limit for the X-ray emission from this object of LX −2.5] are very rare (less than 0.7% of the time). Flares like the one detected by Chandra [Log(LX/Lbol)=−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.