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 \pm 0.081 ) , but it was not detected during the \approx 48 ks observation with the X-ray telescopes .
We determine a 3 \sigma upper limit for the X-ray emission from this object of L _ { X } < 3.1 \times 10 ^ { 23 } ergs \cdot s ^ { -1 } , equivalent to a luminosity ratio upper limit of \log { ( L _ { X } / L _ { bol } ) } \leq - 6.28 .
This measurement improves by a factor of 3 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 ( L _ { X } / L _ { bol } ) = - 4.1 ] have a duty cycle of about 6 % , which is lower than the radio flare duty cycle ( \sim 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 .