X-ray detections of two young bona-fide brown dwarfs

I report the detection of two bona-fide brown dwarfs by XMM-Newton: [GY92] 141 in the rho-Ophiuchus star-forming region and DENIS J155601-233809 in the Upper Scorpius OB association. The two objects have been detected with luminosities of L-X = 8.35 +/- 2.86 x 10(28) erg s(-1) and L-X = 6.54 +/- 1.35 x 10(28) erg s(-1) respectively, corresponding to luminosity ratios of log (L/L-bol) = -2.07 and log (L-X/L-bol) = -2.69. The two sources were close to the limit of detection of the instruments at only 2similar to3-sigma above the background level, and no significant flare or variation Could be detected during the 48.3 ks and 33.9 ks observations. [GY92] 141 had already been observed 10 months earlier with Chandra by Imanishi et al. (2001) with a luminosity similar to14 times fainter than the one reported here, indicating that the X-ray emission of this object is strongly variable.