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

Max Planck für Extraterrestrische Phyisk, Giessenbachstraße, 85748 Garching bei München, Germany e-mail: hbouy@mpe.mpg.de Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 414 rue de la piscine, 38400 Saint Martin d'Hères, France European Southern Observatory, Karl Schwarzschildtstraße 2, 85748 Garching bei München, Germany

Received: 6 January 2004
Accepted: 17 April 2004

Abstract

I report the detection of two bona-fide brown dwarfs by XMM-Newton: [GY92] 141 in the ρ -Ophiuchus star-forming region and DENIS J155601-233809 in the Upper Scorpius OB association. The two objects have been detected with luminosities of erg s^-1 and erg s^-1 respectively, corresponding to luminosity ratios of log ( and log (. The two sources were close to the limit of detection of the instruments at only 2~3-σ 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 [CITE] with a luminosity ~14 times fainter than the one reported here, indicating that the X-ray emission of this object is strongly variable.