X-ray emission from a brown dwarf in the Pleiades

We report on the first detection of X-ray emission from a brown dwarf in the Pleiades, the M7-type Roque 14, obtained using the EPIC detectors on XMM-Newton. This is the first X-ray detection of a brown dwarf intermediate in age between ~12 and ~320 Myr. The emission appears persistent, although we cannot rule out flare-like behaviour with a decay time-scale >4 ks. The time-averaged X-ray luminosity of L_X~ 3.3 +/- 0.8 × 10²⁷ erg s^-1 and its ratios with the bolometric (L_X/L_bol~ 10^-3.05) and Hα (L_X/L_Hα~ 4.0) luminosities suggest magnetic activity similar to that of active main-sequence M dwarfs, such as the M7 old-disc star VB 8, although the suspected binary nature of Roque 14 merits further attention. No emission is detected from four proposed later-type Pleiades brown dwarfs, with upper limits to L_X in the range 2.1-3.8 × 10²⁷ erg s^-1 and to log (L_X/L_bol) in the range -3.10 to -2.91.