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 approximate to12 and approximate to320 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 approximate to 3.3 +/- 0.8 x 10(27) erg s(-1) and its ratios with the bolometric (L-X/L-bol approximate to 10(-3.05)) and Halpha (L-X/L-Halpha approximate to 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 x 10(27) erg s(-1) and to log (L-X/L-bol) in the range -3.10 to -2.91.