We report 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 \approx 12 and \approx 320 Myr .
The emission appears persistent , although we can not rule out flare-like behaviour with a decay time-scale > 4 ks .
The time-averaged X-ray luminosity of L _ { X } \approx 3.3 \pm 0.8 \times 10 ^ { 27 } { erg } { s } ^ { -1 } , and its ratios with the bolometric ( L _ { X } / L _ { bol } \approx 10 ^ { -3.05 } ) and H \alpha ( L _ { X } / L _ { { H } \alpha } \approx 4.0 ) luminosities suggest magnetic activity similar to that of active main-sequence M dwarfs , such as the M7 old-disc star VB 8 , though 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 \times 10 ^ { 27 } { erg } { s } ^ { -1 } and to \log ( L _ { X } / L _ { bol } ) in the range -3.10 to -2.91 .