摘要：We report the discovery of a flaring X-ray source 7″ from the center of the globular cluster NGC 6540 obtained during the EXTraS project devoted to a systematic search for variability in archival data of theXMM–Newtonsatellite. The source had a quiescent X-ray luminosity on the order of ~1032erg s−1in the 0.5–10 keV range (for a distance of NGC 6540 of 4 kpc) and showed a flare lasting about 300 s. During the flare, the X-ray luminosity increased by more than a factor 40, with a total emitted energy of ~1036erg. These properties, as well asHubbleSpace Telescope photometry of the possible optical counterparts, suggest the identification with a chromospherically active binary in the cluster. However, the flare luminosity is significantly higher than what is commonly observed in stellar flares of such a short duration, leaving open the possibility of other interpretations.
关键词：englobular clusters: individual: NGC 6540stars: flareX-rays: binariesX-rays: bursts
