2018A&A...616A..36M

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 the XMM-Newton satellite. The source had a quiescent X-ray luminosity on the order of ∼10³²erg/s in the 0.5-10keV range (for a distance of NGC 6540 of 4kpc) and showed a flare lasting about 300s. 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 as Hubble Space 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.