EXTraS discovery of a peculiar flaring X-ray source in the Galactic globular cluster NGC 6540

¹ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via E. Bassini 15, 20133 Milano, Italy
e-mail: sandro@iasf-milano.inaf.it
² Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
³ IUSS, Piazza della Vittoria 15, 27100 Pavia, Italy
⁴ INFN, Sezione di Pavia, Via A. Bassi 6, 27100 Pavia, Italy

Received: 23 March 2018
Accepted: 1 June 2018

Abstract

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 ~10³² erg s⁻¹ in 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 ~10³⁶ erg. 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.