Faint ultraviolet objects in the core of M13: optical counterparts of the low-luminosity X-ray source?

Abstract

The core of the Galactic globular cluster M13 (NGC 6205) has been observed with WFPC2 on the Hubble Space Telescope through visual, blue, and mid- and far-UV filters in a programme devoted to studying the UV population in a sample of Galactic globular clusters. In the UV colour-magnitude diagrams derived from the HST images we have discovered three faint objects with a strong UV excess, which lie significantly outside the main loci defined by more than 12 000 normal cluster stars. The positions of two of the UV stars are nearly coincident (7 and 1 arcsec) with those of a low-luminosity X-ray source recently found in the core of M13 and with a 3.5-sigma peak in the X-ray contour map. We suggest that the UV stars are physically connected to the X-ray emission. The UV stars are very similar to the quiescent nova in the globular cluster M80, and they might be a, perhaps new, subclass of cataclysmic variable.