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 study the UV population in a sample of Galactic globular clusters. In the UV Color 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" & 1") to those of a low luminosity X-ray source recently found in the core of M13 and to 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.