A Probable Dwarf Nova Outburst in the Core of the Globular Cluster M22

Abstract

We reexamine an event that was previously reported by Sahu et al. as a microlensing event of a Galactic bulge star by a star of M22. We find strong evidence that the event was actually a dwarf nova outburst of a cataclysmic variable in the cluster. The primary evidence consists of the star's proper motion, Hα emission, X-ray luminosity, and variable brightness in quiescence. The star is in the cluster core but can be monitored from the ground for future outbursts to confirm its nature. This is one of a very small number of confirmed or probable dwarf nova eruptions seen in globular clusters and the first to be found in such a low-concentration cluster.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.