The infrared counterpart of the Z source GX 5-1

Abstract

We have obtained UK Infrared Telescope infrared observations of the field of the bright Galactic Z source GX 5-1. From an astrometric plate solution tied to Tycho-ACT standards we have obtained accurate positions for the stars in our field which, combined with an accurate radio position, have allowed us to identify the probable infrared counterpart of GX 5-1. Narrow-band photometry marginally suggests excess Brγ emission in the counterpart, supporting its association with an accretion-disc source. No significant variability is observed in a limited number of observations. We compare the H and K magnitudes with those of other Z sources, and briefly discuss possible sources of infrared emission in these systems.