The Infrared Counterpart of the Microquasar Grs 1758−258

We present revised infrared (2.2μm) astrometry of the field containing the Galactic microquasar GRS 1758−258, using observations at the Keck I 10-m telescope. We find three candidates for the microquasar within a 3σ error circle, but none within 2σ. We show that if the 18.4 day X-ray period of GRS 1758−258 is due to a binary orbit, then only one of the three candidates, an early K-type giant, is large enough to power the microquasar via Roche lobe overflow. We therefore identify this star as the infrared counterpart of GRS 1758−258, which we classify as a low mass X-ray binary. Long term infrared monitoring of this source should provide further information about the microquasar system, including a confirmation of the X-ray period and an estimate of the compact object’s mass. Subject headings: infrared: stars — X-rays: stars — stars: individual (GRS 1758−258) — black hole physics