The infrared counterpart of the microquasar GRS 1758-258

We present revised infrared (2.2 μm) astrometry of the field containing the Galactic microquasar GRS 1758-258, using observations at the Keck I10 m telescope. We find three candidates for the microquasar within a 3 σ error circle but none within 2 σ. 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.