The Quiescent Optical and Infrared Counterpart to EXO 0748-676 = UY Vol

TL;DR

This study reports the first optical and infrared observations of the low-mass X-ray binary EXO 0748-676 in 24 years of quiescence, revealing orbital modulation dominated by accretion disk and heated face emissions, complicating mass measurements.

Contribution

First optical and infrared photometry of EXO 0748-676 in quiescence, showing orbital modulation consistent with the X-ray period and dominance of accretion-related emission.

Findings

Detected optical/infrared counterpart at R=22.4, J=21.3.

Observed orbital modulation matching X-ray period within 0.01%.

No double-peaked ellipsoidal modulation observed.

Abstract

We present optical and infrared photometry of the low-mass X-ray binary EXO 0748-676 in quiescence for the first time in 24 years since it became X-ray active in 1985. We find the counterpart at average magnitudes of R=22.4 and J=21.3. We monitored the source approximately nightly through 2008 November to 2009 January. During this time there was considerable night-to-night optical variability but no long term trends were apparent. The night-to-night variability reveals a periodicity of P=0.159331+/-0.000012d, consistent with the X-ray orbital period to within 0.01%. This indicates that the quiescent optical modulation is indeed orbital in nature rather than a superhump. Interestingly, the modulation remains single-peaked with a deep minimum coincident with the times of X-ray eclipse, and there is no indication of a double-peaked ellipsoidal modulation. This indicates that even in `quiescence' emission from the accretion disk and/or X-ray heated inner face of the companion star dominate the optical emission, and implies that obtaining an accurate dynamical mass estimate in quiescence will be challenging.