Optical/infrared observations of the X-ray burster KS1731-260 in quiescence

TL;DR

This study confirms the optical/infrared counterpart of the X-ray burster KS1731-260 in quiescence, analyzing its brightness and color changes over time to infer properties of the donor star and accretion disk.

Contribution

It provides improved positional data and photometric measurements of KS1731-260 in quiescence, and discusses the nature of its donor star and accretion disk emission.

Findings

Confirmed the counterpart's identification and position.

Observed a ~1.7 magnitude decline in H-band brightness from outburst to quiescence.

Optical colors suggest a G-type secondary star with excess infrared emission.

Abstract

We performed an optical/infrared study of the counterpart of the low-mass X-ray binary KS1731-260 to test its identification and obtain information about the donor. Optical and infrared images of the counterpart of KS1731-260 were taken in two different epochs (2001 and 2007) after the source returned to quiescence in X-rays. We compared those observations with obtained when KS 1731-260 was still active. We confirm the identification of KS1731-260 with the previously proposed counterpart and improve its position to RA=17:34:13.46 and DEC=-26:05:18.60. The H-band magnitude of this candidate showed a decline of ~1.7 mags from outburst to quiescence. In 2007 April we obtained R=22.8+-0.1 and I=20.9+-0.1 for KS1731-260. Similar optical brightness was measured in June 2001 and July 2007. The intrinsic optical color R-I is consistent with spectral types from F to G for the secondary although there is a large excess over that from the secondary at the infrared wavelengths. This may be due to emission from the cooler outer regions of the accretion disk. We cannot rule out a brown dwarf as a donor star, although it would require that the distance to the source is significantly lower than the 7 kpc reported by Muno et al. 2000.