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

¹ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 3800 La Laguna, Spain e-mail: czurita@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³ ISOC, ESA, European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Cañada (Madrid), Spain
⁴ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
⁵ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁶ Department of Astrophysics, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁷ Department of Astronomy, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 0213, USA
⁹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 4 August 2009
Accepted: 18 December 2009

Abstract

Aims. We performed an optical/infrared study of the counterpart of the low-mass X-ray binary KS 1731–260 to test its identification and obtain information about the donor.

Methods. Optical and infrared images of the counterpart of KS 1731–260 were taken in two different epochs (2001 and 2007) after the source returned to quiescence in X-rays. We compared these observations with those obtained when KS 1731–260 was still active.

Results. We confirm the identification of KS 1731–260 with the previously proposed counterpart and improve its position to :34:13.46 and :05:18.60. The H-band magnitude of this candidate showed a decline of ~1.7 mag from outburst to quiescence. In 2007 April we obtained and I = 20.9 ± 0.1 for KS 1731–260. Similar optical brightness was measured in June 2001 and July 2007. The intrinsic optical color 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, ApJ, 542, 1016).