ASTRONOMY AND ASTROPHYSICS The Infrared Counterpart of the X-ray Burster KS

We present JK ′ infrared images of the X-ray transient KS 1731–260, obtained in 1997 and in 1998 with IRAC2b at the ESO/MPI 2.2-m telescope at La Silla. Using as a reference the recent Chandra position, we confirm the identification of the X-ray source with the previously proposed counterpart (Wijnands et al. 2001b), for which we measure J = 17.32 ± 0.2 and K ′ = 16.36 ± 0.18. The source was entering a low X-ray state at the epoch of our observations, and the accretion disk was still dominating the infrared flux. Indeed, when compared with the only published magnitudes in the J-band (Orosz et al. 2001), obtained with the source in quiescence, our photometry confirms the fading of the counterpart during the decay of the RXTE/ASM lightcurve. A&A manuscript no.