1998A&A...329..965B

We report on the ROSAT HRI (High Resolution Imager) observation of the X-ray burster KS 1731-260. The observation was split in two parts; the first took place in 1995, March 16 lasting for ∼1 kilosecond and the second one between September 3rd and 14th, 1996 for about 5 kiloseconds. In both observations, KS 1731-260 was clearly detected with a count rate of 0.82 and 7.9cts/s respectively. From the first observation we found that the X-ray source is located at a position α= 17h34m13.5s and δ= -26°05'16.8" (equinox 2000) with an associated error radius of 10.1 arcsec (90% confidence level combining the attitude and centroid uncertainties). The position derived from the second part of the observation is consistent with the previous one. The ROSAT HRI position rules out the two counterparts that have been proposed so far. Our CFHT J and H infrared imaging of the HRI error box reveals at least 13 possible candidates. As expected from a comparison with other X-ray bursters, the upper limits derived for the infrared brightness of KS 1731-260 imply the presence of a low mass companion for the neutron star. We also report on a refined spectral analysis of the PSPC data taken during the all sky survey in September 1990. These data, together with data recorded by TTM and PCA/RXTE indicate that the column density towards KS 1731-260 varies between ∼1 to 6x10²²H atoms/cm²; thus suggesting that the source is characterized by variable intrinsic absorption. The change in the HRI count rate could be better explained by a change in the column density between the two observations rather than by a true source intensity variation. The numerous detections of the source since its discovery in 1988 together with its recent monitoring by the RXTE/ASM show that KS 1731-260 is a persistent source rather than a soft X-ray transient, as originally thought.