1995A&A...303..526B

We report on the ROSAT-PSPC all sky survey and pointed observations of GS1826-24, performed on September-October 1990 and October 1992, respectively. The source was detected by ROSAT on both occasions, as well as in the course of a June 1992 observation of the nearby globular cluster NGC 6642. The ROSAT spectrum can be adequately described by a power law with photon index in the range 1.5-1.8, consistent with earlier measurements. The unabsorbed 0.1-2.4keV flux measured by ROSAT remained constant around 1.3x10^–10 ergs/s/cm2. The total column density towards GS1826-24 is ∼5x10²¹Hatoms/cm2. No X-ray bursts were detected during any of the ROSAT observations representing a total observing time of about 8 hours. Since its discovery in 1988 by GINGA, the source was detected in 1989 by TTM and from 1990 through 1992 by ROSAT at almost the same level of X-ray intensity. This rules out the hypothesis that the source is a soft X-ray transient. We also report on the follow-up optical study of the source which led to the identification of a V=19 optical counterpart located at α=18h 29mn 28.2s, δ=-23deg47'49".1 (equinox 2000.0±2"). The optical faintness of the companion indicates that GS1826-24 is a low-mass X-ray binary system. We estimate the distance to be larger than 10kpc thus implying that its persistent X-ray luminosity exceeds 10³⁷ergs/s. GS1826-24 could be an X-ray burst source because it lies inside the large error box of an unidentified X-ray burster, and its basic X-ray properties (i.e. strong flickering, and hard spectrum in X-rays) have been observed from low-luminosity X-ray bursters. However, its rather large X-ray luminosity together with the non-detection of X-ray bursts favor the black hole hypothesis for GS1826-24.