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Astron. Astrophys. 322, 229-233 (1997)

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1. Introduction

In the soft [FORMULA] -ray band of the SIGMA telescope aboard GRANAT, 16 out of some 60 previously known X-ray sources have been detected in the direction of the Galactic Bulge. Six exhibit transient behaviour and thus have a negligible impact on the long term averaged [FORMULA] /X-ray emission from this region (Vargas et al. 1996, Goldwurm et al. 1994, Churazov et al. 1994). SLX 1735-269 is one of the remaining ten which are, instead, characterized by "persistent" high energy emission. Within this group, some sources show temporal variations (burst or pulsations) while others, also through analysis at other wavelengths, are suspected to be associated with blackholes (1E 1740.7-2942 and GRS 1758-258). However, the physical nature of SLX1735-269 remained unknown. In order to evaluate what type of object is associated with SLX1735-269, a low energy spectrum was obtained using the ASCA telescope and the results of these observations are reported here.

SLX 1735-269 was first detected in the range 3-30 keV in 1985 in a survey performed by Skinner et al. (1987) during the Spacelab 2 mission. This source was later observed between October 1988 and February 1992 by the TTM telescope in the energy band 2-28 keV (Int 't Zand 1992). The TTM spectra were modeled by a thermal bremsstrahlung with a plasma temperature of [FORMULA] keV. The total integrated flux was given as [FORMULA]. Although the TTM results did not show any significant variability, the reported flux level is roughly twice that obtained by the Spacelab 2 XRT, [FORMULA]. Further observations from 1990 March 24 to 1992 April 7 between 4-20 keV with the ART-P telescope resulted in a total averaged flux of [FORMULA], close to that measured by Spacelab 2 (Pavlinsky et al. 1994). SLX 1735-269 was first detected by the SIGMA telescope above 35mCrab in 1992 in the 35-150 keV band (Goldwurm et al. 1996). Spectral fits by a simple powerlaw yield a photon index of -2.87 [FORMULA] 0.23 while a thermal bremstrahlung resulted in a plasma temperature of [FORMULA] keV. The SIGMA data showed no significant long term variations.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998