Long - Term Variability of the Hard X - ray Source GRS 1758 – 258 : GRANAT / SIGMA Observations

The results of GRANAT/SIGMA hard X-ray observations of GRS 1758–258 in 1990–1998 are presented. The source lies at ∼ 5 from the Galactic Center and was within the SIGMA field of view during the GRANAT surveys of this region. The total exposure time of the Galactic Center was 11×10 s. The regular SIGMA observations revealed strong variability of the source: the 40–150 keV flux varied at least by a factor of 8 on a time scale of a year, between less than 13 mCrab and ≈ 100–110 mCrab. The average flux was ≈ 60 mCrab in 1990–1998. The source’s spectrum is well fitted by a power law with a photon index α ≈ 1.86 in the energy range 40 to 150 keV and becomes steeper at energies above ∼ 100 keV. The radio and hard X-ray properties of GRS 1758–258 are similar to those of another Galactic Center source, 1E1740.7–2942. GRS 1758–258 and 1E1740.7–2942 are the two brightest hard X-ray sources in the Galactic Center region. Both sources have radio jets, similar X-ray luminosities (∼ 10 erg/s), and spectra, and exhibit variations in the hard X-ray flux on long times scales by a factor of ∼ 10 or more . In contrast to most of the known black hole candidates, which are X-ray transients, GRS 1758–258 and 1E1740.7–2942 were detected by SIGMA during most of the observations in 1990–1998. Assuming that this behavior of the sources implies the suppression of accretion-disk instability in the region of partial hydrogen ionization through X-ray heating, we impose constraints on the mass of the optical companion and on the orbital period of the binary system.