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Long-Term Variability of the Hard X-ray Source GRS 1758-258: GRANAT/SIGMA Observations | | S.I. Kuznetsov
; M.R. Gilfanov
; E.M. Churazov
; R.A. Sunyaev
; A.V. Dyachkov
; N.G. Khavenson
; B.S. Novikov
; R.S. Kremnev
; P. Goldoni
; A. Goldwurm
; P. Laurent
; J. Paul
; J.-P. Roques
; E. Jourdain
; L. Bouchet
; G. Vedrenne ( Space Research Institute
; RAS
; Moscow
; Moscow Physical-Technical Institute
; Dolgoprudny
; Russia
; MPI fuer Astrophysik
; Garching
; Germany
; Service d`Astrophysique
; DAPNIA/DSM
; CEA Saclay
; France
; Centre d`Etude Spatiale des Rayonnements
; | | Date: |
13 Mar 1999 | | Subject: | astro-ph | | Affiliation: | 1,2), M.R. Gilfanov (3,1), E.M. Churazov (3,1), R.A. Sunyaev (3,1), A.V. Dyachkov , N.G. Khavenson , B.S. Novikov , R.S. Kremnev , P. Goldoni , A. Goldwurm , P. Laurent , J. Paul , J.-P. Roques , E. Jourdain , L. Bouchet , G. Vedrenne ( Space Researc | | Abstract: | The results of GRANAT/SIGMA hard X-ray observations of GRS 1758-258 in 1990-1998 are presented. The source lies at ~5arcdeg 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 11x10^6 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 alpha ~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 (~10^37 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. | | Source: | arXiv, astro-ph/9903212 | | Services: | Forum | Review | PDF | Favorites | |
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