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

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dc.creatorKuznetsov, S. I.
dc.creatorGilfanov, M. R.
dc.creatorChurazov, E. M.
dc.creatorSunyaev, R. A.
dc.creatorDyachkov, A. V.
dc.creatorKhavenson, N. G.
dc.creatorNovikov, B. S.
dc.creatorKremnev, R. S.
dc.creatorGoldoni, P.
dc.creatorGoldwurm, A.
dc.creatorLaurent, P.
dc.creatorPaul, J.
dc.creatorRoques, J. -P.
dc.creatorJourdain, E.
dc.creatorBouchet, L.
dc.creatorVedrenne, G.
dc.date1999-03-13
dc.date1999-03-16
dc.date.accessioned2026-07-07T02:30:50Z
dc.date.available2026-07-07T02:30:50Z
dc.descriptionThe results of GRANAT/SIGMA hard X-ray observations of GRS 1758-258 in 1990-1998 are presented. The source lies at ~5\arcdeg 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 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 (~10^37 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.
dc.description18 pages with 5 figures. Astronomy Letters, 1999, 25 (in press)
dc.identifierhttps://arxiv.org/abs/astro-ph/9903212
dc.identifierhttp://arxiv.org/abs/astro-ph/9903212
dc.identifier.urihttp://salesiana.dossiersoluciones.com/handle/123456789/13953
dc.subjectAstrophysics
dc.titleLong-Term Variability of the Hard X-ray Source GRS 1758-258: GRANAT/SIGMA Observations
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