Review of Galactic Center Observations with Granat
Seven sets of Galactic Center (hereafter GC) observations in hard X-Rays were performed by GRANAT in 1990–1993. Coded mask telescopes ART-P and SIGMA obtained high angular resolution (~ 5 ÷ 15 arcminutes) images of this region over broad energy band from 3 to 1000 keV. No evidence for presence of supermassive black hole (AGN-like) at the dynamic center of our Galaxy has been found in these observations. The contribution of central few tens of parsecs around the Galactic Center to the X-Ray luminosity of the whole GC region is very small.
The extended source (occupying ~1° region around GC) has been detected by ART-P telescope. Above ~ 10 keV the geometry of the source exhibits some correlation with distribution of molecular clouds, suggesting that at these energies diffuse emission can originate from Thomson scattering of the emission from compact sources residing in GC region.
Most of the X-ray variability found during previous non-imaging observations can be explained by the variability of several point sources scattered over several degrees region near Galactic Center. At energies higher than 60 keV contribution from two sources 1E1740.7-2942 and GRS1758-258 dominates. On the basis of hard spectra resembling that of Cygnus X-1, these two sources have been included to the list of black hole candidates.
No significant evidence for strong persistent flux in narrow 511 keV line from any point source in the region has been found with typical 2σ upper limit of ~ 3.5 × 10-4 phot/s/cm2. However on few occasions transient broad high energy excesses (200–500 keV) probably related to electron-positron annihilation process have been detected in the spectrum of 1E1740.7-2942.
Key wordsX-Rays general — Galaxy center — Gamma Rays observations
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