A Persistent High‐Energy Flux from the Heart of the Milky Way:INTEGRAL’s View of the Galactic Center

The Ibis/Isgri imager on Integral detected for the first time a hard X-raysource, IGR J17456-2901, located within 1' of Sgr A* over the energy range20-100 keV. Here we present the results of a detailed analysis of ~7 Ms ofIntegral observations of the GC. With an effective exposure of 4.7 Ms we haveobtained more stringent positional constraints on this HE source andconstructed its spectrum in the range 20-400 keV. Furthermore, by combining theIsgri spectrum with the total X-ray spectrum corresponding to the same physicalregion around SgrA* from XMM data, and collected during part of the Integralobservations, we constructed and present the first accurate wide band HEspectrum for the central arcmins of the Galaxy. Our complete analysis of theemission properties of IGR shows that it is faint but persistent with novariability above 3 sigma contrary to what was alluded to in our first paper.This result, in conjunction with the spectral characteristics of the X-rayemission from this region, suggests that the source is most likely notpoint-like but, rather, that it is a compact, yet diffuse, non-thermal emissionregion. The centroid of IGR is estimated to be R.A.=17h45m42.5,decl.=-28deg59'28'', offset by 1' from the radio position of Sgr A* and with apositional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10^35erg/sec. Very recently, Hess detected of a source of ~TeV g-rays also locatedwithin 1' of Sgr A*. We present arguments in favor of an interpretationaccording to which the photons detected by Integral and Hess arise from thesame compact region of diffuse emission near the central BH and that thesupernova remnant Sgr A East could play an important role as a contributor ofvery HE g-rays to the overall spectrum from this region.Comment: 14 pages, 11 figures, Accepted for publication in Ap