Résumé du preprint DAPNIA-06-11

A persistent high-energy flux from the heart of milky way : Integral's view of the galactic center.
G.Belanger, A.Goldwurm, M.Renaud, R.Terrier, F.Melia, N. Lund, J.Paul, G.Skinner and F.Yusef-Zadeh
Highly sensitive imaging observations of the Galactic center (GC) at high energies 
with an angular resolution of order 10\' is a very recent development in the field of 
high-energy astrophysics. The IBIS/ISGRI imager on the INTEGRAL observatory 
detected for the first time a hard X-ray source, IGR J17456-2901, located 
within 1\' of Sagittarius A* (Sgr A*) over the energy range 20-100 keV. 
Here we present the results of a detailed analysis of approximately 7106 s of 
observations of the GC obtained since the launch of INTEGRAL in 2002 October. 
Two years and an effective exposure of 4.7106 s have allowed us to obtain more 
stringent positional constraints on this high-energy source and to construct its 
spectrum in the range 20-400 keV. Furthermore, by combining the ISGRI spectrum 
with the total X-ray spectrum corresponding to the same physical region 
around Sgr A* from XMM-Newton data collected during part of the gamma-ray 
observations, we constructed and present the first accurate wideband high-energy 
spectrum for the central arcminutes of the Galaxy. Our complete and updated analysis 
of the emission properties of the INTEGRAL source shows that it is faint but persistent 
with no variability above 3 sigma, contrary to what was alluded to in our first paper. 
This result, in conjunction with the spectral characteristics of the soft and hard X-ray 
emission from this region, suggests that the source is most likely not pointlike but 
rather that it is a compact yet diffuse nonthermal emission region. The centroid of 
IGR J17456-2901 is estimated to be R.A.=17h45m42.5 s, decl.=-28deg59\'28\'\' (J2000.0), 
offset by 1\' from the radio position of Sgr A* and with a positional uncertainty of 1\'. 
Its 20-400 keV luminosity at 8 kpc is L=(5.37+/-0.21)1035 ergs s-1. A 3 sigma upper 
limit on the flux at the electron-positron annihilation energy of 511 keV from the direction 
of Sgr A* is set at 1.910-4 photons cm-2 s-1. Very recently, the HESS collaboration 
presented the detection of a source of ~TeV gamma-rays also located within an arcminute 
of Sgr A*. We present arguments in favor of an interpretation that the photons detected 
by INTEGRAL and HESS arise from the same compact region of diffuse emission near the 
central black hole and that the supernova remnant Sgr A East could play an important 
role as a contributor of very high energy gamma-rays to the overall spectrum from this 
region. There is also evidence for hard emission from a region located between the 
central black hole and the radio arc near l~0.1d along the Galactic plane and known 
to contain giant molecular clouds.


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