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 7×106 s of
observations of the GC obtained since the launch of INTEGRAL in 2002 October.
Two years and an effective exposure of 4.7×106 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 1.9×10-4 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. |
