Our Galaxy is a bright source of gamma-rays that has been observed over many decades of energy. The interstellar medium is probably responsible for most of the Galactic emission below 10 keV, and above 300 keV. However, in the soft gamma-ray domain (>20 keV)the nature of the Galactic emission was still uncertain and a matter of debate. The lack of sensitive, high resolution observations did not allow for the relative contribution of compact sources and the interstellar medium to be determined and the best imaging experiment up to 2000 revealed a few point source that could only account for about 50% of the total galactic flux.
A major contribution from point sources would have important consequences on the diffuse emission models.
The INTEGRAL gamma-ray observatory was launched in October 2002 from Baïkonour, carrying two major co-aligned coded-masks instruments: the IBIS imager and the SPI spectrometer. To lessen the instrumental background contribution that dominates at high-energies (E > 100 keV), both experiments are actively shielded but SPI's shielding is much thicker and larger. As a result, SPI is less sensitive to the internal background that dominates the other contributions at high energy (E>100 keV). Coded mask imaging acts as a high-pass spatial-frequency filter, strongly attenuating structures larger than the resolution. The angular resolution of IBIS is 13' while that of SPI is 2.6°. This makes IBIS with its ISGRI camera very well suited to measure the emission from compact sources, and SPI the overall emission of a few degrees thick Galactic disc. Below 200 keV, The SPI sensitivity reach a level of few millicrab for a 106s observation and ISGRI is 2-3 times better.
INTEGRAL observed the Galactic centre region in the spring and fall 2003 as part of its core programme (guaranteed time) and during two target of opportunity observations. The total observing time amounts to about 1.5 106 s in the central part of the survey. A composite ISGRI image illustrating both the intensity and the spectral hardness of sources, has been constructed and is shown in Figure 1. A catalogue of excesses above 6 sigma has been elaborated from this image featuring 91 entries. An attempt has been made to identify each source using SIMBAD at the Centre des Données Stellaires in Strasbourg leaving 26 unidentified sources. Not surprisingly for a region containing the Galactic bulge, i.e. an old star population feature, most (40) of the identified sources are accreting binary systems with low mass companions. Only 7 are high-mass systems. Two radio pulsars and two SNRs are also found. A millisecond pulsar, a soft gamma-ray repeater and a Seyfert 1 galaxy complete the list of the identified sources of known type. The identified binaries contribute to 86%, 78%, 77% and 74% to the source contribution. The global spectrum of the other sources is thus significantly harder. This may indicate a new emerging population of hard sources.
Ref:
1) P. Umbertini et al., Astronomy &Astrophysics Letters, 2003, 411, L131
2) F. Lebrun et al., Nature, 2004, 428, 293
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