Abstract

The two persistent soft gamma-ray sources in the galactic centre region are black hole candidates of stellar mass with comptonized accretion disks that radiate 1037−38 erg s−1. They appear as microquasar stellar remnants from which emanate double-sided radio jets that extend over distances of a few parsecs.

The galactic centre in soft γ-rays

Contrary to the standard X-ray band (below 20keV), where many sources have been detected at less than 5° from the galactic centre, at higher energies (35-500 keV) the field is dominated by only two persistent sources: 1E1740.7-2942 and GRS1758-258 located respectively at ∼ 50′ and ∼ 5° from Sgr A (see Fig. 1). In the 30-500 keV band these sources radiate a few ×1037 erg s−1, near the Eddington limit of collapsed objects of stellar mass. Although persistent, they are time variable. Since no γ-ray source has been detected from Sgr A, if there is a super-massive black hole at the dynamic centre of the Galaxy, at present it is in sepulchral silence.

The telescope SIGMA on board GRANAT detected a powerful burst around 420 keV from the strongest γ-ray source 1E1740.7-2942 which is interpreted as the redshifted (probably gravitationally) 511 keV line from the annihilation of e+e− pairs. Since this Einstein source can produce and annihilate 10 billion (1010) tonnes of positrons in just one second, it is now known as the “Great Annihilator”. No annihilation line has yet been detected from GRS1758-258, the second strongest soft γ-ray source in that region.