SIGMA/[ITAL]GRANAT[/ITAL] Discovery of GRS 1739−278, A Hard X-Ray Transient in the Galactic Bulge

While the SIGMA telescope performed its twelfth observing campaign on the Galactic center region, a new hard X-ray transient source was discovered. The source, named GRS 1739-278, was bright during the whole campaign with a hard spectrum. Thanks to the accurate position provided by the coded-mask imaging technique, the GRS 1739-278 counterparts at other wavelengths have been promptly identified. The collected observations and similarities of the source behaviors with those of Nova Muscae 1991 suggest that GRS 1739-278 may contain a black hole located in the Galactic bulge. The fact that the hard X-ray peak luminosity of GRS 1739-278 is similar to that of GRS 1730-312 (another hard X-ray transient observed by SIGMA in the bulge) leads us to consider the hard X-ray transient peak luminosity as a possible distance indicator.