Spring, 1997 GRANAT/SIGMA observations of the Galactic Center: discovery of the X-ray nova GRS 1737-31

The results of GRANAT/SIGMA hard X-ray observations of the Galactic center in March, 1997 are reported. One new source - GRS 1737-31 and two previousl y known sources - 1E 1740.7-2942 and GRO J1744-28 were detected. The spectr al and temporal behavior of GRS 1737-31 suggests that this source may be a distant X-ray Nova and a black hole candidate. The comparison of the hard X-ray transient detection rates by GRANAT/SIGMA and currently operating all-sky monitors (CGRO/BATSE, GRANAT/WATCH) leads t o following conclusions regarding the spatial and luminosity distributions of the hard X-ray transients: (i) the spatial distribution of X-ray Novae exhibits a concentration toward s the Galactic Center similar to the distribution of the visible matter in the Galaxy. In particular, it is more concentrated than Galactic disk popul ation. (ii) if hard X-ray transients follow distribution of visible mass in the Ga laxy then their peak luminosity in the 35-150 keV band is close to 10(37) e rgs(-1) with a relatively small scatter.