Spring, 1997 GRANAT/SIGMA observations of the Galactic center: Discovery of 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 - 1E1740.7-2942 and GROJ1744-28 were detected. The spectral and temporal behavior of GRS 1737-31 suggests that this source may be a di stant X-ray Nova and a black hole candidate. Comparison of the hard X-ray t ransient detection rates by GRANAT/SIGMA and currently operating all-sky mo nitors (CGRO/BATSE, GRANAT/WATCH) leads to following conclusions regarding the spatial and luminosity distributions of the hard X-ray transients: (i) spatial distribution bf X-ray Novae exhibits concentration towards the Gala ctic Center similar to the distribution of visible matter in the Galaxy. In particular, it is more concentrated than Galactic disk population; (ii) if hard X-ray transients follow distribution of visible mass in the Galaxy th en their peak luminosity in the 35 - 150 keV band is close to 10(37) erg s( -1) with a relatively small scatter.