VARIABILITY OF THE GALACTIC-CENTER IN THE HARD X-RAY RANGE

We report the first evidence of strong flux variability in the hard X-ray range from IE 1740.7-2942 obtained during an observation of the region on 1989 May 17. The measurement was performed with a high-sensitivity array of multiwire proportional counter operative in the energy range 15-180 keV, and with a field of view of 1.9-degrees FWHM. The balloon-borne observation, lasting for 8100 s including background data, was pointed on 1E 1740.7-2942 as the reference source, following the suggestion by Skinner et al. and Cook et al. that this object was the point source responsible for the high-energy emission from the Galactic center. Our data, when compared with previous data in the same energy range obtained either by narrow field of view or imaging instruments, imply a flux variability of a factor of 3 at 40 keV and 2.5 at 100 keV. The spectrum on 1989 May 17 can be described by a single power law with spectral index of 1.95 and intensity of 2.85 x 10(-4) cm-2 s-1 keV-1 at 30 keV. Because of the 1.9-degrees field of view, we cannot rule out a low-energy contribution due to the presence of the well-known source A1742-2942 and the new GRANAT sources identified in the 4.3-degrees x 3.7-degrees ART-P images of the Galactic center in the energy range 3-17 keV. Since the newly discovered very hard sources GRS 1758-258 and GX 1 + 4 were not within the instrument's field of view during our observation, we can assume the low detected value of hard X-ray emission as evidence of flux variability of 1E 1740.7-2942. The detection of 1E 1740.7-2942 in a condition very close to the low state of Cyg X1 confirms the similarity between these two sources, thus supporting the black hole nature of the Galactic center high-energy emitter.