BROAD-BAND X-RAY OBSERVATIONS OF NGC-4151 BY THE ART-P AND SIGMA TELESCOPES IN 1990-92

During GRANAT operation in 1990-92, a total of 35 and 207 hours of NGC 4151 observations were accumulated by the ART-P and SIGMA telescopes, respectively. The steep spec trum of NGC 4151 in hard X-rays, first detected by SIGMA in 1990 (Jourdain et al. 1992), was also found in later SIGMA observations. Comparing the results of quasi simultaneous observations of the ART-P, SIGMA and OSSE telescopes in July 1991 showed that the spectrum exhibits at least two breaks at energies above 8 keV. The broad band spectrum can be satisfactorily described by a model with exponential cut-off above E similar to 50 keV. This result argues in favor of a thermal origin of the hard X-ray emission from the source. We further note that the spectrum of NGC 4151 observed by GRANAT meets the requirements for potential sources constituting the cosmic X-ray background in the 3-100 keV band.