1991•The Astrophysical JournalRequires access
Low-flux hard X-ray observation of Cygnus X-1
P. Ubertini, A. Bazzano, C. La Padula, R. Manchanda, V. F. Polcaro, R. Staubert, E. Kendziorra, F. Perotti
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Abstract
Results are reported of observations of the black hole candidate Cygnus X-1 at a low flux level on August 5, 1985, with the POKER experiment in the hard X-ray range (15-120 keV). During the same flight the Crab Nebula was also observed as an in-flight calibration source, and the spectral results are reported. The measurement shows a low flux from Cygnus X-1, at a level between the ones reported as gamma-1 and gamma-2 during 1979 observations. The photon spectrum is well described by a single power law, with a photon index of 1.87 and an intensity of 0.00257 photons/sq cm s keV at 50 keV, corresponding to 1.11 Crab. The low hard X-ray flux of the POKER observations is well matched at lower energies with Exosat data collected on August 12, 1985. Applying the model of Sunyaev and Titarchuk (1980) to an accretion disk surrounding a black hole, the emission from a few keV up to the MeV region is consistent with a single temperature Comptonized spectrum.