LOW-FLUX HARD X-RAY-OBSERVATION OF CYGNUS X-1

The black hole candidate Cygnus X-1 was observed at a low flux level on 1985 August 5 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 the 1979 observations by the Gamma-Ray Spectrometer on board HEAO 3, and similar to the more recent observations by the MIFRASO balloon experiment (1987) and by the HEXE detector on board the MIR-KVANT module during 1987 July-1988 April. The photon spectrum is well described by a single power law, with a photon index of alpha = 1.87 and an intensity of 2.57 x 10(-3) photons cm-2 s-1 keV-1 at 50 keV, corresponding to 1.11 Crab. The low hard X-ray flux of the POKER observation is well matched at lower energies with the EXOSAT data, collected on 1985 August 12 even though the two observations are not contemporary and a source transition cannot be ruled out. Compared with other available high-statistics measurements our observation shows a source behavior characterized by a quite hard spectral shape and low intensity, just above the one considered typical of the gamma-1 state. Applying the Sunyaev and Titarchuk model 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.