SUPERLOW-STATE HARD X-RAY-OBSERVATION OF CYGNUS X-1

We report evidence of a "low state" observation of the black hole candidate Cyg X-1 in the hard X-ray range. The data were obtained with MWPCs and the scintillator arrays on board the balloon-borne telescope MIFRASO. During a single drift-scan observation, Cyg X-1 was detected in the energy range 15-300 keV with a high degree of statistical significance. Our data show evidence of a very low flux from the source, similar to the one reported as gamma-1 during the end of the 1979 observation by the Gamma-Ray Spectrometer on board HEAO 3. The photon spectrum is well described by a Comptonized model characterized by an electron temperature of kT(e) = 56 keV and an optical depth of tau = 2/3. consistent with an increase of the electron temperature of the hot plasma surrounding the collapsed source and a lower optical depth of the accretion disk since the 1979 observation. Our data set is also compatible with a single power law, with a photon index of alpha = 1.92 and an intensity of 5.44 x 10(-4) photons cm-2 s-1 keV-1 at 100 keV, corresponding to 1.10 Crab. The very low hard X-ray flux confirms that during the 1987 July observation, the source was in the "superlow" state as supported by the Ginga 1987 August 5-8 low-energy data. These data sets, together with at least four different high-energy observations, suggest this "superlow" state to be a usual behavior for the source when low emission at low energy is present.