Spectral analysis of the 1975 May transition of Cygnus X-1

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Abstract
X-ray spectra from Ariel V of the 1975 May active (HI) state and the subsequent transition of Cyg X-1 are examined. The presence of a blackbody soft component (kT ∼ 0.3 keV) in the spectrum is revealed. During the transition the spectral index of the hard, power-law component decreases from 2.8 to 2.0. The possibility of a simultaneous variation of the blackbody temperature is discussed. A weak emission feature may be present at 6–7 keV. The data are found to be in general agreement with the predictions of inverse Compton models for Cyg X-1. An estimate of the electron temperature and the optical thickness for Thomson scattering of the emitting region is attempted on the basis of recent theoretical calculations.