Identification of the soft X-ray excess in Cygnus X-1 with disc emission

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We present results of a study of the soft X-ray excess in the black hole candidate Cygnus X-1 made with the Rosat PSPC, using observations taken during persistent emission at orbital phases close to 0.5. The soft excess can be well fitted as a blackbody with temperature kTbb = 0.13± 0.02 keV. kTbb did not vary appreciably with intensity of the source. By assuming that the distance of the source is its lower limit of 2.5 kpc, a luminosity of the soft excess of 4.7· 10³⁶ erg s⁻¹ was obtained. From this, disc temperatures were calculated as a function of radius, assuming the compact object to be a 10 M black hole, in particular, the temperature at 7 Schwarzschild radii expected to be highly representative of the total disc emission. This was found to be 0.13 keV, in very good agreement with the spectral fitting result. This good agreement strongly supports the identification of the soft excess with emission from the disc around a black hole.