EXOSAT observations of Cygnus X-1 : study of the soft X-ray excess.

Abstract

The paper presents results of a spectral analysis of the Cyg X-1 data collected during 1984-1985 by the Medium Energy instrument onboard the Exosat satellite. Emphasis is on two issues: the soft X-ray excess in the range below 3 keV and the iron-line intensity. The presence of a soft component in addition to the high-energy power-law component is confirmed in all spectra, including during the dips. The soft component can be approximated by a black-body spectrum with a variable intensity and a temperature of 0.25 keV. Variations in the absorption by cold matter for the soft and hard component are found to be sufficient to explain the changes in the X-ray spectra during dips.