The Complex 0.4--12 keV X-Ray Spectrum of Cygnus X-1

Simultaneous observations of the 0.4-2 keV and 2-12 low-state spectrum of Cygnus X-1, made with the low-energy transmission grating spectrometer and gas scintillation proportional counter on board EXOSAT, are presented. At least four spectral components are required to describe the data: a hard (energy spectral index alpha about 0.4) power law dominating the emission above 2 keV, a broad (FWHM 170 eV) emission line near 6.3 keV, a steep (alpha = 2.2) continuum below 3 keV, and a broad emission feature between 600 and 800 eV. Conventional viscous accretion disk models (alpha disks) cannot simultaneously explain both the spectral shape and the luminosity of the steep soft continuum component.