Rxte Observation of Cygnus X-1: Spectral Analysis

We present the results of the spectral analysis of the broad-band spectrum of Cygnus X-1 from 5.5 to 200 keV, using data from a 10ksec observation by the Rossi X-ray Timing Explorer. The spectrum is well described by a power law with a photon index Γ = 1.43± 0.02, modified by an exponential cut-off with an e-folding energy Ef = 152 +8 −7 keV, and a weak soft excess component that can be modeled as a thermal blackbody having a temperature kTbb = 1.0± 0.1 keV. The inclusion of a reflection component does not improve the fit. We apply the accretion disk corona models of Dove, Wilms & Begelman (1997) — where the temperature of the corona is determined self-consistently — to the observation. A slab-geometry accretion disk corona model, where a hot corona sandwiches a cold disk, predicts a spectrum that is too soft to explain the observations. On the other hand, a spherical corona (with an optical depth τ = 2.1± 0.1 and an average temperature kTc = 66± 3 keV) that is surrounded by an exterior cold disk provides a good description of the data. Such “photon-starved” coronal geometries are therefore more promising for explaining the accretion process of Cygnus X-1. Subject headings: stars: individual (Cygnus X-1) – methods: data analysis – accretion – radiation mechanisms: nonthermal – X-rays: stars – black hole physics