RXTE Observation of Cygnus X-1: Spectral Analysis
Abstract
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 10,ksec observation by the Rossi X-ray Timing Explorer. The spectrum is well described by a power law with a photon index \Gamma = 1.43 \pm 0.02, modified by an exponential cut-off with an e-folding energy Efold = 152^{+8}_{-7} keV, and a weak soft excess component that can be modeled as a thermal blackbody having a temperature kTbb =1.0 \pm 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 \tau = 2.1 \pm 0.1 and an average temperature kT_c = 66 \pm 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.Keywords
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