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Abteilung Astronomie

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Preprint 17b/97

RXTE Observation of Cygnus X-1: Spectral Analysis

J. B. Dove(1), Jörn Wilms(2), M. A. Nowak(3), Brian Vaughan(4), and M. C. Begelman(3)

(1) Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
(2) Institut für Astronomie und Astrophysik, Abt. Astronomie, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3) JILA, University of Colorado and the National Institue of Standards and Technology, Campus Box 440, Boulder, CO 80309, USA
(4) Space Radiation Laboratory, California Institute of Technology, 220-47 Downs, Pasadena, CA 91125

Contribution to The Active X-ray Sky: Results from Beppo-SAX and RXTE (L. Scarsi, H. Bradt, P. Giommi, F. Fiore, eds.), Nucl. Phys. B, Proc. Suppl.

Abstract. We present the results of the analysis of the broad-band spectrum of Cygnus X-1 from 3.0 to 200 keV, using data from a 10ksec observation by the Rossi X-ray Timing Explorer. Although the spectrum can be well described phenomenologically by an exponentially cut-off power law (photon index Gamma = 1.45(+0.01)(-0.02), e-folding energy E_fold=162(+9)(-8)keV, plus a deviation from a power law that formally can be modeled as a thermal blackbody with temperature kT_BB=1.2(+0.0)(-0.1)keV), the inclusion of a reflection component does not improve the fit. As a physical description of this system, we apply the accretion disc corona models of Dove, Wilms & Begelman, where the temperature of the corona is determined self-consistently. A spherical corona with a total optical depth tau = 1.6 +/- 0.1 and an average temperature kT_c = 87 +/- 5keV, surrounded by an exterior cold disc, does provide a good description of the data (reduced chi^2 = 1.55). These models deviate from the data by up to 7% in the 5-10keV range, and we discuss possible reasons for these discrepancies. However, considering how successfully the spherical corona reproduces the 10-200 keV data, such `photon-starved' coronal geometries seem very promising for explaining the accretion processes of Cygnus X-1.

Paper (79k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de)
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