Institut für Astronomie und Astrophysik

Abteilung Astronomie

Preprint C/98

RXTE Observation of Cygnus X-1: I. Spectral Analysis

James B. Dove(1,2), Jörn Wilms(3,1), Micheal A. Nowak(1), Brian A. Vaughan(4), and Mitchell C. Begelman(1,2)

(1) JILA, University of Colorado and the National Institue of Standards and Technology, Campus Box 440, Boulder, CO 80309, USA
(2) Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0391, USA
(3) Institut für Astronomie und Astrophysik, Abt. Astronomie, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(4) Space Radiation Laboratory, California Institute of Technology, 220-47 Downs, Pasadena, CA 91125

1998, Mon. Not. R. Astron. Soc., 298, 729

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 10 ksec observation by the Rossi X-ray Timing Explorer. The spectrum can be well described phenomenologically by an exponentially cut-off power law with a photon index Gamma = 1.45(+0.0)(-0.02) (a value considerably harder than typically found), 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. Although the 3-30 keV portion of the spectrum can be fit with a reflected power law with Gamma = 1.81 +/- 0.01 and covering fraction f = 0.35 +/- 0.02, the quality of the fit is significantly reduced when the HEXTE data in the 30-100 keV range is included, as there is no observed hardening in the power law within this energy range. As a physical description of this system, we apply the accretion disc corona models of Dove, Wilms & Begelman (1997) - 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 +/- 5 keV, 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 - 10 keV 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.

Key words: radiation mechanisms: non-thermal - radiative transfer - X-rays: binaries - accretion, accretion discs

Paper (118k gzip'ed Postscript including figures)

[Home Page] [Preprints 1998] [Quick Reference] [Feedback]

Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de) Last updated 12-Aug-1998