Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany
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Preprint I/05

Long Term Variability of Cygnus X-1. IV. Spectral Evolution 1999-2004

J. Wilms (1), M. A. Nowak (2), K. Pottschmidt (3), G. G. Pooley (4), S. Fritz (5)

(1) Department of Physics, University of Warwick, Coventry, CV7 1AL, UK
(2) MIT-CXC, NE80-6077, 77 Massachusetts Ave., Cambridge, MA 02139, USA
(3) CASS, University of California at San Diego, La Jolla, CA 92093-0424, USA
(4) MRAO, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK
(5) IAA Tübingen, Universität Tübingen, Sand 1, 72076 Tübingen, Germany

To be published in: A&A

Abstract. Continuing the observational campaign initiated by our group, we present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15 GHz. We present ~200 pointed observations taken between early 1999 and late 2004 with the Rossi X-ray Timing Explorer and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., by Titarchuk (1994, compTT) and by Coppi (1999, eqpair), can reproduce this simplicity; however, the success of the phenomenological broken power law models cautions against "overparameterizing" the more physical models. Broken power law models reveal a significant linear correlation between the photon index of the lower energy power law and the hardening of the power law at ~10 keV. This phenomenological soft/hard power law correlation is partly attributable to correlations of broad band continuum components, rather than being dominated by the weak hardness/reflection fraction correlation present in the Comptonization model. Specifically, the Comptonization models show that the bolometric flux of a soft excess (e.g., disk component) is strongly correlated with the compactness ratio of the Comptonizing medium, with L_disk ~ (l_h/l_s)^-0.19. Over the course of our campaign, Cyg X-1 transited several times into the soft state, and exhibited a large number of "failed state transitions". The fraction of the time spent in such low radio emission/soft X-ray spectral states has increased from ~10% in 1996-2000 to ~34% since early 2000. We find that radio flares typically occur during state transitions and failed state transitions at l_h/l_s ~ 3 and that there is a strong correlation between the 10-50 keV X-ray flux and the radio luminosity of the source. We demonstrate that rather than there being distinctly separated states, in contrast to the timing properties the spectrum of Cyg X-1 shows variations between extremes of properties, with clear cut examples of spectra at every intermediate point in the observed spectral correlations.
 
Key words: stars: individual (Cyg X-1) - binaries: close - X-rays: binaries - black hole physics

Preprint (482 kb PDF file including figures)
Astrophysics (astro-ph): astro-ph/0510193

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