Institut für Astronomie und Astrophysik
Abteilung AstronomieSand 1, D-72076 Tübingen, Germany
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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 Ldisk ~ (lh/ls)-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 lh/ls ~ 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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Last modified 30 Jan 2013