A Transition Disk Model Fit to Cygnus X-1

Abstract

During the last two decades, the X-ray/γ-ray spectrum of the black hole candidate Cygnus X-1 has been extensively studied by, e.g., HEAO1, EXOSAT, Ginga and the Compton GRO. This system settles into two distinct long term configurations, namely the so-called soft and hard states. These spectral states have also been observed in other black hole candidates. In the hard state, the X-ray spectrum is dominated by 2 – 200 keV emission, which has been interpreted by some to be the result of the inverse Compton scattering of cold external photons by a hot (kT ≈ 50 keV) low density plasma. This plasma could be the innermost region of the accretion disk (Shapiro, Lightman & Eardley 1976), or it could be a corona overlying a relatively cold disk (Liang & Price 1977; Haardt & Maraschi 1993). An alternative model is one in which the emission is a sum of multi-temperature Wien peaks from the transition region of the disk (Maraschi & Molendi 1990; Misra & Melia 1996). All these models predict a similar spectrum which is more or less a power-law with an exponential cutoff.