A self-consistent model for the long-term gamma-ray spectral variability of Cygnus X-1

Abstract

The long-term transitions of the black hole candidate Cygnus X-1 (between the states gamma_1, gamma_2, and gamma_3) include the occasional appearance of a strong ~ MeV bump (gamma_1), whose strength appears to be anti-correlated with the continuum flux (~ 400 keV) due to the Compton upscattering of cold disk photons by the inner, hot corona. We develop a self-consistent disk picture that accounts naturally for these transitions and their corresponding spectral variations. We argue that the bump is due to the self-Comptonization of bremsstrahlung photons emitted predominantly near the plane of the corona itself. Our results suggest that a decrease by a factor of approx 2 in the viscosity parameter alpha is responsible for quenching this bump and driving the system to the gamma_2 state, whereas a transition from gamma_2 to gamma_3 appears to be induced by an increase of about 25 % in the accretion rate Mdot. In view of the fact that most of the transitions observed in this source seem to be of the gamma_2 to gamma_3 variety, we conclude that much of the long term gamma-ray spectral variability in Cygnus X-1 is due to these small fluctuations in Mdot. The unusual appearance of the gamma_1 state apparently reflects a change in the dissipative processes within the disk.