A Self-Consistent Model For The Long-Term Gamma-Ray Spectral Variability of Cygnus X-1

The long-term transitions of the black hole candidate Cygnus X-1 (between the states γ1, γ2, and γ3) include the occasional appearance of a strong ∼ MeV bump (γ1), 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 ≈ 2 in the viscosity parameter α is responsible for quenching this bump and driving the system to the γ2 state, whereas a transition from γ2 to γ3 appears to be induced by an increase of about 25% in the accretion rate Ṁ . In view of the fact that most of the transitions observed in this source seem to be of the γ2 − γ3 variety, we conclude that much of the long term gamma-ray spectral variability in Cygnus X-1 is due to these small fluctuations in Ṁ . The unusual appearance of the γ1 state apparently reflects a change in the dissipative processes within the disk. Subject headings: accretion disks—black hole physics—radiation mechanisms: bremsstrahlung—radiation mechanisms: inverse Compton—relativity—stars: individual: Cygnus X-1