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 γ 1 , γ 2 , and γ 3 ) include the occasional appearance of a strong ∼ MeV bump (γ 1 ), whose strength appears to be anticorrelated 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 γ 2 state, whereas a transition from γ 2 to γ 3 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 γ 2 -γ 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 Ṁ. The unusual appearance of the γ 1 state apparently reflects a change in the dissipative processes within the disk.