Test of the inverse Compton model for Cygnus X-1

Abstract

The X-ray spectrum of Cygnus X-1 from ∼1 keV to ∼200 keV exhibits two distinct components. The spectrum below ∼10 keV is soft and may be the tail of comptonised black body if the turnover at ∼1 keV is real, while that above ∼10 keV is a simple power law^1,2. The spectra above 200 keV remain uncertain with some showing a cutoff but others a continuation of the power law³. The power law hard spectrum is thought to be the result of unsaturated Compton up-scattering of soft photons by hot thermal electrons⁴. The comptonisation region may be the optically thin inner part of the disk or spherical accretion flow near the black hole or a hot corona surrounding a thin disk¹. The unsaturated Compton model is attractive as it accounts for the power law spectrum with index α ∼ 1 (I_v ∝ v^−α) and the alleged high electron temperature (T_e >10⁹K). However, there are still questions about the origin of the soft photon supply and stability. As important conclusions are drawn from this model (for example, interpretation of the high energy cutoff as a measure of the electron temperature and α as a measure of the Komponeet parameter y ≡ (4kT_e/m_ec²)τ²_es ≃ 4/(3α + α²), τ_es ≡ electron scattering depth)^4,5, it is crucial to have independent verification of this idea. We propose here a more critical test and show that the hard X-ray data available to date strongly support this model.