A Compton-cooled feedback mechanism for Cygnus X-1

Abstract

The spectrum of the X-ray source Cygnus X-1 appears to exhibit two states. The source spends most of its time in the so-called ‘low state’ in which it is characterized by a power-law spectrum, N(ɛ)∝ɛ^−α, of photon index 1.6 and high-energy cutoff ∼100 keV (ref. 1). Occasionally it makes a transition to a ‘high state’² during which the soft flux (1–3 keV) increases by a factor of a few and the X-ray spectrum changes to an index steeper than 2.5. Most of the interpretation of the spectrum of Cyg X-1 has centred on the comptonization of soft photons by a hot gas in an accretion disk surrounding a black hole^1,3. It has been assumed that the hot gas is held at a fixed temperature, despite the obvious high degree of variability observed in the source on time scales ranging from milliseconds to months^4,5. We consider here the effect of relaxing this assumption of constant temperature because a Compton-cooled gas, in which the gas temperature drops in response to the comptonization of soft photons, can readily produce a two-state time-averaged spectrum from variations in the soft flux⁶. We speculate on a feedback mechanism that may effectively lock the source in either of the two states.