X-ray time variations from Cygnus X-1 and implications for the accretion process

Abstract

THE binary system Cygnus X-1 is thought to contain a black hole that accretes matter from an O-type supergiant companion. Gravitational energy of the matter in the accretion disk is transformed to thermal energy, resulting in the emission of X-rays. These are emitted in successive bursts ('shots')¹. Here we demonstrate the existence of a time delay between X-rays in different energy ranges; this is manifest as a double-peaked structure in the phase lag between two X-ray energy ranges for Fourier periods between 0.2 and 20 s. Taken together with our analysis of the power density spectrum, this indicates that shots tend to occur with two preferred durations and that the X-ray energy spectrum becomes harder as the shot progresses. This is consistent with an accretion model in which clumps of matter in the disk have two preferred sizes, and are heated by the release of gravitational energy as they drift from the outer to the inner region of the disk before disappearing into the black hole.