A physical model for the variability properties of X-ray binaries

Abstract

Emission from X-ray binaries is variable on a wide range of
timescales. On long timescales, changes in mass accretion rate drive changes in spectral state. There is also rapid variability, the power spectrum of which consists of a low frequency quasi-periodic oscillation (QPO) superimposed on a broad band noise continuum. Here I investigate a model intended to quantitatively explain the observed spectral and variability properties. I consider a truncated disc geometry whereby the inner regions of an optically thick, geometrically thin accretion disc evaporate to form an optically thin, large scale height accretion flow.
The QPO is driven by Lense-Thirring precession of the entire hot flow and the broad band noise is due to fluctuations in mass accretion rate which propagate towards the central object. Mass conservation ties these two processes together, enabling me to define a model for the QPO and broad band noise which uses only one set of parameters. I am thus able fit the model to data. The accretion rate fluctuations drive fluctuations in the precession frequency, giving rise to a quasi-periodic oscillation rather than a pure periodicity. The model thus predicts recent observations which show the QPO frequency to
correlate with flux on short timescales. I then investigate a more unique model prediction. As the flow precesses, the patch of the disc preferentially illuminated by the flow rotates such that a non face on observer sees a quasi-periodic shift between blue and red shift in the iron K alpha line. An observation of such an effect would constitute excellent evidence for the model.