Time Lags and Reflection in BH X-ray Binaries
Abstract
It has been known for almost two decades that black hole X-ray binaries exhibit time lags between energy bands, where the hard emission usually follows the soft one (Miyamoto et al., 1991). These lags can naturally be thought of as arising due to light-travel time delays, e.g. due to distant reflection or Compton upscattering in the corona, but recently models have been considered where the lags are produced by accretion variations propagating through the emitting region. Kotov et al. (2001) have shown that the energy dependence of the lags in the hard state of Cyg X-1 contains structures around the iron line, suggesting a reflection origin for at least part of the lag, but they ruled out models where the lag corresponded to the disc inner truncation radius, instead favoring an accretion propagation model. However, recent multi-wavelength observations have shown that in several BHXRBs there is clear evidence for X-ray-optical reprocessing from the outer disk, which implies that there should also be an X-ray reflection signature from large radii. This large-scale reflector may contribute significantly to the lags because, even if the solid angle of the reflector is small, the light-travel times to the outer disk are large. The observed drop in time lag towards higher Fourier-frequencies may then result from the smearing of the faster variations that is expected at such large light-travel times. Here we use the constraints from optical and X-ray observations to predict the contribution of the outer disc to the observed lags and fit a self-consistent model for this component to both the energy-dependence and the Fourier-frequency-dependence of the lags in several BHXRBs.
- Publication:
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AAS/High Energy Astrophysics Division #11
- Pub Date:
- March 2010
- Bibcode:
- 2010HEAD...11.4308C