The effects of spectral hardness changes on reverberation lags
Abstract
Accreting black holes show characteristic reflection features in their X-ray spectrum, including an iron Kα line, which results from hard X-ray continuum photons illuminating the accretion disk. Measuring the reverberation lag resulting from the difference in path length between direct and reflected emission, and the spectral distortions to the iron line caused by rapid orbital motion and gravitational redshift, provides a powerful tool to probe the innermost regions around the black hole. Previous reverberation studies, both for supermassive and stellar-mass black holes, have largely ignored spectral variability of the continuum. However, this is a potentially important effect, since a hardening of the continuum spectrum causes non-linear changes in the shape of the reflection spectrum as different transitions in the disk are excited and the ionisation balance is changed. We have studied the effect of a pivoting continuum power-law on the reverberation lag spectrum, assuming the lamppost geometry, and developed an analytic description. Our model accounts self-consistently for both continuum and reverberation lags. We have applied it to Cygnus X-1 cross-specta for the 0.015-32 Hz Fourier frequency range, obtaining good fits and constraining the continuum and reflection parameters.
- Publication:
-
The X-ray Universe 2017
- Pub Date:
- October 2017
- Bibcode:
- 2017xru..conf..141M