X-ray reverberation studies of AGN

The broad iron line seen in the X-ray spectra of Active Galactic Nuclei (AGN) is a unique probe of the very near environment of the black hole where strong field general relativistic effects are important. The iron fluorescence line is produced when the X-ray continuum emission, which originates in the accretion disk corona, back-scatters or "reflects" off the disk. The line is sensitive to Doppler, transverse Doppler shifts and special relativistic boosting from the motion of the disk and gravitational redshifting from being deep in the potential well of the black hole. The sudden activation of X-ray flares in the corona (the presence of which are suggested by observation) provides a variable illumination pattern over the disk, and the reflected "echo" from these flares can be used to map out the astrophysics of this region in detail. This is the technique of reverberation mapping.