2019ApJ...878...20C

Characteristic signatures that X-ray reverberation from an extended corona can manifest in the observed PSD of AGN are investigated. The presence of two X-ray blobs illuminating an accretion disk can cause the interference between two reprocessing-echo components and produce distinct physical features in the PSD. The oscillatory structures (e.g., dips and humps) are seen, but, contrarily to the lamp-post case, the strongest dip is not always the one at the lowest frequency. Instead, we find that the frequency where the strongest dip is seen is associated with the lower-source height while the lowest frequency where the first dip appears is linked to the upper-source height. This is because the reverberation timescales increase with the source height. Accurate modeling of the PSD then helps put constraints on the lower and upper limits of the corona extent. Furthermore, the reverberation signatures are less pronounced with increasing number of sources that do not produce reflection (e.g., additional X-rays from fast, relativistic outflows). The amplitude of the oscillations also depends on the amount of dilution contributed by the X-ray sources, and thus encodes information about their relative brightness. Due to stronger dilutions, robust detection of these signatures with current observations will become even more difficult if the corona is extended. Future observations made by Athena will enable us to fit these characteristics with statistically significant details, and to reveal the nature of the disk-corona system.