| Abstract |
To this day, the physics and geometry of the inner-most region in active galactic nuclei (AGN) are largely unknown. According to the currently-accepted paradigm, the inner-most region consists of a super-massive black hole (BH) surrounded by an accretion disc. Perhaps the only way of indirectly probing this region is to study the highly variable X-ray emission of AGN. This X-ray emission is thought to consist of a power-law continuum component from a primary X-ray source, plus “reprocessed” continuum emission by the accretion disc.
This scenario predicts that reprocessed disc emission should also vary in response to the intrinsically variable continuum emission, but with a delay (this is known as “X-ray reverberation”). The search for correlations, including delays, between light curves in different energy bands has revealed evidence in support of the X-ray reverberation picture, although the results are mostly “hand-waving.” This is mainly because the statistical properties of the available tools, i.e. the “time-lag spectrum” and “intrinsic coherence function,” are unknown.
In my work I developed, for the first time, prescriptions for reliably and accurately calculating time-lag and intrinsic coherence estimates. I then applied these prescriptions to estimate the time-lags and intrinsic coherence between X-ray light curves in different energy bands, for a sample of ten X-ray bright and highly variable AGN. To this end, I downloaded and reduced all available XMM-Newton archival data for these sources. The results of my study regarding the time-lags and intrinsic coherence of my sample are summarised as follows: (a) The “iron line vs. continuum” (i.e. 5-7 vs. 2-4 keV) time-lags, which have the most widely-accepted reverberation origin, are consistent with a “point-like” X-ray source located a few gravitational radii above the BH. However, it is not possible to constrain other important parameters such as the BH mass and spin, or the inclination angle of the system. (b) The low-frequency “continuum” time-lags have a universal energy- and frequency-dependence, and scale approximately with the square root of the Eddington accretion rate. (c) The intrinsic coherence has a universal energy- and frequency-dependence, although it scales neither with the BH mass, nor with the Eddington accretion rate.
Result (a) is the best that can be done with current data. It is also unlikely to significantly improve in the future, since that would require a substantial improvement in the data quality (e.g. many more XMM-Newton observations). Results (b) and (c) are broadly consistent with popular AGN X-ray variability models, although detailed theoretical predictions need to be compared to my results in order to constrain the models.
|
Variability studies of the iron line emission in AGN
