The variability of warm absorbers in Active Galactic Nuclei

Abstract

This thesis presents three studies of warm (photoionized) absorber variability in Active Galactic Nuclei (AGN) using high-resolution X-ray spectra provided by the Chandra High Energy Transmission Grating (HETG). The first study is a single observation of the AGN MR 2251-178, which is known to have a highly variable warm absorber (WA). We find an unusually thin effective hydrogen column density along the line of sight compared to previous observations. Strong line emission without corresponding absorption indicates significant WA geometric structure. Strong absorption features in the spectrum are evidence of a highly-ionized, high-velocity outflow, which could be carrying a large amount of mass and energy out of the AGN. In the second study, we search for absorption lines variability in the well-studied WA of MCG -6-30-15. We find a significant anti-correlation over time between at least two ions, with suggestions of additional time variation in other ions. At least one line, the is - 2p resonance line of Mg XII, varies as a function of 2-10 keV continuum luminosity. Luminosity-driven ionization changes alone are insufficient to explain the observed variation.

(cont.) Either multiple factors influence line strength on observable time scales, or the line of sight to the central source varies over time through a structured absorber. In the third study, we survey spectra from the HETG data archive. We model the normalized excess variance (NEV) spectrum of a varying WA and find that it does not explain high-energy (> 2 keV) spectral variation, nor does it generally fit NEV spectra at lower energies (< 2 keV). We also search through each spectrum at high resolution (AA = 0.01 A) for bins which vary more than expected due to normal Poissonian fluctuations. We find some evidence for such variation in the aggregate sample, though not in AGN individually. Our results show that WA structure is more complicated than pictured in contemporary models. Future high-resolution spectroscopic variability studies are certainly warranted. AGN models should eventually consider the effects of WA structure and the influence of continuum variation on the WA.