The long-term x-ray, UV and optical variability of active galactic nuclei

Abstract

The variability of emission from active galactic nuclei (AGN), both intrinsic, due to changes in the emitting regions, and extrinsic, due to absorption and scattering from material near to the black hole, can reveal a wealth of information about their geometry and accretion behaviour. In this thesis, I use analysis of the spectral variability of the Xray, UV/optical emission from AGN to probe the properties of the material surrounding supermassive black holes at their centres.

Firstly, I present constraints on the variability of the photon index of the X-ray spectrum of 24 Swift AGN from the Palomar sample of galaxies. The change in the behaviour of AGN from ‘softer-brighter’ to ‘harder-when-brighter’ at a critical accretion rate, seen previously in other samples of AGN, is found. ‘Harder-when-brighter’ behaviour is measured in several sources individually, achieved previously in only one AGN.

I present a study of the long-term (months-years) X-ray spectral variability of three Seyfert AGN known to exhibit strong absorption variations. Trends in the long-term behaviour of the absorbers, in particular a strong anti-correlation between the observed absorbing column and the intrinsic luminosity of NGC 1365, are explained using a variable wind model. In this model, the radius at which the wind arises is dependent on the luminosity of the central engine, leading to correlated long-time-scale changes in the observed absorption properties.

Finally, time lags between the X-ray and UV/optical variability of the Seyfert AGN NGC 4395 are measured, using cross-correlation techniques. The lags fit the ?4/3 dependence expected from reprocessing of X-rays in the accretion disc. Modelling of the expected lags from a standard Shakura-Sunyaev thin disc with the previously-measured physical parameters of the system is then carried out. The results are consistent with the data, further supporting reprocessing as the principle source of UV/optical variability in NGC 4395.

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