X-ray/UV/optical Variability and Mapping the Inner Structures of Active Galactic Nuclei.

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The origin of UV and optical variability from Active Galactic Nuclei (AGN) is not well understood. It may be caused by reprocessing of X-ray emission, coming from very near to the central supermassive black hole (SMBH), by the surrounding accretion disc and broad line region (BLR) gas. Alternatively it might be caused by accretion rate fluctuations in the disc. These possibilities can be tested by measuring the time lags between the X-ray emission and the emission in the UV and optical bands. Different physical scenarios result in different time lag patterns. In the reprocessing scenario, the lags map out the temperature structure of the accretion disc and the geometry of the surrounding gas, in a process known as 'reverberation mapping'. These structure are far too small to map by direct imaging or any other method.Southampton is one of the major world research groups in the study of AGN variability and we lead many of the largest X-ray/UV/optical monitoring programmes, eg using the Swift multiwaveband observatory and the XMM-Newton X-ray/UV observatory. In particular, a huge observational program with Swift will be complete by early 2018, ideally timed for a new student.The current project consists of both observation and computer modelling. The successful student will analyse these world-leading datasets to map out the inner structures of AGN and will also build a computer model of the expected emission from the accretion disc and BLR gas. By comparing observation with theory we will determine the inner geometry of AGN, including both the X-ray source geometry and that of the inner part of the accretion disc.