Mullard Space Science Laboratory

C. E. Ashton
A study of warm absorbers in active galactic nuclei
MPhil 2006 (supervisor: G. Branduardi-Raymont)

This thesis explores the ‘warm absorber’ phenomenon observed in Active Galactic Nuclei (AGN). Warm absorbers are clouds of ionised gas within AGN, that cause absorption at soft X-ray wavelengths. They are observed in half of all Type 1 AGN, hence they play an important part in the framework of our understanding of Active Galactic Nuclei. Observations with the satellite XMM-Newton have given us the highest signal-to-noise data yet.

XMM-Newton observations of the quasars PG 1114+445 and PG 1309+355 are studied. Both quasars exhibit evidence for absorption by warm material in the line-of-sight. We define a ‘phase’ of absorption to have a single ionisation parameter and column density. From fits to the data, the absorption in PG 1114+445 is found to be in two phases, a ‘hot’ phase with a log ionisation parameter ξ of 2.57 and a column of 1022 cm-2, and a ‘cooler’ one with log ξ of 0.83 and a column of 1021 cm-2. The absorption in PG 1309+355 consists of a single phase, with log ξ of 1.87 and a column of 1021 cm-2. The absorbing gas lies at distances of 1019 – 1022 cm from the continuum radiation sources in these AGN, suggesting origins in a wind emanating from a molecular torus, according to the ‘Standard Model’ of AGN. The kinetic luminosities of the outflowing absorbers represent insignificant fractions (< 10-3) of the energy budgets of the AGN.

Using data for the Seyfert 1 H 0557-385, the warm absorption is characterised by two phases, a phase with log ξ of 0.48 and a column of 1021 cm-2, and a phase with log ξ of 1.63 and a column of 1022 cm-2. Neutral absorption is also present in the source, and possible origins for this are discussed.

For a large sample, observations of warm absorbers are collated and compared with models.


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