Outflow and feedback in active galactic nuclei: high-resolution X-ray spectroscopy and variability

Abstract

Active Galactic Nuclei (AGN) are accreting supermassive black holes at the centers of galaxies. They can have a profound impact on the evolution of their host galaxies and surroundings by means of their outflows of ionized gas. However we still lack a solid understanding of how strong their outflows are and where they originate. Without this, we can not quantify the effect of feedback in a proper way. This thesis focusses on investigating the properties of the outflows by using high-resolution X-ray spectroscopy and variability studies. These outflows show their presence by blue-shifted absorption lines. A single, good quality observation can be used to characterize the properties of the outflow, such as outflow velocity, ionization structure and column densities of the different ions detected in the outflow. However, multiple observations of the same source are needed to constrain its location. By constraining the timescale on which changes in the ionization structure of the outflow occur, the distance to the outflow can be constrained. This has been done for two sources in this thesis, NGC 5548 and Mrk 509, leading to distance limits of several lightyears in both cases. AGN not only have outflows, but also show emission lines in their spectra. Whether these lines are produced by the same gas as that responsible for the absorption (i.e. the outflow) is unclear at the moment. The emission lines in NGC 5548 changed in flux between two observations and this has allowed us to for the first time constrain the geometry and location of the emitting region, showing that the emission lines could indeed be produced by the outflowing gas. A further systematic study including other sources did not show any evidence for variable emission lines however, mostly due to the limited signal to noise ratio of most observations.