X-ray Reflection as a Probe of Accreting Black Holes

24/02/2014 — CITA National Fellows' Meeting, University of Toronto (24/02/2014)
14/02/2013 — Astrophysics Seminars, University of Bristol, UK (14/02/2013)
11/01/2013 — Astronomy & Physics Colloquia, St. Mary's University, Halifax, NS (11/01/2013)

Active galactic nuclei (AGN) are some of the most luminous objects we see in the Universe, powered by the accretion of matter onto a supermassive black hole in the centre of a galaxy, yet many of the physical processes by which the energy is released and injected into the surroundings remain a mystery.

X-rays are emitted from a 'corona' of energetic particles surrounding the black hole and as well as being observed directly, they are seen to be reflected from the accreting disc, producing a number of spectral features including emission lines that are broadened by relativistic effects in the proximity of the black hole. I will discuss how detailed measurement of the reflected X-rays from the accretion disc can be used to probe the innermost regions of accretion flow and corona, right down to the innermost stable orbit and the event horizon.

Novel spectral analysis techniques allow us to reconstruct, from the observed relativistic X-ray reflection spectrum, the spatially resolved illumination pattern of the accretion disc and I will discuss how comparing this to the results of systematic general relativistic ray tracing simulations I have developed, we are able to constrain the location and geometry of the X-ray emitting corona and understand the dramatic change of the narrow line Seyfert 1 galaxy 1H 0707-495 into an extremely low flux state in terms of a collapse in the corona as well as the changes AGN undergo all the time, driving their extreme variability in the emitted X-rays.

I will discuss how measurements of the X-ray variability, specifically the reverberation time lags that are observed between variability in the directly observed X-rays from the corona and those reflected from the accretion disc add a further dimension to the study of accreting black holes, letting us not only build up a three dimensional image of the immediate vicinity of the black hole but also to probe mechanisms by which the energy is released from the accretion flow; techniques that will let us exploit not just current instrumentation but future proposed X-ray observatories to really put theories of black holes and accretion to the test and understand such extreme objects and how they shape Universe.