A Tomographic Scan of the Accretion Disk Around a Supermassive Black Hole

Student: Matthew Emery

Major: Astrophysics

Mentors: Dr. George Chartas

Department: Physics and Astronomy

A Tomographic Scan of the Accretion Disk Around a Supermassive Black Hole

Quasars are thought to be powered by accretion of gas onto supermassive black holes. As gas accretes onto the black hole it emits intense radiation with wavelengths ranging from radio to X-rays. The X-ray emission is thought to be emitted near the event horizon and thus provides a probe of studying the effects of strong gravity and testing general relativity. A novel method that allows one to perform a tomographic scan of a quasar's accretion disk is called X-ray microlensing. Stars in intervening galaxies may occasionally produce magnification patterns that cross the accretion disk resulting in magnifications of selective regions of the disk. One of the manifestations of X-ray microlensing is the gravitational redshift of Fe emission lines emitted from parts of the accretions disk very close to the event horizon.

We present results from the analysis of ~ 50 X-ray observations of the gravitationally lensed quasar RXJ1131 taken with the Chandra X-ray Observatory. Our main goal is to employ gravitational lensing to infer the structure near the event horizon of its supermassive black hole. Our simulations show that gravitationally shifted Fe lines should be accompanied by similarly shifted fluorescent disk lines of Mg, Si and S at lower energies. We report the first detection in a quasar of shifted Mg, Si and S fluorescent disk lines which we identify as the origin of the "soft" excess observed in the X-ray spectra of many other quasars. The properties of these shifted lines are used to infer the ionization properties of the accretion disk of RXJ1131 as a function of distance from the event horizon.