14 - Probing strong gravity effects with X-ray polarimetry

Summary

Light coming from the innermost regions of active galactic nuclei or galactic black hole systems is heavily influenced by the strong gravity of their central compact body. High velocities of the emitting matter also modify the properties of the light received by the detector. In this contribution the influence of the combined special and general relativistic effects on the observed intensity and polarization is summarized. The application on the systems with a geometrically thin and optically thick Keplerian accretion disc is shown.

Introduction

The light emitted in the vicinity of the compact object has different properties when absorbed by a detector at infinity. First of all the photons are usually emitted by fast-moving matter – orbiting, falling into or being ejected from the central body with very high speeds. Thus the effects of Einstein's special relativity, the Doppler shift and aberration, change the photon energy and the direction of its polarization. The beaming effect in the direction of emitting matter motion may be quite significant as well. It is worth mentioning that although these effects are those of special relativity, the high velocities causing them are due to large gravity of the central object. Therefore we need to use general relativity in order to evaluate them properly.

However, general relativity has even more direct impact on the properties of these photons. They move from a strong gravity region, often dragged along by the rotating space-time if the compact body has large angular momentum.