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.