Publikace ÚTF

Effect of cold plasma on the form of rays propagating in the equatorial plane of a rotating black hole is investigated. Two kinds of regions in the radius-impact parameter plane allowed for the rays are constructed: for radiation with a given frequency at infinity and for radiation with a given "telescope frequency" seen by a local observer. The form of allowed regions for locally nonrotating observers as well as observers falling freely from infinity is established. The allowed regions contain rays which directly reach the horizon, or there exists a "neck" connecting the forbidden regions such that the rays coming from infinity cannot reach the horizon. In case we considered a set of observers at various radii instead of the neck we find two different regions - from one the rays reach the horizon and not infinity and from the other one they reach infinity, but not the horizon. The results are analyzed by analytical methods and illustrated by figures constructed numerically.