Lecture 32 - The Gamma Ray Universe
Gamma rays, representing the most energetic part of the electromagnetic spectrum, are produced by the hottest and the most energetic objects in the universe. Because of their extremely small wavelengths, gamma-rays can not be captured and reflected by mirrors unlike visible light or X-rays.
Moreover, gamma-rays coming from space are mostly absorbed by the Earth's atmosphere. So gamma-ray astronomy could not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. And we finally developed the ability to detect gamma-rays only in the 1960s, opening up a new window to the Universe.
One of the new windows to the universe is Gamma Ray Astronomy. This branch of astronomy came of age during the past twenty years. Today, one can detect gamma rays with energies from MeV to TeV. Low energy gamma rays come from the interstellar medium, supernova remnants, etc. High energy gamma rays are associated with very energetic sources, such as Pulsars, Quasars, Active Galactic Nuclei, etc. In this lecture I shall discuss various sources of gamma rays, as well as various emission mechanisms, in particular Compton scattering.