AGN are galaxies which contain a central massive black hole. These black holes can contain up to 109 times as much matter as the sun. The black hole itself generates no electromagnetic radiation, however the matter spiraling into it releases a great deal of energyas radiation, from radio waves all the way up to high energy gamma rays.
Supernova Remnants such as the pulsar lying in the heart of the crab nebula are of particular interest to gamma ray astronomers. These systems are believed to be well understood and predictions can be made and checked about their gamm ray properties based on their properties in other wavelengths.
Gamma Ray Bursts are a complete mystery, no one understands what sourceis responsible for these anomalous gamma ray sources. Seemingly distributedevenly over the sky, each burst lasts for a few minutes and then fades. Thesebursts have been detected by satellites only, the nature of the ACT limitsground based astronomy to sources with longer term variability.