Gamma-Ray Astronomy in the Compton Era: The Universe in Gamma Rays

The Compton instruments combine to provide many different views of the high- energy universe. Perhaps the most interesting, and one representing a primary mission goal for Compton, is the all-sky survey. The first two and one half years were devoted to viewing the entire gamma-ray sky with the two imaging detectors, EGRET and COMPTEL. The remarkable images which resulted from this survey show for the first time what the sky looks like over the medium (1-30 MeV) and high (>100 MeV) energy ranges.

   This COMPTEL all-sky map
   is the first ever at MeV energies.
   This image complements the EGRET
   all- sky map which is shown on the home page.

The dominant feature is the wide strip across the middle of the image which is the gamma-ray glow of the Milky Way. This is shown particularly well by the EGRET map on the home page. This diffuse emission gives astronomers insight into the interactions of cosmic rays and interstellar material. Unfortunately, the emission also makes it more difficult to detect point sources of gamma rays which lie in the galactic plane. The other main feature of these maps is the occasional island of enhanced emission seen over the diffuse background. These are the gamma-ray emitting pulsars and galaxies which are of considerable interest

to astronomers. Many of these point sources are not obvious from simply inspecting the map. For instance, the EGRET point source catalog contains 157 individual sources. Advanced statistical methods are required to distinguish the weaker sources from mere fluctuations and to make the best estimates of the source characteristics such as true location, total flux, and variability or a more detailed look, we can focus in on a smaller region of sky. The anticenter region (so-called because it is 180 degrees away from the direction to the galactic center)

is a good place to start. The COMPTEL image of the anticenter shows four different types of objects which represent the variety of the gamma-ray sky. The Crab pulsar, the active galaxy PKS 0528+134, the transient gamma-ray source GRO J0422+32, and the gamma-ray burst GRB 910503 were all detected by COMPTEL in this region. Much of the sky is similar, look closer and you will find several sources, each with distinct characteristics. It is worth spending a little time discussing all these types of sources in more detail to understand the contributions of the Compton instruments.

The COMPTEL view of the galactic anticenter. The diversity of the gamma-ray sky is apparent even from this limited region.