Supernova remnants and plerions in the Compton Gamma-Ray Observatory era

Due to observations made by the Compton Gamma-Ray Observatory over the last six years, it appears that a number of galactic supernova remnants may be candidates for sources of cosmic gamma-rays. These include shell-type remnants such as IC443 and γ Cygni, which have no known parent pulsars, but have significant associations with unidentified EGRET sources, and others that appear to be composite, where a pulsar is embedded in a shell (e.g. W44 and Vela), or are purely pulsar-driven, such as the Crab Nebula. This review discusses our present understanding of gamma-ray production in plerionic and non-plerionic supernova remnants, and explores the relationship between such emission and that in other wavebands. Focuses include models of the Crab and Vela nebulae, the composite nature of W44, the relationship of shell-type remnants to cosmic ray production, the relative importance of shock-accelerated protons and electrons, constraints on models placed by TeV, X-ray and radio observations, and the role of electrons injected directly into the remnants by parent pulsars. It appears as if relic electrons may be very important in the Vela and Crab remnants. The recent observation of the TeV hot spot in the Vela remnant, which is offset from the current pulsar position, is attributed to relic electrons that were left at the birthplace of the pulsar, the offset being due to the proper motion of the Vela pulsar during its 11,000 year lifetime. We also discuss the role of freshly-injected electrons in the remnants around the Crab, Vela and PSR B1706-44 pulsars. These electrons can acquire energies that tap up to at least 10 percent of the full pulsar polar potential, and can produce prominent synchrotron and inverse Compton radiation signatures. The various recent models for predicting gamma-ray emission in shell-type remnants are summarized. The constraining upper limits to TeV emission from such remnants obtained by the Whipple Observatory indicate that either the emission due to particles accelerated at remnant shocks is too faint to be detected by EGRET, or that conditions near their shells (e.g. high density, low magnetic field) limit the acceleration of particles to below a few TeV.