What Are the Unidentified Galactic
Gamma-Ray Sources?
Only 45% of the gamma-ray sources observed with the EGRET (Energetic
Gamma Ray Experiment) on the Compton Gamma Ray Observatory (CGRO) have
been identified with sources known at lower frequencies. The identities
of the remaining 55% constitute a major mystery in high-energy astrophysics.
The distribution of these unidentified sources suggests a largely Galactic
population; however, there are some at very high Galactic latitudes. It
is reasonable to anticipate that those sources are similar to, or at least
related to, the extragalactic blazars identified so far.
The unidentified sources in or near the Galactic plane are a more difficult
problem. Only seven (six high-energy) gamma-ray sources near the Galactic
plane have been identified as pulsars. Although some of the unidentified
sources may be radio-quiet pulsars like Geminga, and others, which show
substantial time variations, are undoubtedly gamma-ray emitting blazars
similar to those seen at high latitudes they are too numerous and too bright
for all of them to be explained in this manner. Some probably represent
a new class of Galactic gamma-ray objects where extreme particle acceleration
processes are occurring.
The potentially most exciting discovery from a future gamma-ray mission
would be the identification of these objects as a new class of gamma-ray
sources. This will require an instrument with improved sensitivity and
the best possible angular resolution.
