Unidentified Gamma-ray Sources
There are a large number of gamma-ray sources whose identity is
completely unknown. These aren't the mysterious gamma-ray bursts which briefly
flash on at random points in the sky. Rather, they tend to be steady
emitters of gamma-rays. Many of these sources appear to be associated with
the Milky Way Galaxy. This component of the gamma-ray sky has been known
since the mid-1970s. Recent observations have compounded the problem by
greatly increasing the number of detected sources in this class. Other
wavelength bands may have a number of sources which are unidentified, but at
gamma-ray energies this class is over half of the total! So far, efforts to
identify the nature of these sources have been largely unsuccessful.
![EGRET all-sky map at E > 100 MeV](../../../Images/basic/gamma/egret_sources.gif)
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In this plot of unidentified gamma-ray sources and gamma-ray pulsars, the
unidentified sources tend to cluster along the plane of the Milky Way
Galaxy which runs horizontally through the center of the map.
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The main reason these sources are unidentified is that they are typically
detected at high energies (E > 100 MeV) where source localization is
somewhat poor. Each source may have a positional error of up to 1 degree.
This makes it very difficult to identify the counterparts at other wavelengths
such as optical or X-ray. An X-ray image covering the source position may
show dozens of possible counterparts, and an optical image hundreds. Only by
detecting correlative variability or a distinctive signature like pulsar
emission can a definitive association be easily made.
There have been a few successes. One of the earliest unidentified
sources, a very bright object called "Geminga", was determined to be
a gamma-ray pulsar. In this case, a search for pulsed emission using
X-ray data found the characteristic spin period of this neutron star
(spinning 4.22 times a second). Gamma-ray observations confirmed
this pulsation which settled the question of the nature of this
particular source. Some scientists believe that many of these sources
will turn out to be radio-quiet pulsars. There is statistical evidence
that some of these sources are associated with supernova remnants or
OB star associations. Some of these sources, especially those not in the
plane of our Galaxy, may turn out to be active galaxies. Some may even
represent a new class of sources altogether! Progress in solving this mystery
depends upon building ever better gamma-ray instruments and combining the
gamma-ray observations with sensitive observations at other wavelengths.
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