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.

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.

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.