Unidentified Gamma-ray Sources

More than half of the sources detected by EGRET are not identified with known objects seen at lower energies. Scientists refer to these sources as unidentified, as they do not know why these objects emit gamma-rays. Some may be blazars, while others may be radio-quiet pulsars, similar to Geminga, which emits copious gamma-ray and x-ray radiation, but no pulsation in other parts of the electromagnetic spectrum.

An image  of EGRET's all-sky survey showing a bright strip of gamma-ray emission located along the central strip of the image, which represents  the plane of our galaxy.  The isolated patches of brightness at various other locations are outside of the plane of our galaxy.

The image is a projection of the gamma-ray sky expected to be seen by GLAST.  The projectioon is similar to the all-sky map seen by EGRET, except there are some smaller patches of gamma-ray brightness because of GLAST's higher resolution, which can break a big patch into several smaller gamma-ray sources.

The above pictures illustrate how well GLAST will image the sky in comparison with EGRET. On the left is EGRET's resulting image of its last all sky survey. On the right is an image of the gamma-ray sky as seen by GLAST.

GLAST will be able to greatly improve the positions of sources of gamma-rays so that they may be more easily identified. Solving the puzzle of the unidentified sources (especially those that appear to be in our Galaxy) may be one of GLAST's biggest contributions.