National Aeronautics and Space Administration

Lightcurve of Geminga

The Geminga pulsar is one of the brightest gamma-ray sources, but is only a weak X-ray source and has little (if any) radio or optical emission. This figure shows how the gamma-ray brightness of Geminga changes during one complete rotation of the neutron star, which takes about 1/4 second.
Top figure: the number of high-energy gamma rays seen by the EGRET instrument on the Compton Gamma Ray Observatory during the neutron star rotation. Although Geminga is particularly bright twice in each rotation, it never completely "turns off."
Lower figure: map of the Geminga region of the sky as the pulsar rotates. Geminga (upper left) changes brightness dramatically, while the Crab pulsar (lower right) appears to remain steady (the Crab rotates much faster than Geminga, so its variation cannot be seen in this set of figures).

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