Source: NASA, Compton Gamma Ray Observatory
In 1974, the SAS-2 satellite conducted a gamma ray survey of the sky. The second brightest was found in the constellation Gemini but was not known to emit any radiation at any other wavelengths. The name Geminga was adopted both because as an appreviation for the "Gemini gamma ray source" and because Geminga signifies "it is not there" in Milanese dialect.
For nearly 20 years, the nature of Geminga was unknown. Then, in March 1991, a periodicity of 0.237 sec was detected by the ROSAT satellite in soft X-ray emission. Therefore, Geminga is most likely a neutron star; for whatever reason, it is not visible as a radio pulsar, perhaps because its beams of radio radiation do not sweep past the Earth.
A comparison of images of the suspected optical counterpart taken over an eight year period shows a proper motion that is consistent with a distance to Geminga of about 100 parsecs. Geminga is believed to have formed in a supernova explosion about 300,000 years ago; this nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System.
``Discovery of Soft X--ray Pulsations from the $\gamma$--ray Source Geminga'', Halpern, J.P. and Holt, S.S. 1992, Nature 357, 222.
``The Proper Motion of Geminga's Optical Counterpart'', Bignami, G.F., Caraveo, P.A. and Mereghetti, S., 1993, Nature 361, 704. (See also the photo on page 695).
``The Geminga Supernova as a Possible Cause of the Local Interstellar Bubble'', Gehrels, N. and Chen, W. 1993, Nature 361, 706.
``Galactic Gamma--Ray Sources'', Bignami, G. F. and Hermsen, W. 1983, Annual Review of Astronomy and Astrophysics 21, 67.
See also the News Notes section of Sky and Telescope 1992, Feb and Aug.
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