Discovery of soft X-ray pulsations from the γ-ray source Geminga

THE nature of the γ-ray source 'Geminga' (2CG195 + 04) is a problem of considerable importance in high-energy astrophysics. First discovered in 1972 by the SAS-2 satellite¹, Geminga emits virtually all its power at energies above 50 MeV, and at energies above 100 MeV is the second brightest source in the γ-ray sky survey made by the Cos-B satellite². It eluded identification at all other wavelengths until the Einstein Observatory found an unusual soft X-ray source, 1E0630 + 178, in its error box³. This source also has a claimed twenty-fifth magnitude optical counterpart^4-6. This distinctive set of properties is reminiscent of the Vela pulsar, except for the absence of radio emission⁷ or a synchroton nebula³. We have made a more sensitive soft X-ray observation of the Geminga field using Rosat, and have detected coherent pulsations from 1E0630+178 at a period of 0.237s. This result confirms suggestions^3-6,8,9 that Geminga is, like Vela, a γ-ray pulsar. We speculate that Geminga is somewhat the older of the two. With this discovery we consider the mystery of Geminga largely solved.