Pulsed high-energy γ-radiation from Geminga (1E0630+178)

HALPERN and Holt¹ have recently reported the detection of coherent pulsations with a period of 237 ms from the soft X-ray source 1E0630 +178, which lies in the error box of the γ-ray source known as Geminga (2GC195 + 04). This observation provides compelling evidence that Geminga, an object whose nature has hitherto been mysterious, is an X-ray pulsar. Prompted by this discovery, we have searched the data from EGRET, the Energetic Gamma Ray Experiment Telescope on the Compton Gamma Ray Observatory, for a comparable signal in the γ-radiation from this part of the sky. We now report the detection of pulsed γ-rays, with energy >50MeV, from 1E0630 + 178, confirming the identification of Geminga with this X-ray source. The period derivative, (11.4 +/- 1.7) x 10^-15ss^-1, suggests that Geminga is a nearby, isolated, rotating neutron star with a magnetic field of 1.6 x10¹² gauss, a characteristic age of 3 x 10⁵ yr and a spin-down energy loss rate of 3.5 x 10³⁴ erg s^-1.