Gamma Rays from the Geminga Pulsar: Variations with Time and Phase

Pulsed gamma radiation from the Geminga pulsar was recorded by the COS B satellite from 50 MeV to 5 GeV between 1975 and 1982. It has been analyzed to derive the source spectral properties as a function of time and phase. The two main peaks are separated by 0.50 +/- 0.01 in phase. Significant pulsed emission has also been detected in both phase regions between the two main peaks. Significant spectral differences with phase have been found; the emission from the first peak is slightly softer than from the second one and the interpeak 2 radiation is extremely soft. The first peak emission remained stable within the statistics over 7 years while the flux from the other phase intervals changed significantly with time. The phase dependence of the variability implies that all the observed source emission should be pulsed and that it consists of four discrete beams with different apertures and spectra. The beams characteristics are strikingly similar to those of four gamma-ray beams generated by the Vela pulsar.