Ia. Grenier et al., GAMMA-RAYS FROM THE GEMINGA PULSAR - VARIATIONS WITH TIME AND PHASE, The Astrophysical journal. Supplement series, 90(2), 1994, pp. 813-816
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 a
nalysed 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 reg
ions between the two main peaks. Significant spectral differences with
phase have been found; the emission from the first peak is slightly s
ofter than from the second one and the Interpeak 2 radiation is extrem
ely soft. The first peak emission remained stable within the statistic
s over 7 years while the flux from the other phase intervals changed s
ignificantly with time. The phase dependence of the variability implie
s that all the observed source emission should be pulsed and that it c
onsists of four discrete beams with different apertures and spectra. T
he beams characteristics are strikingly similar to those of four gamma
-ray beams generated by the Vela pulsar.