High energy gamma rays from the Vela pulsar: Long-term variability and energy distribution

New results on the temporal and spectral characteristics of the high energy (50 MeV to 5 GeV) gamma ray emission from the Vela pulsar are presented. A new sensitive analysis method using maximum likelihood techniques has been applied to the COS-B data. The whole pulsed flux is found to exhibit long term variability. The data show strong evidence for a spectral break at approximately 300 MeV in the energy spectrum of this total emission. Five discrete emission regions within the pulsar lightcurve have been identified, with the spectral characteristics and long term behaviour being different. These results support the idea that various production regions simultaneously exist in the pulsar magnetosphere and that the physical processes generating the gamma rays differ with location.