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 final COS-B dataset. The whole pulsed flux is found to exhibit long term variability. The energy spectrum of this total emission cannot be described by a single power law and the data show strong evidence for a spectral break at approximately 300 MeV. Five discrete emission regions within the pulsar light-curve have been identified, with the spectral characteristics and long-term behaviour being different. These results support the idea that various source regions simultaneously exist in the pulsar magnetosphere and that the physical processes generating the gamma rays in these sites differ with the location.