OBSERVATION OF THE VELA PULSAR, PSR-0833-45, AT 0.2-6.0 MEV WITH THE FIGARO-II EXPERIMENT

Results are reported of a 7.75 hr observation of PSR 0833-45 in Vela with FIGARO II (French Italian GAmma Ray Observatory) performed during a balloon flight from Charleville (Queensland, Australia) on 1988 November 25. Two sigma upper limits for the pulsar flux are given in the gamma-ray energy range 0.2-6.0 MeV (at 1 MeV: 7 × 10-5 photons cm-2 s-1 MeV-1, at 2 MeV: 4.5 × 10-5 photons cm-2 s-1 MeV-1). The values are significantly lower than the measured flux obtained by the Riverside experiment as reported by Tümay Tümer et al. in 1984, 31 days after a large glitch in the pulsar period. The FIGARO data also show a departure from the extrapolation below 50 MeV of the time-averaged flux measured by the COS B satellite, suggesting the existence of a break in the "quiescent" energy spectrum for values at or above 5-10 MeV. Implications on the emission mechanism are briefly discussed.