Abstract
The high-mass X-ray binary Vela X-1 has been observed using the Narrabri 300 GeV gamma-ray telescope. The results show evidence for pulsed very high energy (VHE) gamma-ray emission, with a conservative overall Rayleigh chance probability of 0.00011. The long-term period derivative during our observations of 1986-1988 was negative, indicating a spin-up similar to that noted between 1975 and 1979. The time-averaged luminosity in VHE gamma rays is estimated to be about 3.6 x 10 to the 34th ergs/s.
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