TEV GAMMA-RAY PROPERTIES OF VELA X-1

Evidence is presented, from 5 years of observation, that Vela X-1 is a persistent emitter of pulsed TeV gamma-rays at the expected X-ray period. The significance of this effect is 99.3%, and an average, constant, pulsed signal of 2.3% of the cosmic-ray background was detected from 1986 to 1990. The light curve is a mixture of the fundamental period (283 s) and the second harmonic. No orbital modulation was detected, and there are indications of gamma-ray emission during X-ray eclipse. Enhancements in the signal strength of the order of seven and a duration of up to eight pulsar rotations occurred. These enhancements occurred on average once every 12.9 hr. We conclude that the TeV gamma-ray emission mimics the X-ray behavior of the source. The gamma-rays are probably produced during periods of higher accretion when the density of the accreting material produces a sufficiently dense target for the high-energy protons produced at the poles of the neutron star. Such a scenario can also explain the emission during X-ray eclipse.