Time Variation of the Pulse Period of VELA-X-1

X-ray pulsar Vela X-l was observed with the X-ray astronomy satellite HAKUCHO in five occasions between March 1979 and March 1981. An increase of the pulsation period at an average rate of P/P ≃ 3.0 × 10^-4 yr^-1 was observed over the time span of two years. Besides, variations of the pulse period in the time scale of ∼10 days were resolved in superposition on the secular spin-down trend. The observed rate of change P ∼- 3 × 10^-8, for both spin-up and spin-down, is an order of magnitude greater than the secular spin-down rate.