Observation of Vela X-1 by HAKUCHO

The X-ray pulsar Vela X-1 has been observed by HAKUCHO in two occasions from March 6 to 27, 1979, and from March 5 to 23, 1980. The time variation of the X-ray intensity and the long term variation of pulsar period were analyzed in detail. The X-ray intensity was significantly variable with time, and the correlation between the mean X-ray intensity and spectral hardness ratio was obtained. The good correlation between them existed when the X-ray intensity was lower than a certain value. In the present status of Vela X-1 observation, the pulsar period was 282.801 +- 0.001 sec, and showed increasing trend with time. This indicates the spin-down of the Vela X-1 pulsar. Some spin-down models are proposed. (Yoshimori, M.)