VELA X-1

M. Oda and the Hakucho Team, Institute of Space and Aeronautical Science, University of Tokyo, telex: "Observations of the x-ray pulsar Vela X-1 (= 4U 0900-40) with the Hakucho satellite indicate a high rate of spin-down, as data obtained 1980 Dec.-1981 Jan. reveal a pulse period of 282s.87 +/- 0s.O2 (preliminary). In 1979 Mar., the pulsar's heliocentric pulse period (282s.745 +/- 0s.006) was consistent with its long-term trend of decreasing period of ~ 0.04 s/yr (cf. Rappaport et al. 1980, Ap. J. 235, 570). However, by 1980 Mar. the period had increased to 282s.7874 +/- 0s.0014. In view of this peculiar behavior, optical data of its companion, HD 77581, are very important. Spectroscopic data have not been published since those given by Hutchings (1974, Ap.J. 192, 685); immediate spectroscopic observations of HD 77581 are strongly urged. Those who have unpublished spectroscopic data of this object are requested to contact N. Oda, Institute of Space and Aeronautical Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, Japan (telex Japan 24550 SPACETKY J); or J. Jugaku, Tokyo Observatory, 2-21-1 Ohsawa, Mitaka City, Tokyo, Japan."