Spin-down episode of the X-ray pulsar Vela X-1

Abstract

Since the discovery1 of a 283-s pulsation of an X-ray binary Vela X-1, several observations2–8 between 1975 and 1978 have revealed that its pulse period P was decreasing with an average value of Ṗ/P ≍ −1.5 × 10−4 yr−1. The long-term spin-up which is considered to be caused by the angular torque of accreting matter, is a common feature of X-ray pulsars. The pulse periods of Vela X-1 were measured by the Japanese satellite Hakucho in March 1979 and March 1980, and the period in the latter was found to be significantly greater than in the earlier period despite the general spin-up between early 1976 and early 1979. Moreover, we have measured for the first time the spin-down rate Ṗ/P = (2.6 ± 0.2)× 10−3 yr−1 in March 1980. This spin-down phenomenon is of great importance, as it is considered to be a manifestation of the dynamical behaviour caused by the internal structure of a neutron star.