Variations of pulse periods of X-ray pulsars

Among several X-ray pulsars thus far observed by HAKUCHO, complex time variations of pulse periods have been obtained for Cen X-3, Vela X-1 AO535+26, and GX301-2, which are known to be members of binary systems of finite eccentricities. The rates of long-term pulse period change for these pulsars are not constant, if incorporated with earlier observations, and sometimes change their signs. For Vela X-1 the rates of period change were obtained for 11 data sets, each extending over about one orbital periods. A large variation of pulse period from one orbital period to another indicates that a long-term period variation results from an average of short-term period fluctuations. The mechanism of period fluctuations is discussed in connection with the matter accretion fed by stellar wind.