The recent pulse period evolution of SMC X-1

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We report observations of SMC X-1 in three new high-intensity states with ROSAT HRI in December 1995, May 1997, and March 1998 in which pulsations with a period of 0.70769+/-0.00006 sec, 0.70706+/-0.00001 sec, and 0.706707+/-0.00001 sec respectively were detected. Combining the pulse periods from observations with ROSAT PSPC in high-intensity states in October 1991 and March 1998, respectively, we obtain the spin-up rate of the pulsar in recent 6.5 years, -dP/dt = 1.18+/-0.06 10-11 s s-1, consistent with the average spin-up rate -dP/dt = 1.2 10-11 s s-1 determined from previous measurements indicating that the stable spin-up has continued. Pulsations with a period of 0.709103+/-0.000003 sec were also detected 2 weeks after an X-ray turn-off during an X-ray low-intensity state in October 1991, and the period derivative derived within 10 days is -dP/dt (1.1+/-0.7) 10-11 s s-1. This is consistent with a constant accretion torque in sign and magnitude. The magnitude of the magnetic moment of the pulsar is discussed based on different description of the apparent spin-up behavior.