The recent pulse period evolution of SMC X-1

We report observations of SMC X-l in three new high-intensity states with R OSAT HRI in December 1995, May 1997, and March 1998 in which pulsations wit h a period of 0.70769 (+/- 0.00006) a, 0.70706 (+/- 0.00001) s, and 0.70670 7 (+/- 0.00001) s respectively were detected. Combining the pulse periods f rom observations with ROSAT PSPC in high-intensity states in October 1991 a nd March 1998, respectively, we obtain the spin-up rate of the pulsar in re cent 6.5 years, -(P) over dot = (1.18 +/- 0.06) 10(-11) s s(-1), consistent with the average spin-up rate -(P) over dot = 1.2 10(-11) s s(-1) determin ed from previous measurements indicating that the stable spin-up has contin ued. Pulsations with a period of 0.709103 (+/- 0.000003) s were also detect ed similar to 2 weeks after an X-ray turn-off during an X-ray low-intensity state in October 1991, and the period derivative derived within similar to 10 days is -(P) over dot similar or equal to (1.1 +/- 0.7) 10(-11) ss(-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 di fferent description of the apparent spin-up behavior.