1999A&A...345..117K

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)s, 0.70706 (±0.00001)s, and 0.706707 (±0.00001)s 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, -{dot}(P)=(1.18±0.06)x10^–11s/s, consistent with the average spin-up rate -{dot}(P)=1.2x10^–11s/s determined from previous measurements indicating that the stable spin-up has continued. Pulsations with a period of 0.709103 (±0.000003)s 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 ∼10days is -{dot}(P)≃(1.1±0.7)x10^–11s/s. 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.