The recent pulse period evolution of SMC X - 1

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, −Ṗ = (1.18± 0.06) 10−11 s s−1, consistent with the average spin-up rate −Ṗ = 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) s were also detected ∼2 weeks after an X-ray turn-off during an X-ray lowintensity state in October 1991, and the period derivative derived within ∼ 10 days is −Ṗ ' (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 different description of the apparent spin-up behavior.