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Astron. Astrophys. 345, 117-120 (1999)

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The recent pulse period evolution of SMC X-1

P. Kahabka 1 and X.-D. Li 2

1 Astronomical Institute and Center for High Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Department of Astronomy, Nanjing University, Nanjing 210093, P.R. China (lixd@netra.nju.edu.cn)

Received 23 December 1997 / Accepted 4 February 1999


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 ([FORMULA] 0.00006) s, 0.70706 ([FORMULA] 0.00001) s, and 0.706707 ([FORMULA] 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, [FORMULA], consistent with the average spin-up rate [FORMULA] determined from previous measurements indicating that the stable spin-up has continued. Pulsations with a period of 0.709103 ([FORMULA] 0.000003) s were also detected [FORMULA]2 weeks after an X-ray turn-off during an X-ray low-intensity state in October 1991, and the period derivative derived within [FORMULA] days is [FORMULA]. 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.

Key words: accretion, accretion disks – stars: individual: SMC X-1 (SMC) – stars: magnetic fields – stars: neutron – stars: rotation – X-rays: stars

Send offprint requests to: ptk@astro.uva.nl

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999