The observations reported in this paper have been performed with the PSPC and HRI detectors of the ROSAT satellite (Trümper 1983). In Table 1 a log of the observations analysed in this work is given. The observations were centered on SMC X-1. The October-1991 observations and the June-1993 observation have been retrieved from the public ROSAT archive in November 1997. The ROSAT HRI observations were made by the first author of this paper. The recently discovered transient RX J0117.6-7330 located southeast of SMC X-1 (Clark, Remillard & Woo 1996) has not been detected in these observations.
Table 1. ROSAT observations of SMC X-1
2.1. High- and low-intensity X-ray states
SMC X-1 has been observed during the ROSAT all-sky survey (Kahabka & Pietsch 1996). The source was in a low-intensity state in a first pointed observation, and was found in a high state in a ROSAT PSPC pointing in October 1991, preceding the low-intensity state by 12 days. This limits the duration of this specific X-ray turn-off phase to less than 2 weeks. In this paper the pulse periods of SMC X-1 during three X-ray high states observed with the ROSAT HRI 4, 5.5, and 6.5 years after the 1991 high state observation are reported. Pulse period determinations from the ROSAT observations are summarized in Table 1.
2.2. Pulse periods and period derivatives
We have searched for the pulse period in the data from four high state data and from the low state data following the first high state. In the present analysis the event times have been projected from the spacecraft to the solar-system barycenter with standard EXSAS software employed (Zimmermann et al. 1994). They have also been corrected for arrival time delays in the binary orbit by use of the ephemeris and the orbital solution given in Levine et al. (1993). This takes into account the change in the length of the orbital period and of the mid eclipse ephemeris due to orbital decay (Wojdowski et al. 1998). Period uncertainties have been determined from the relation , with the exposure time given in Table 1 and the number of phase bins .
Periods of P=0.709113 ( 0.000003) s (=3000, 9 degrees of freedom), P=0.708600 ( 0.000002) s (=980, 9 degrees of freedom), P=0.70769 ( 0.00006) s (=56, 9 degrees of freedom), P=0.707065 ( 0.000010) s (=113, 9 degrees of freedom) and P=0.70670 ( 0.00002) s (=250, 9 degrees of freedom) have been obtained during the October-1991, June-1993, December-1995, May-1997 and the March-1998 high-intensity states, respectively (cf. Fig. 1 and Table 1). From the October-1991 to the December-1995 high state a change in pulse period with a mean and from the June-1993 to the March-1998 high state a mean are derived.
The period derivative derived over the 6 year interval from October-1991 to Mar-1998 is consistent with the mean derived from previous observations (Levine et al. 1993). The evolution of the pulse period with Julian date using data from Henry & Schreier (1977), Kunz et al. (1993), Levine et al. (1993), Wojdowski et al. (1998) and the results from this work is given in Fig. 2. Also shown are the residuals compared to a linear best-fit with a . It is very evident that the pulse period of SMC X-1 undergoes a period walk with a time scale of a few 1000 days (a few years). But the amplitude of this period walk is small (). It may be suspected that somewhere at the end of 1994 the "positive" deviation from the mean was largest (cf. Fig. 2). After this time the mean may have increased. It is not clear in which way the period walk continues. An explanation of this period walk in terms of a "free" precessing neutron star is unlikely (cf. Bisnovatyi-Kogan & Kahabka 1993).
A pulse period search has also been performed in an observation during a low-intensity state performed in the time interval 16 to 19 Oct-1991 (cf. Table 1 and Fig. 1). A period of P=0.709103 0.000003 s (=71, 9 degrees of freedom) has been determined (cf. Fig. 1). This period is close to the period determined during the 7-Oct to 8-Oct-1991 high-intensity state and consistent with the long-term negative value. The significance of this period is . The period derivative between the high and low state in Oct-1991 (with a time interval days) is .
Table 2. Periods and period residuals from least-square linear fit for observations of SMC X-1. The periods derived from the Einstein and HEXE observations have not been corrected for orbital decay.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999