New orbital period, plus clarification on pulse periods, for X-ray pulsars in the SMC

ATel #674; J. L. Galache (Southampton University), R. H.D. Corbet (GSFC and USRA), M. J. Coe (Southampton University), S. Laycock (CfA), C. B. Markwardt (GSFC & University of Maryland) and F. E. Marshall (GSFC)
on 13 Dec 2005; 16:30 UT
Credential Certification: Jose L. Galache (jlg@astro.soton.ac.uk)

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The Small Magellanic Cloud pulsar SXP169 (XTE J0054-720, RX J0052.9-7158) was discovered by Lochner et al. (1997, IAUC # 6814 ). SXP165 was reported by Corbet et al. (2004, AIP Conf. Proc. 714, 337). Using data from the weekly monitoring campaign of the SMC carried out with the RXTE Proportional Counter Array, a search was carried out for pulsations around a central period of 167.35s, and encompassing both pulsars. We find that SXP169 and SXP165 can be identified as a single pulsar that has undergone spin-up with a change in pulse period of ~3s over 7.5 years. The implied luminosity is Lx ~ 1.7e36 erg/s, consistent with that expected from Type I outbursts. The resulting light curve shows regular outbursts at a period of 68.6 +/- 0.2 days, which we propose to be the orbital period. The ephemeris we derive are:

T = MJD 52172 +/- 2 + n x 68.6 +/- 0.2

where T is the epoch of outburst and n is the outburst cycle number.

We suggest keeping the SXP169 name for historical reasons.

The SMC pulsar SXP46.6 (1WGA 0053.8-7226, XTE J0053-724) was reported on January 6, 1998 (Corbet et al. 1998, IAUC # 6803 ); its orbital period was later proposed as 139 +/- 6 days (Laycock et al. 2005, ApJSup 161, 96). SXP46.4 (XTE SMC 46.4s Pulsar) was reported by Corbet et al (2002, IAUC # 7932 ). Employing the techniques outlined above on the same archival XTE data we find these two sources to be one single pulsar that has been slowly spinning up for the past 9 years (~0.25s in total). This change in period implies a luminosity of Lx ~ 1.8e36 erg/s, which is consistent with the regular Type I outbursts it displays. Timing analysis provides a period of 137.4 +/- 0.4 days, which is consistent with the value of Laycock et al. We derive the following ephemeris:

T = MJD 52156.6 +/- 1.4 + n x 137.4 +/- 0.4