The Orbital Period of the SMC X-ray Pulsar 2E 0050.1-7247

ATel #277; R. H.D. Corbet (NASA/GSFC and USRA), M. J. Coe, W. R.T. Edge (Southampton University), S. Laycock (CfA), C. B. Markwardt (NASA/GSFC and University of Maryland), F. E. Marshall (NASA/GSFC)
on 12 May 2004; 21:12 UT
Credential Certification: Robin Corbet (corbet@gsfc.nasa.gov)

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Monitoring of the Small Magellanic Cloud with the RXTE Proportional Counter Array has revealed a periodicity in the X-ray pulsar 2E 0050.1-7247 (Israel et al. 1997, ApJ, 484, L141). An extended outburst from this source was detected on 2003 January 13 and weekly monitoring showed that this particular outburst continued until 2003 February 18. The source was subsequently detected less frequently in continued weekly monitoring and detections of 2E 0050.1-7247, excluding the early 2003 outburst, occurred on the following MJDs:
52849, 52904, 52933, 52940, 52961, 52993, 53019, 53044, 53068, 53075, 53102, 53128, 53136.

These detections show a pattern which is well described by an ephemeris of:
T = MJD 52850 +/- 2 + n x 28.0 +/- 0.3
where T is the epoch of outburst and
n is the outburst cycle number.

This regular pattern of outbursts strongly suggests that they show the orbital period of this system. A plot showing the difference between predicted and observed times of source detection is available at: http://lheawww.gsfc.nasa.gov/users/corbet/pulsars/2e0050.1_omc.ps

We also examined the MACHO light curves of several stars within the ROSAT error circle and found no modulation at the X-ray period for any object.