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A Transient 7.92s X-ray Pulsar in the SMC (SXP 7.92)

ATEL # 1600; R.H.D. Corbet (UMBC & NASA GSFC), M.J. Coe (Southampton), F.E. Marshall (NASA GSFC), V.A. McBride (Southampton), M.P.E. Schurch (Southampton)
on 2 Jul 2008; 22:15 UT
Password Certification: Robin Corbet (

Subjects: X-ray, Binaries, Neutron Stars, Transients, Variables, Stars

Observations of the Small Magellanic Cloud with the Proportional Counter Array on board the Rossi X-ray Timing Explorer have shown the presence of a pulsar with a period of 7.92 s during three consecutive observations performed on 2008-06-13, 2008-06-20, and 2008-06-28 with durations of 10ks in all cases. During the last two observations pulsations were also detected at 46.4s from the previously known source SXP 46.6 (= SXP 46.4, Galache et al. 2008, ApJS, 177, 189). On 2008-06-13 the pulsed semi-amplitude was 0.76 +/-0.08 counts/s/Proportional Counter Unit, equivalent to 0.36 mCrab, and the barycentered pulse period was 7.9183 +/- 0.0003 s.

The PCA observations were centered on RA = 14.3 deg., -72.3 deg. and the PCA has a field of view of 2 degrees FWZI. The PCA field of view contains two known X-ray pulsars with similar periods to the detected source: SMC X-3 with a period near 7.78s, and the Anomalous X-ray Pulsar SXP 8.02 (= CXOU J010043.1-721134; Tiengo et al. 2008, ApJ, 680, L133). These sources are 0.41 and 0.29 degrees from the center of the PCA FOV respectively.

A target of opportunity observation was made using the Swift XRT on 2008-06-29 centered on RA = 13.318 deg., dec = -72.407 deg., with a duration of 1.8 ks to determine whether SMC X-3 was active. SMC X-3 was not detected and the only strong source seen in the Swift 23.6 x 23.6 arc minute FOV was SXP 46.6. In order for SXP 8.02, which was not inside the Swift FOV, to be the source of the 7.92s pulsations it would have to have exhibited significant spin-up, behavior that is not expected from an AXP. As the XRT FOV is much smaller than that of the PCA, the Swift non-detection of the 7.92s source does not necessarily mean that this source was not active at the time of the Swift observation.

We therefore propose that the 7.92s source ("SXP 7.92") is a previously unknown transient X-ray pulsar, possibly a Be star system undergoing a large amplitude "Type II" outburst. RXTE observations of this region of the SMC will continue.

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