RXTE Detection of an 85.4 s X-ray Pulsar in the Direction of the SMC (SXP 85.4)

ATel #2813; R. H.D. Corbet (UMBC/NASA GSFC), E. S. Bartlett, M. J. Coe, V. A. McBride, L. J. Townsend (Southampton), M. P.E. Schurch (UCT), F. E. Marshall (NASA GSFC)
on 24 Aug 2010; 14:26 UT
Credential Certification: Robin Corbet (Robin.Corbet@nasa.gov)

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RXTE PCA observations in the direction of the Small Magellanic Cloud made on 2010-08-16 and 2010-08-21, as part of a long-term monitoring program, show the presence of pulsations with a period of 85.4 +/- 0.1 s. The modulation semi-amplitude on 2010-08-21 was approximately 0.15 cts/s/PCU. This period does not coincide with the fundamental period of any known pulsar in the PCA field of view (a compilation of SMC pulsar periods is given at http://www.astro.soton.ac.uk/~mjc/smc/ ).

Although 85.4 s is close to the 2nd harmonics of SXP 169 and SXP 172, these objects do not appear likely to be the source of the 85.4 s pulsations. Recent measurements of SXP 169 have shown a spin-up trend that has produced a period significantly different from twice 85.4 s (e.g. Galache et al. 2008, ApJS, 177, 189) and the detection times are not consistent with the 68.5 day orbital period of SXP 169. The collimator response to SXP 172 is only ~5%, making this source unlikely to be the origin of the pulsations unless it is exceptionally bright. In addition, we see no evidence for modulation near twice 85.4 s in the power spectra of the light curves. This therefore appears likely to be a new source, which we call SXP 85.4 following the convention of Coe et al. (2005, MNRAS, 356, 502). In addition to modulation at 85.4 s, the power spectra also show the presence of the 2nd harmonic of this period at 42.7 s. A search for previous detections of this pulsar in earlier observations of this location shows the presence of modulation at 85.4 s and 42.7 s on 2010-05-25. This is again not consistent with the 68.5 day orbital period of SXP 169.

Because the RXTE PCA is not an imaging instrument, the location of this pulsar cannot yet be determined to better than within the 2 degree FWZI FOV of the PCA centered on R.A. = 00h 57m 12s, decl. = -72o 18' 00" (J2000). However, this pulsar has not been detected in a second regularly monitored region of the SMC centered on R.A. = 00h 50m 00s, decl. = -73o 06' 00" (J2000).

The transient nature of this pulsar suggests that, in common with all other X-ray pulsars in the SMC with known optical counterparts, except for SMC X-1, it is a Be/neutron star high-mass X-ray binary. The direction of this pulsar will continue to be monitored with the RXTE PCA.