RXTE finds a new 723s pulsar in the SMC (SXP723)
ATel #3538; L. J. Townsend (Southampton), M. J. Coe (Southampton), R. H.D. Corbet (UMBC/NASA GSFC), J. L. Galache (CfA), M. P.E. Schurch (UCT)
on 9 Aug 2011; 15:58 UT
Credential Certification: L. J. Townsend (ljt203@soton.ac.uk)
Subjects: X-ray, Binary, Pulsar
An RXTE observation performed as part of our long-term programme to monitor the SMC (see Galache et al., 2008, ApJS, 177, 189 for details) on 2011-06-20 (MJD 55732.22) has revealed highly significant (>4 sigma) pulsations at 723 +/- 2s. This period was detected along with its 2nd and 3rd harmonics at 361.4s and 240.5s respectively. The pointing position of RXTE for the 11ks observation (ObsID: 96037-04-25) was RA = 12.5 degrees, dec = -73.1 degrees (J2000.0). It is not thought that any known pulsar within the 2 degree field of view (FOV) could produce the observed pulsations. Therefore we announce the discovery of a new 723s pulsar in the SMC, which we designate SXP723. A search of archived observations of this position reveal 6 other detections of a periodicity in the range 715-728s above the 99.99% confidence limit in the last 13 years. It is not clear if all of these detections belong to SXP723, though one or more harmonics are seen in nearly all observations suggesting this is highly likely. No orbital information can as yet be derived from the current data set.
Whilst the position of SXP723 is not well constrained, the luminosity lower limit estimated from the pulsed flux measurement is 3 x 10^37 erg/s. This assumes the source is at the centre of the FOV, has a pulsed fraction of 0.1 (which has been estimated from the folded light curve) and is at 60kpc. Unfortunately, because another pulsar (SXP8.80) was also active during this observation we cannot derive spectral information on SXP723. We note that, of the 7 apparent detections of this source, the most recent one is much more significant and the only one in which the fundamental period was the strongest. As such, the pulse profile evolves significantly between single, double and triple peak dominated structure between each detection.
The transient nature and X-ray luminosity suggest this is yet another Be/X-ray binary system in the SMC, although until the position and/or spectral information is known this cannot be confirmed.
