The binary period and outburst behaviour of the Small Magellanic Cloud X-ray binary pulsar system SXP504

A probable binary period has been detected in the optical counterpart to the X-ray source CXOUJ005455.6-724510 = RXJ0054.9-7245 = AXJ0054.8-7244 = SXP504 in the Small Magellanic Cloud. This source was detected by Chandra on 2002 July 4 and subsequently observed by XMM-Newton on 2003 December 18. The source is coincident with an Optical Gravitational Lensing object in the light curves of which several optical outburst peaks are visible at similar to 268-d intervals. Timing analysis shows a period of 268.6 +/- 0.1 d at > 99 per cent significance. Archival Rossi X-ray Timing Explorer data for the 504-s pulse-period have revealed detections which correspond closely with predicted or actual peaks in the optical data. The relationship between this orbital period and the pulse period of 504 s is within the normal variance found in the Corbet diagram.