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 CXOU J005455.6-724510 = RX J0054.9-7245 = AX J0054.8-7244 = 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 ~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.