Binary period in Optical counterpart to CXOU J005455.6-724510 = RXJ0054.9-7245 = AX J0054.8-7244 = SXP504
ATel #426; W. R.T. Edge, M. J. Coe, J. L. Galache and V. A. McBride (Southampton University), R. H.D. Corbet (GSFC and USRA), C. B. Markwardt (GSFC and University of Maryland), S. Laycock (CfA), F. E. Marshall (GSFC)
on 24 Feb 2005; 18:17 UT
Credential Certification: W R T Edge (wrte@astro.soton.ac.uk)
Subjects: X-ray, Binary, Transient, Pulsar
A probable binary period has been detected in the optical counterpart to the X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AXJ0054.8-7244 = SXP504 in the Small Magellanic Cloud. This source was detected by Chandra on 04 Jul 2002 (MJD 52459) and first reported in ATEL#217. It was subsequently observed by XMM-Newton on 18 Dec 2003 (MJD 52991) (ATEL#219). The source is coincident with the Optical Gravitational Lensing (OGLE) object numbered 47103 (in Phase II) and 36877 (in Phase III) (Udalski et al., 1998). Several probable optical outburst peaks are visible in these light curves at ~268 day intervals. Timing analysis shows a period of 261 ± 7 days at >99% significance. A search of archival RXTE PCA data for the 504s pulse-period revealed detections at MJDs 50802, 51652, 51870, 52165, 52612 and 52978 which correspond closely with predicted or actual peaks in the optical data. This source is also coincident with MACHO object 207.16245.16 (Alcock et al., 1999) in which there is some evidence for a weak period at around 268 days. All these observations can be described by an ephemeris of T= (MJD 50556 ± 3) + n (268 ± 1.4) where T is the epoch of the outburst and n is the outburst cycle number. Nearly all the X-ray detections are at the predicted optical ephemeris, but because of non-continuous X-ray coverage others may well have been missed. The relationship between this orbital period and the pulse period of 504s is within the normal variance found in the Corbet diagram (Corbet, 1984).
