Photometric Period of XTE J0053-724 (SXP46.6 = 1WGA J0053.8-7226)

ATel #1316; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 30 Nov 2007; 19:36 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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We have analyzed 7 years of OGLE-III photometry for the SMC X-ray pulsar XTE J0053-724 (SXP46.6). The optical counterpart is star 30 of field SMC108.8 in the OGLE-III database. It corresponds to the Be star marked as Star B on the finding chart of Buckley et al. (2001, MNRAS, 320, 281). M. Coe has pointed out on his web site (www.astro.soton.ac.uk/~mjc/smc/Finders/sxp46.6.jpg) that this star is most consistent with the Chandra X-ray position (00:53:53.8; -72:26:35). The I-band light curve shows a downward trend from I = 14.45 to 14.70 over the first 3 years of observation. During that time outbursts of ~0.06 mag are apparent in the raw light curve. In season 4 the source dropped abruptly by ~0.25 mag, and it rose by the same amount during the following season (typical of some Be stars). The outbursts were absent or very weak in seasons 4 through 7. Phase dispersion minimization was used to analyze the first 3 seasons of I data. It reveals two prominent periods at 69.2 +/- 0.3 and 138.4 +/- 0.9 days. The folded light curves show outbursts lasting about 0.2P, but it is unclear which period best fits the data. Laycock et al. (2005, ApJS, 161, 96) determined an X-ray period of 139 days (consistent with our longer period), which they suggested could be the orbital period. Their folded light curve shows strong outbursts at phase 0 and weaker ones near phase 0.5. This suggests the period could be half their value (consistent with our shorter period), if there is variation in the strength of individual outbursts. Galache et al. (2005, ATEL #674) refined the X-ray period to be 137.4 +/- 0.4 days. Presumably their folded light curve also shows secondary outbursts, since they used much of same data as Laycock et al. did. Because the X-ray and optical observations overlap in dates, we were able to determine that the outbursts in both X-ray and I band are approximately coincident in time. The presence of similar periods in both the X-ray and optical data confirms that the optical identification with Star B is correct. With the current I-band data we cannot distinguish between periods of 69.2 and 138.4 days. There is an indication of weak secondary outbursts when the I data are plotted on the longer period, although these are less prominent than those seen in the X-ray data.