OGLE-III counterpart to SMC X-2 reveals 18.62 day binary period

ATel #1670; M. Schurch (southampton), A. Udalski (Warsaw), M. Coe (Southampton)
on 20 Aug 2008; 19:08 UT
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)

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The OGLE-III optical data of the two possible counterparts to SMC X-2 (SXP2.37) have been extensively analysed. The first 5 years of OGLE-III data for both these stars is presented in Schmidtke el al 2006 (AJ,132,971). They find no significant periodicities in either star and propose the brighter more northern star to be the true counterpart due to the large variations in its light curve. Two further years of OGLE-III data for both stars are now available making a total of 7 years coverage. In order to search for periodicities in these data it is essential to first detrend the data. However, the first three years of data were not included in this analysis because the changes were too rapid to satisfactorily remove. So only the most recent 4 years of data were first detrended by fitting a straight line to each individual year of data to remove the long term global changes. Then the resultant detrended 4 year light curves were then searched for periodicities using Lomb-Scargle analysis. From these data it is clear that the additional two years of data have contributed strongly to a periodicity in the northern counterpart (SMC107.5-25) at 18.62 +/- 0.02 days, with harmonics at 9.31 and 6.21 days. No periodicity is seen in the southern star (SMC107.5-260). Combining this period with the well known pulse period of 2.37s places SMC X-2 on the edge of the distribution of Be/X-ray binaries in the Corbet diagram. We thus propose that the counterpart is the northern star and that 18.6d is the binary period of the Be/X-ray binary system.