The Optical Period of XMMU J005011.2-730026 (SXP214) from MACHO and OGLE-II Observations

ATel #3305; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University)
on 25 Apr 2011; 21:34 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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Recently, Coe et al. (2011, arXiv:1103.0195v1, MNRAS in press) announced the discovery of 214.05-s pulsations from the transient X-ray source XMMU J005011.2-730026. They identified the optical counterpart with a V~15 star that has weak H-alpha emission. The star is present in the MACHO, OGLE-II, and OGLE-III data bases. From analysis of only OGLE-III observations, these authors found a 4.520-day optical periodicity, which they tentatively attributed to non-radial pulsations of the Be star. We have examined periodograms of the MACHO and OGLE-II observations and find the strongest power in the combined data set occurs at P=0.81813 days (normalized power of 50), which is an alias of the 4.520-day periodicity (power of 15). We also note that the shorter period is more consistent with pulsation periods found in other Be/X-ray systems. Although the 0.81813-day periodicity is stable over ~9 years of observations, the amplitude of the sinusoidal signal is variable. This is particularly evident during seasons 1-3 of OGLE-II, in which the full amplitude doubles from 0.015 to 0.030 mag. As shown in Figure 7 of Coe et al., the amplitude further increases in OGLE-III data.