The Probable Orbital Period of SXP214

ATel #4936; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 2 Apr 2013; 00:34 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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The X-ray pulsation period of SXP214 was discovered by Coe at al. (2011, MNRAS, 414, 3281) in a transient SMC source (XMMU J005011.2-730026). They identified the optical counterpart as a Be star and determined a photometric period of 4.52 days. They also showed a plot of OGLE-II and -III observations (see their Fig. 7). However, Schmidtke & Cowley (2011, ATel #3305) found the primary period is 0.818 days, a stronger alias of the 4.52-day period. The folded OGLE-II light curve is sinusoidal with an amplitude increasing with time from 0.015 to 0.030 mag. The amplitude further increased to 0.060 mag during the first six seasons of OGLE-III. Throughout OGLE-II and the beginning of OGLE-III the source remained at a mean brightness of I~15.15 mag.

During the last four OGLE-III seasons, SXP214 gradually faded, reaching I~15.6. The source remained faint during the first two OGLE-IV seasons, but began to re-brighten in the most recent data posted on the XROM system (Udalski, 2008, Acta Astron., 58, 187). The character of the light curve completely changed when SXP214 was in its faint state. The 0.818-d optical pulsations disappeared, and a period of 29.91 +/- 0.04 days became apparent. The folded light curve, when the source was faint, shows a broad maximum with a deep eclipse-like feature superimposed (see URL link below). Minimum light occurs at JD 2455806 +/- 1. The shape of this curve light is similar to that of SXP291, as shown in Fig. 21 of Schmidtke et al. (2013, MNRAS, in press, doi: 10.1093/mnras/stt159). Based on the character of the light curve, it is probable that P=29.91 days is the orbital period of SXP214.

OGLE-IV Light Curve for SXP214