The Orbital Period of the Be/X-ray Binary SXP6.85 (XMMU J010253.1-724433)
ATel #7498; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 12 May 2015; 21:38 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)
Subjects: Optical, X-ray, Binary
SXP6.85 was identified with a Be star in the Small Magellanic Cloud by Haberl, Pietsch, & Kahabka (2007, ATel #1095). The source shows long-term optical variability of ~0.5 mag with a recurrence time of ~658 d (see Fig. 7 of McGowan et al., 2008, MNRAS, 384, 821). Schmidtke & Cowley (2007, ATel #1181) showed that the source is reddest ~100 d after maximum light. The light curve is similar to the type-3 Be stars discussed by Mennickent et al. (2002, A&A, 393, 887). There have been a variety of orbital periods suggested for this source ranging from 21.9 to 114 d, but little agreement between authors (see summary in Schmidtke, Cowley, & Udalski, 2012, MNRAS, 431, 252).
Nikolajuk, Bozzo, & Ferrigno (2015, ATel #
7481) described a recent X-ray outburst by SXP6.85. We felt it might be worth while to look at the most recent OGLE data and combine them with earlier photometry. Hence, we have examined I-band observations from OGLE II, III, and IV. A pattern of small variations is present at nearly all epochs, especially when the source is bright. After subtracting polynomial fits of the underlying long-term variations in bright states, the detrended data were searched for periodic signals.
Using periodogram analysis, a strong signal was found near P~24.8 d (and its one-day aliases). Because the light curve, folded on this period, is asymmetrical in shape, we used the PDM technique to derive an improved ephemeris of P=24.789+-0.003 d with maximum light at T=HJD2456997.9+-0.8. The resulting light curve, using only the bright states of all OGLE photometry, is shown in the accompanying URL link. The full amplitude is ~0.04 mag, with a slow rise and more rapid decline. Although only bright portions of the long-term light curves were used in this analysis, similar variations are often seen in the rising segments of the light curve. Because the 24.789-d period is stable over many years, this is likely the orbital period of the binary system. Based on this new ephemeris, the two recent X-ray observations occurred at orbital phases 0.84 and 0.04, close to the optical maximum.
OGLE light curve for SXP6.85
