IGR J015712-7259 (= SXP11.6): Optical Confirmation of the Orbital Period

ATel #5473; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 15 Oct 2013; 19:39 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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The transient X-ray source IGR J015712-7259, first seen in INTEGRAL data and subsequently observed with Swift and RXTE, shows X-ray pulsations with a period of 11.6 s (Coe et al., 2008, ATel #1882; McBride et al., 2010, MNRAS, 403, 709). Using long-term monitoring with the BAT instrument (Swift), Segreto et al. (2013, A&A, 557, A113) announced the discovery of a X-ray period of P=35.6 +/- 0.5 d. This is likely to be the orbital period.

We have examined three seasons of OGLE-IV I-band observations for the optical counterpart, which falls in two Magellanic Bridge fields: MBR102.01 (star # 984) and MBR108.25 (star #11). The XROM light curves (Udalski, 2008, Acta Astron., 58, 187) are similar, but they differ by 0.1-0.2 mag in zero-point level. The brightness was relatively constant at the beginning of season 1, dimming slightly (0.05 mag) at the end. Small outbursts (<0.1 mag) were superimposed throughout. The outbursts disappeared during season 2 when the star faded by ~0.4 mag. The source was at its faintest level in season 3, with relatively large photometric scatter. Overall, the brightness changed by 0.6-0.7 mag from seasons 1 through 3.

The OGLE-IV observations for season 1 from each data set were detrended and then combined. We used phase-dispersion minimization to determine the optical period: P=35.1 +/- 0.1 d, with maximum light at JD 2455379.9 +/- 0.7. The folded light curve shows a narrow, asymmetrical outburst (width ~0.15P), with a sharp rise (~0.1 mag) and gradual decline, that is typical of Be/X-ray binary systems (see URL link for a figure). There is some evidence for a slight level change near phase 0.5 as well as a small dip just prior to the outburst. These features should be monitored with further observations. Data from seasons 2 and 3 were searched, but no significant optical periodicities were found.

Using the X-ray ephemeris of Segreto et al. (2013), we find the X-ray and optical outbursts are nearly coincident, differing by <0.1P in phase. This reinforces the interpretation that they are orbital signatures. The lack of optical outbursts when the source is faint is consistent with the shrinking of circumstellar material surrounding the primary star. The observed spin and orbital periods are surprising similar to those of SXP11.5 (Townsend et al., 2011, MNRAS, 410, 1813) and place the system in the expected region of the Corbet diagram.

OGLE-IV Light Curve for SXP11.6