Optical and X-ray variability of two Small Magellanic Cloud X-ray binary pulsars - SXP46.6 and SXP6.85

Abstract

We present long-term optical and RXTE data of two X-ray binary pulsars in the Small Magellanic Cloud, SXP46.6 and SXP6.85. The optical light curves of both sources show substantial (?0.5-0.8 mag) changes over the time-span of the observations. While the optical data for SXP6.85 do not reveal any periodic behaviour, by detrending the optical measurements for SXP46.6 we find an orbital period of ?137 d, consistent with results from the X-ray data. The detection of Type I X-ray outbursts from SXP46.6, combined with the fact that we also see optical outbursts at these times, implies that SXP46.6 is a high orbital eccentricity system. Using contemporaneous optical spectra of SXP46.6, we find that the equivalent width of the Ha emission line changes over time indicating that the size of the circumstellar disc varies. By studying the history of the colour variations for SXP6.85, we find that the source gets redder as it brightens which can also be attributed to changes in the circumstellar disc. We do not find any correlation between the X-ray and optical data for SXP6.85. The results for SXP6.85 suggest that it is a low-eccentricity binary and that the optical modulations are due to the Be phenomenon.