The Photometric Period of SXP1062 (CXO J012745.97-733256.5)
ATel #4399; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 19 Sep 2012; 19:15 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)
Subjects: Optical, X-ray, Binary, Neutron Star, Pulsar
The SMC Be/X-ray pulsar SXP1062 was discovered by Henault-Brunet et al. (2012, MNRAS, 420, L13). It is coincident with the Be star 2dFS 3831 and appears to be associated with the young supernova remnant NGC 602. Haberl et al. (2012, A&A 537, L1) have shown the pulsar has a very high spin-down rate of 0.26 s/d. We have examined I-band data of the optical counterpart from the OGLE-IV XROM system (Udalski, 2008, Acta Astron., 58, 187). Two seasons of observations are currently available.
The brightness was at I~14.18 (on the provisional magnitude scale) for 80 days at the beginning of the first season, followed by a sudden brightening of 0.6 mag and an exponential decline lasting about 90 days. In the second season there was a very slow decline in brightness, centered on I~14.15. The shape of the large brightening is very similar to the outbursts present in XROM data for SXP756 (see http://ogle.astrouw.edu.pl/ogle4/xrom/xrom.html ), which is known to have a ~394-d orbital period (Cowley & Schmidtke, 2003, AJ, 126, 2949). Henault-Brunet et al. have suggested the orbital period of SXP1062 to be ~300 d, based on placement of the X-ray pulsar in the Corbet diagram (Corbet et al., 2009, Proc. IAU Symp. 256, 361). However, the OGLE-IV light curve shows only one outburst occurred in two observing seasons, suggesting the orbital period is likely to be >500 d. Further photometry will be needed to define this value.
We have searched for short-period variations in the existing data, both before and after the large outburst. For observations taken between JD 2455414 and 2455494 (season 1), the periodogram shows pronounced periods at 0.9007+-0.0005 and 9.37+-0.05 d, which are aliases of each other. Light curves folded on these periods are sinusoidal with an amplitude of 0.026 mag. From JD 2455699 to 2455993 (season 2), the most prominent periods are at 0.9010+-0.0003 and 9.257+-0.03 d. Although the folded light curves are again sinusoidal, the amplitude is only 0.012 mag. (We note there is an additional alias in both data segments at ~1.12 d, but its weakness in the second periodogram implies it cannot be the true signal.) Because a variable 9-d period is difficult to understand from a physical viewpoint (e.g. it cannot be an orbital signal), we believe the sinusoidal modulations come from non-radial pulsations of the Be star, with a period near 0.9 d. Behavior like this is commonly seen in Be stars and Be/X-ray pulsars.
