Nonradial Pulsations in SXP202A
ATel #9320; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 4 Aug 2016; 21:14 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)
Subjects: Optical, X-ray, Binary
Referred to by ATel #: 9348
The X-ray pulsar SXP202A (00:59:21, -72:23:17) was discovered by Majid et al. (2004, ApJ 609, 133) but still little is known about the system's basic properties. Galache et al. (2008, ApJS, 177, 189), linking the times of 6 X-ray outbursts, noted a possible ~91-d periodocity. Optical studies using MACHO, OGLE-II, and OGLE-III data, however, have not confirmed this period (see summary by Schmidtke et al. 2013, MNRAS, 431, 252). The only reported optical periodicity is a weak ~71.9-d signal that was suggested by Rajoelimanana et al. (2011, MNRAS, 413, 1600).
Prompted by the recent announcement (Coe et al., ATel #9307) of the detection of a very bright X-ray outburst from SXP202A, we have undertaken a new period analysis of the optical counterpart using OGLE-IV photometry. Six full seasons of I-band measurements are available. The mean brightness is I~14.5, with long-term variations of about 0.1 mag. Visual examination of seasons 2-6 shows slightly enhanced scatter and some minor brightenings, suggesting there might be an underlying periodicity. We removed the long-term variations by subtracting a quadratic fit to the observations. A periodogram of flattened data shows a prominent peak near 36.2 d and its aliases at 1.025, 0.971, 0.506 d, etc. A sinewave fit to each of these reveals a low-amplitude (~0.012 mag) signal. Examination of individual seasons shows small changes in the mostly likely period (from 0.971 to 0.994 d). Although the power at 36.2 d is slightly stronger than that at 0.971 d (37.7 vs. 31.6) in the combined data set, we argue that the shorter period (formally, P=0.9706 +/- 0.0006 d) is preferred and represents nonradial pulsations (NRPs) of the primary star. The low amplitude and sinusoidal shape of the folded light curve is most consistent with pulsations. Furthermore, Schmidtke et al. have shown that small variations in NRP signals over multiple observing seasons result in artificially enhanced power at the longest period. We note the 71.9-d periodocity suggested by Rajoelimanana et al. is approximately double the 36.2-d period present in the most recent OGLE observations.
A periodogram of OGLE-IV data from seasons 2-6 and a light curve of these data folded on the 0.9706-d period is given at the URL below.
OGLE-IV light curve for SXP202A