SXP 182: The Unusual Light Curve and Confirmation of the Orbital Period

ATel #16330; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 10 Nov 2023; 19:29 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

Tweet

Coe et al. (ATel #16321) announced the detection of X-ray pulsations (P=182 s) in Swift J010902.6-723710 and identified the likely optical counterpart (a Be star) in the OGLE-IV data base. Hence, this newly found SMC Be/X-ray pulsar has the designation SXP 182. Coe et al. also reported that I-band data (2010 to present) show an orbital period of 60.670 d with strong eclipses.

We have examined the historical behavior of SXP 182 using archival data from Phase II (1997-2000) of the OGLE program. The source is coincident with star smc_sc11_i_107571. The I-band light curve is shown in the top panel of the accompanying figure. The plot reveals a series of prominent dips (presumably the eclipses found in more recent data by Coe et al.) that are superimposed on a system brightness that slowly varies between I~16.2 and I~16.3.

Long-term trends in individual observing seasons were removed by subtracting a quadratic fit to the non-dipping points. A phase dispersion minimization (PDM) plot of the detrended I* data, covering trial periods between 10 and 1000 d, is shown in the middle panel of the figure. The period with least variance is 60.66+/-0.18 d, consistent with the period given by Coe et al. We also calculated periodograms of data from the flat portion of the I* light curve to search for non-radial pulsations. No significant power was found for periods in the range 0.2 to 5.0 d.

The light curve of I* data folded on the adopted ephemeris is shown in the bottom panel, where T_0 is set to the time of the faintest observation (JD 2450762.60). The brightness is relatively flat between phases 0.05-0.70, after which the system brightens slightly. The brightening is interrupted by two distinct, but overlapping, dimmings. The first is broad (~9 d, or 0.15P) and shallow (0.1 mag), while the second is very sharp (~1.5 d, or 0.025P) and much deeper (0.3 mag). The centers of these dips are offset in phase by 0.1P.

The broad dip in SXP 182 is like that found in other Be/X-ray pulsars, for example SXP 325/327 (see Fig. 22 of Schmidtke et al. (2013, MNRAS, 431, 252)). The narrow eclipse, however, is present in only a few such systems. In the light curve for SXP 65.8, Schmidtke et al. (ATel #13033) found a brief occultation (<0.01P), which may be caused by a transit of the neutron star and its possible accretion disk in front of the Be star. We note the occultation in that system lies 0.7P after maximum light, which is very different than the phasing of the narrow dip present in SXP 182. Multicolor photometry and spectroscopy of SXP 182 through the entire orbit may help determine the nature of the broad minimum and the source of the deep eclipse.

The OGLE-II Light Curve of SXP 182