SXP 1062: A Refined Orbital Period and Changes in Non-radial Pulsations

ATel #12890; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 26 Jun 2019; 01:41 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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SXP 1062 was discovered by Henault-Brunet et al. (2012, MNRAS, 420, L13) and appears to be associated with supernova remnant NGC 602. This Be/X-ray pulsar is one of only a few such systems with a very long orbital period. Schmidtke, Cowley, and Udalski (2012a, ATel #4596) used OGLE-IV data to derive a probable orbital period of 656 +/- 2 d by matching declining portions of 2 optical outbursts. More recent I-band data show 2 additional outbursts in Seasons 5 and 8 (see http://ogle.astrouw.edu.pl/ogle4/xrom/sxp1062.html ), while another outburst was missed completely as it fell between Seasons 6 and 7. Because the outburst peaks are narrow and the data spacing can be several days apart, the peaks are under-sampled. In particular, the peak brightness in Seasons 3 and 5 most likely occurred during large gaps (14 and 35 d, respectively) in the observing sequence. Therefore, in order to refine the orbital period we used the overall profile to align all 4 observed outbursts, noting that the rate of decline is extremely similar from one epoch to another. A plot of the folded light curve is shown in http://www.public.asu.edu/~atpcs/SXP/SXP1062_OGLEIV_outburst.pdf . We derive a value of P(orb)=656.5 +/- 0.5 d, with peak brightness occurring at JD2455500.0 +/- 0.5. The next optical outburst should be visible later this year, peaking on 2019 October 26.

It would be interesting to further analyze the coincidence between X-ray and optical outbursts as suggested by Gonzalez-Galan et al. (2018, MNRAS, 475, 2809), but more complete data sampling is needed. Using the refined optical ephemeris given above, we note the brightest Swift/XRT observations near the Season 3 OGLE outburst occur at optical phase ~0.085 (see Fig. 1 of Sturm et al. (2013, A&A, 556, A139)). Hence, the X-ray and optical outbursts do not appear to be coincident.

In an examination of data between outbursts Schmidtke, Cowley, and Udalski (2012b, ATel #4399) found low-amplitude optical variations near 0.901 d, which are likely due to non-radial pulsations (NRP) of the Be star. We have now examined all available OGLE-IV data and discovered significant changes to the character of these NRP. Excluding data taken near outbursts, we find the 0.901-d pulsations in Seasons 1 and 2 are also present in Season 4. There are insufficient data in Season 3 to detect short-period variations. No NRP were detected in Seasons 5-8. However, in Season 9 new pulsations with P=0.8313 +/- 0.0003 d are present. A comparison of periodograms and NRP light curves from different epochs is shown in http://www.public.asu.edu/~atpcs/SXP/SXP1062_OGLEIV_NRP.pdf . This behavior of optical pulsations, in which periods change and/or disappear, is found in many Be/X-ray systems.