Non-radial Optical Pulsations in SXP 175 (RX J0101.8-7223)

ATel #13128; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 24 Sep 2019; 22:14 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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SXP 175 was discovered as an X-ray pulsar by Townsend et al. (ATel #3311). The source's position was coincident with RX J0101.8-7223, which had been identified with the emission-line star MA[93] 1288. Townsend et al. (2013, MNRAS, 433, 23) confirmed the optical counterpart is an early Be star. Using data from MACHO, OGLE II, III, and 2 seasons of IV they derived a period of ~87 d, which they proposed to be the orbital period. Now that 7 additional seasons of OGLE IV data are available (http://ogle.astrouw.edu.pl/ogle4/xrom/sxp175.html), we have studied the low amplitude optical variations more closely by breaking the data into sets which include a few seasons each. We find that there are changing periodicities near 0.99 or 1.01 d for which the 87-d period is an alias. Schmidtke, Cowley, & Udalski (2013, MNRAS, 431, 252, see Appendix) demonstrated that when short period non-radial pulsations (NRP) are both present and variable, then excess power will be detected at the long period alias rather than from the NRP. We believe this is the case for SXP 175.

In order to remove long term changes in system brightness, we detrended all of the OGLE measurements by subtracting a linear fit to individual seasons of observations prior to period analysis. From the 4 seasons of OGLE II data as well as the first 5 seasons of OGLE III, no significant periods were found. In Fig. 1 (http://www.public.asu.edu/~atpcs/SXP/SXP175_OGLEIV_Periodograms.pdf) we show periodograms for the remaining OGLE III seasons and the currently available OGLE IV data. Each periodogram combines data from 3 seasons, as noted on the figure. In all cases there is power near 1 day. In some data sets the observed power is strongest near ~0.99 d while in other data sets the power at ~1.01 d is stronger. Due to the spacing of observations, there is no clear preference for either of the short periods. When they are weak there is little power near P=87 d. The periods derived from formal sine wave fits are marked on the figure. For a given alias the observed variation in stated periods is greater than the fitting errors. We also note that the periodogram from OGLE IV Seasons 1-3 has considerable structure at all alias periods. This would not be expected if a constant 87-d orbital period were present, but it is consistent with a variable NRP interpretation. In Fig. 2 (http://www.public.asu.edu/~atpcs/SXP/SXP175_OGLEIV_NRP.pdf) we show a sample of a folded light curve from Seasons 7-9 of OGLE IV data, using P(NRP)=1.0091 d. The shape is sinusoidal with a full amplitude of only 0.0079 mag. This behavior is typical of that seen in other Be systems where NRP are present.

It appears that the orbital period of SXP 175, suggested to be ~87 d, has not yet been found, but instead this long period is an artifact of variable non-radial pulsations near 1 d.