New Photometric Results for SXP 1323

ATel #17129; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 3 Apr 2025; 22:55 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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RX J0103.6-7201 was identified in ROSAT catalogues of discrete X-ray sources in the SMC (Haberl et al. 2000, A&A Sup. Ser., 142, 41; Sasaki et al. 2000, A&A Sup. Ser., 147, 75). Haberl & Pietsch (2005, A&A, 438, 211) discovered X-ray pulsations with P=1323 s. The emission-line star [MA93] 1393 was suggested to be the optical counterpart by Haberl & Sasaki (2000, A&A 359, 573). Its spectral type is B0 III-V (McBride et al., 2008, MNRAS, 388, 1198). Hence this Be/X-ray pulsar is known as SXP 1323.

Using >20 yr of OGLE data, Treiber et al. (2025, A&A, 694, A43) parameterized the photometric variations in SXP 1323 and assigned Type 5, in which the maximum and minimum extrema from median brightness are nearly the same and there is little evidence for long-term variability. Detailed period analysis of OGLE-II observations revealed strong power at 26.16, 0.88, and 0.41 d (Schmidtke & Cowley 2006, ATel #716; Schmidtke et al. 2006, AJ, 132, 919). They suggested the 26-d period was an orbital signature, while the latter two periods to be caused by non-radial pulsations (NRP) of the Be star. Bird et al. (2012, MNRAS, 423, 3663) analyzed 12 years of OGLE-II and -III data and found periodicities at 26.17 and 7.91 d (the latter being an alias of 0.88 d), but their search was restricted to periods >2 d and not sensitive to finding short-period NRP. Carpano et al. (2017, A&A, 602, A81) also discovered a 26.2-d periodicity in X-ray observations, confirming the orbital interpretation. A short orbital period like this, however, is difficult to reconcile with 1323-s X-ray pulsations (see Corbet 1984, A&A, 141, 91).

We have examined OGLE-IV data for SXP 1323, which are available on the XROM website (https://ogle.astrouw.edu.pl/ogle4/xrom/xrom.html). Periodograms from the first 10 seasons (2010-2020) reveal power at periods similar to previous studies. However, the relative strength of the P=0.88 d signal diminished over time, and by Season 9 there was no significant power at this period. For each season of data we fitted sine waves to measure the (full) amplitude of the NRP. The results are shown in the top panel of the accompanying figure. The amplitude of 0.41-d NRP remained relatively constant, while that of 0.88-d NRP steadily declined.

When OGLE-IV operations resumed after the 2020-2022 coronavirus shutdown, the observing cadence changed so that multiple observations (up to 21) were obtained on many nights, greatly facilitating the photometric study of NRP. A plot of 1619 I-band observations taken on 239 nights between 2022 August 9 and 2024 February 12 is shown in the figure. A prominent modulation with a period near 26 days is present. A periodogram of these data is shown in the figure. There is strong power at P=26.2 d and its aliases (1.04 d, 0.96 d, etc.) and weak power at 0.41 d and its aliases (0.70 d, 0.29 d, etc.). Noticeably absent is the 0.88-d signal that was present in Seasons 1-10.

The two bottom panels of the figure show plots of the 2022-2024 data folded on the 26.2-d (orbital) and 0.41-d (NRP) periods. To construct a clean presentation of the orbital light curve, we subtracted an initial estimate of the 0.41-d signal from the original data. PDM analysis yields a best-fit period of 26.264 +/- 0.032 d, in agreement with previous determinations. The folded curve has an amplitude of 0.05 mag and maximum light near JD 2460026. The shape is asymmetrical with a steeper rise and extended decline that lasts most of the orbital period. We note that, unlike other presentations in the literature, this light curve has greatly reduced scatter and is free of short-period signals. Similarly, we constructed a clean presentation of the NRP light curve by subtracting the 26.2-d orbital modulation from the original data prior to conducting a formal sine wave analysis. The best-fit NRP period is 0.412 +/- 0.001 d. The light curve has a sinusoidal shape with an amplitude of 0.03 mag.

OGLE-IV Light Curves for SXP 1323