A New Analysis of Non-Radial Pulsations in SXP 59.0
ATel #17175; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 8 May 2025; 18:34 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)
Subjects: Optical, X-ray, Binary, Neutron Star, Pulsar
Marshall et al. (1998, IAU Circ., 6818) discovered X-ray pulsations with P=59.0 s in RX J0054.9-7226 (hereafter SXP 59.0). Based on the timing of multiple X-ray outbursts, Galache et al. (2008, ApJS, 177, 189) found a recurrence time of 122.10 d, which is likely to be the orbital period. The optical counterpart is emission-line star [MA93] 810 (Stevens et al. 1999, MNRAS, 309, 421) with a O9 V spectral type (McBride et al. 2008, MNRAS, 388, 1198). Analyses of MACHO, OGLE-II and OGLE-III photometry revealed periods near 15, 20, 30, and 60 d (Coe & Orosz 2000, MNRAS, 311, 169; Schmidtke & Cowley 2005, AJ, 130, 220; Rajoelimanana et al. 2011, MNRAS, 413, 1600), but none of the folded light curves looked reasonable and the 122-d period was not confirmed. Schmidtke et al. (2013, MNRAS, 431, 252) presented evidence for non-radial pulsations (NRPs) at three periods near 1 day. Aliasing and/or beating of these NRPs could explain the âlongâ periods in other photometric studies.
The OGLE-IV light curve for SXP 59.0 is available online (https://ogle.astrouw.edu.pl/ogle4/xrom/sxp59.0.html). During the first seven observing seasons (2010-2017) SXP 59.0 brightened from I=15.5 to I=15.1 with small night-to-night scatter. Thereafter, the source dimmed by 0.2 mag with significantly greater scatter. This prominent change in photometric behavior coincided with the giant X-ray outburst that occurred 2017 March 30 (Weng et al. 2019, MNRAS, 487, 1000). We examine here the data taken after the giant outburst.
To remove long-term trends, a quadratic curve was subtracted from data for each season. The flattened observations were divided into four data sets. The first set contains data from 2017-2020 (three seasons), as there are not enough points in individual seasons to yield meaningful results. Starting in 2022, the observing cadence increased so that up to 21 observations were taken on some nights. Hence, the remaining three sets contain data from individual seasons: 2022, 2023-2024, and 2024-2025.
Periodograms reveal three periods at 0.950, 0.966, and 0.982 d, close to the NRPs reported by Schmidtke et al. (2013). In addition, there are long-period aliases (>20 d) and short-period ones (near 1 d, 1/2 d, 1/3 d, â¦). There is no evidence of a 122-d periodicity. We used a sine-wave fit to refine the period and estimate the full amplitude of each detected signal. When data are folded on these trial periods, all light curves, even those for long-period aliases, are nearly sinusoidal with very low amplitudes. Hence, we are confident in the NRP interpretation. The results are summarized in the table below. The 0.966-d and 0.982-d NRPs from 2022 data are blended and could not be resolved due to data spacing. During that season, the majority of points (461 out of 472) were taken during a span of only 69 days.
The period of each NRP signal in SXP 59.0 is consistent from one set to another, while the amplitude varies. The variation is greatest for 0.950-d NRPs, intermediate for 0.966-d NRPs, and least for 0.982-d NRPs. The presence of closely spaced NRPs is a common feature among Be stars. In a study of 430 classical Be stars using TESS data, Labadie-Bartz et al. (2022, AJ 163, 226) found >91% of early-type stars (B3 and earlier) showed frequency groups.
Data Set | Duration | Npts | NRP #1 | NRP #1 | NRP #2 | NRP #2 | NRP #3 | NRP #3
| | | Period | Amp. | Period | Amp. | Period | Amp.
| (days) | | (days) | (mag) | (days) | (mag) | (days) | (mag)
2017-2020 | 946 | 200 | 0.9499 | 0.0184 | 0.9654 | 0.0158 | 0.9812 | 0.0090
2022 | 127 | 472 | 0.9492 | 0.0124 | blend | blend | blend | blend
2023-2024 | 242 | 1206 | 0.9505 | 0.0351 | 0.9664 | 0.0109 | 0.9820 | 0.0137
2024-2025 | 255 | 1084 | 0.9510 | 0.0168 | 0.9669 | 0.0083 | 0.9816 | 0.0107
