Long-term Monitoring of Non-radial Pulsations in Swift J010745.0-722740
ATel #14724; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 17 Jun 2021; 03:32 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)
Subjects: Optical, X-ray, Binary
The Be/X-ray binary Swift J010745.0-722740 (hereafter X01077) shows recurrent outbursts (P ~1180 d) in X-rays and optical light (Maggi et al. 2014, ATel #5778; Vasilopoulos et al. 2017, ATel #10253), which are thought to arise when an orbiting neutron star impacts the disk of a Be star. The optical counterpart is emission-line star [MA93] 1641, with a brightness that is consistent with SMC membership. Schmidtke et al. (2014, ATel #5781) examined available OGLE photometry for this star. They reported the outbursts show double-peaked profiles and discovered the presence of prominent non-radial pulsations (NRP) between outbursts. These NRP had periods ranging from P=0.3500 to 0.3504 d. Recently, Schmidtke et al. (2021, RNAAS, 5, 130) analyzed a more extensive data set (OGLE-II, -III, and -IV) to refine the orbital period and to estimate the temperature of the impact region from color changes during the outbursts.
Here, we report an extensive new analysis of the changes in NRP from X01077. The data come from 22 seasons of OGLE observations taken between 1997 and 2020. We note that the OGLE-IV I magnitudes were systematically brighter than earlier measurements, so a correction of +0.145 mag was applied to that data set. A plot of the corrected I light curve is shown in the attached figure. The times of outburst are marked, including some that were missed as they fell between observing seasons.
Period analysis of data from individual seasons was performed after removing observations close to the outbursts. The number of useful points per season varied between 33 and 146. We calculated a periodogram for each segment to obtain a preliminary estimate of the NRP period, which was then refined using a formal sine-wave fit. Each periodogram covered a frequency range from 0 to 5 d^-1. Data for season 2 of OGLE-IV is shown as an example of a typical periodogram (see attached figure). The most likely period is ~0.3503 d, but aliases near 0.2593 and 0.5397 d cannot be ruled out. Overall, the ~0.3503 d signal is the most commonly found period in the 22 seasons of data.
A plot of the NRP period vs. time is also shown in the attached figure. There is a clear trend of increasing period from 1997 to 2020, although the rate might not be strictly uniform. There appear to be several plateaus that are unrelated to small variations in the quiescent brightness or orbital phase. This is particularly noticeable in the flattening of NRP period during the last 5-6 seasons. Fitting a straight line to all the NRP data, we find the pulsation period changes by an average rate of 9.2990x10^-8 d d^-1. The full amplitudes of sine-wave fits show no significant change over time, with a mean value of 0.040 mag (low 0.030, high 0.052). We note that changes in NRP periods have been seen in other Be/X-ray binaries, but the X01077 data set is unique since it covers such a long, uninterrupted time interval.
Non-radial Pulsations in Swift J010745.0-722740
