Support for a 244 day Period in XTE J1829-098 from Swift BAT, MAXI and RXTE PCA Observations

ATel #15614; Robin H. D. Corbet (UMBC/NASA GSFC/CRESST/MICA), Joel B. Coley (Howard University/CRESST/NASA GSFC), Keith C. Gendreau (NASA GSFC), S. Guillot (IRAP/CNRS), N. Islam (UMBC/NASA GSFC), Gaurava K. Jaisawal (DTU Space), C. Malacaria (ISSI), Mason Ng (MIT), Katja Pottschmidt (CRESST/UMBC/NASA GSFC), Pragati Pradhan (ERAU, Prescott), Paul S. Ray (NRL), Andrea Sanna (U. Cagliari), J. Wilms (Remeis-Observatory, FAU Erlangen-Nuernberg), Michael T. Wolff (NRL)
on 12 Sep 2022; 19:34 UT
Credential Certification: Robin Corbet (corbet@umbc.edu)

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The transient X-ray pulsar XTE J1829-098 was recently reported to be in outburst from MAXI GSC observations (ATel #15551), confirmed with Swift (ATel #15555) and NICER (ATel #15556) observations.

Markwardt et al. (2009, ATel #2007) from RXTE PCA monitoring proposed a recurrence period of ~246 days with durations of ~7 days. Subsequently, Nakajima et al. (2021, ATel #14554) reported that MAXI outbursts were consistent with a period of 243.66 +/- 3.07 days, centered on MJD 58,341.73. The current outburst is reported to be consistent with this (ATel #15551).

We therefore investigated the long-term light curves of XTE J1829-098 obtained with the RXTE PCA, Swift BAT, and MAXI to confirm the periodicity and further investigate its properties.

The full RXTE PCA scans cover MJD 53,147 to 55,869 (2004-05-22 to 2011-11-04), approximately 1.5 years longer than used by Markwardt et al. However, data obtained after approximately MJD 54,800 (2008 December) do not show flux increases at the predicted times. For the earlier data, the power spectrum shows numerous harmonics of the ~244 day period, with the second harmonic at 121. 58 +/- 1.43 days (equivalent to 243.2 +/- 2.9 days) the strongest peak.

We investigated the Swift BAT transient monitor 15-50 keV orbital light curve covering MJD 53,416 to 59,828 (2005-02-15 to 2022-09-06). While the power spectrum of this light curve from 0.07 to 6412 days (~69,800 statistically independent steps) does not show a large signal near 244 days, even if the PCA “quiescent” interval from MJD 54,800 to 55,869 is removed, there are a number of peaks consistent with harmonics of the 244 day period. The strongest peak is at 40.70 +/- 0.04 days which, if interpreted as the 6th harmonic, implies 244.2 +/- 0.2 days. We also conducted a periodicity search by folding the data into 14 bins and calculating the reduced chi^2 value, which can be more sensitive for non-sinusoidal waveforms. For the same period range the largest chi^2 value is at ~243.8 days, and increases if the quiescent interval is removed.

Folding the BAT data on the Nakajima et al. (2021) ephemeris shows emission concentrated to a narrow range around phase 0 with an average rate of ~0.002 cts/cm^2/s (~10 mCrab). At this low count rate, the outburst duration is difficult to estimate, but is suggestive of a full-width half-maximum (FWHM) of ~15 days.

We also analyzed the MAXI GSC 2-20 keV light curve covering MJD 55,055 to 59,828 (2009-08-12 to 2022-09-06), again removing the PCA quiescent interval. The power spectrum is dominated by the ~72 day precession period of the International Space Station. For periods longer than this, the strongest peak is at 244.5 +/- 2.0 days. Although it would not be significant for a blind search alone, the second and third strongest peaks are at the second and third harmonics. A folding-based period search is complicated by strong sub-harmonics of the ISS precession period. Folding the MAXI data on the Nakajima et al. ephemeris shows a similar profile to the folded BAT light curve with a narrow peak around phase 0. The maximum count rate is ~ 0.01 cts/cm^2/s (~2.7 mCrab) above the out-of-outburst rate, with a FWHM duration of ~20 days.

In summary:
All three instruments show modulation near 244 days although, without the initial PCA detection, it would have been challenging to detect the periodicity with the BAT or MAXI alone.
The outbursts seen with BAT and MAXI, similar to the PCA, are concentrated in a narrow phase range.
The PCA data show a quiescent interval during which strong outbursts ceased.
The later BAT and MAXI observations show that regular outbursts resumed, but more detailed analysis is required to determine when this occurred.
The appearance and disappearance of periodic variability is seen in Be star high-mass X-ray binaries (e.g. Reig 2011, ApSS, 332, 1).
Unless XTE J1829-098 becomes quiescent again, the next outburst is expected near 2023-04-13.