MAXI/GSC observation of series of X-ray outbursts from the X-ray pulsar XTE J1829-098

ATel #16787; M. Nakajima (Nihon U.), T. Mihara (RIKEN), H. Negoro, Y. Kudo, H. Shibui, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Kawamuro, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, H. Nishikawa, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida (Ehime U.), I. Takahashi, M. Niwano, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, Y. Okada (Kyoto U.), M. Yamauchi, Y. Otsuki, T. Hasegawa, M. Nishio (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.)
on 24 Aug 2024; 10:08 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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MAXI/GSC detected a weak X-ray brightening from the Be/X-ray binary pulsar XTE J1829-098 on August 20, 2024. Including this event, MAXI/GSC has observed 13 X-ray outbursts from this source. Since August 2018, MAXI, Swift, and NICER (ATel #11927, #14521, #14554, #15550, #15551, #15555, #15556) have detected a series of 12 outbursts (including marginal detections) synchronized with the claimed periodicity of 243-246 days (ATel#2007, #11927, #14554, #15614). To analyze these outbursts, we extracted the 5-12 keV data using the MAXI on-demand process (http://maxi.riken.jp/mxondem/). In this analysis, we used the default source/background region files. The profiles of the obtained outbursts in the 5-12 keV energy band were approximated by the Gaussian function, and the fitting results are as follows.

No.	peak date(MJD)	peak flux(mCrab)
1	58342.1 ± 1.0	20 ± 2
2	58585.9 ± 2.9	12 ± 5
3	58832.4 ± 1.6	14 ± 2
4	59073.5 ± 1.2	20 ± 3
5	59322.1 ± 1.0	31 ± 3
6	59562.3 ± 2.1	14 ± 2
7	59804.7 ± 1.7	20 ± 2
8	60048.3 ± 1.7	12 ± 4
9	60290.2 ± 1.6	28
10	60533.2 ± 1.8	20 ± 4

As summarized in the above table, each outburst occurred at intervals consistent with the claimed periodicity of 243-246 days (ATel#2007, #11927, #14554, #15551, #15614). The peak of the No.8 outburst in April 2023 is consistent with the prediction made by Corbet et al. 2022 (ATel#15614). Corbet et al. 2022 also noted that the outburst period search using only the MAXI/GSC data might not be significant for a blind search due to the ∼72-day precession of the International Space Station. Since this 72-day precession effect is present in the data obtained from the MAXI/GSC monitoring results (http://maxi.riken.jp/v7l3h/J1829-098/index.html), we recommend that users analyze the data acquired from the MAXI on-demand process (http://maxi.riken.jp/mxondem/) to remove such effects. The 5-12 keV light curve extracted from the MAXI on-demand process is not affected by the 72-day precession, and we used all MAXI/GSC observation data (MJD 55061 to 60545) for the period search analysis. The power spectrum derived from this data shows the peaks of the normal outbursts at ∼244 days without the ∼72 day precession. In addition, the outburst period search is performed by folding the active phase data (MJD 58331 to 60545 ) using the task 'efsearch' provided by FTOOLS. The chi-squared versus outburst period plots are fitted with a Gaussian, and the error was estimated according to Leahy 1987. The updated ephemeris is
T_outburst = MJD 58342.1 + n * (243.8 +/- 1.5 ),
where n is the number of orbits since the outburst in August 2018. This result is consistent with the previous results (ATel#2007, #14554, #15614). We also attempted to fold the 5-12 keV MAXI/GSC data with this ephemeris in the active period (after August 2018). The folded light curve has the maximum flux of 0.015 +/- 0.001 photons/s/cm² (16 ± 1 mCrab) at phase 0, with a FHWM duration of 17 days.