MAXI/GSC observation of possible regular X-ray outbursts from the Be/X-ray binary pulsar 4U 2238+60 (SAX J2239.3+6116)

ATel #16762; 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 9 Aug 2024; 03:36 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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We report possible regular X-ray outbursts from the Be/X-ray binary pulsar 4U 2238+60 (SAX J2239.3+6116) observed by MAXI/GSC. The source was detected by Einstein Probe WXT and FXT (ATel#16750) on 2024-08-01(MJD 60523) and 2024-08-03 (MJD 60525). To investigate the faint X-ray activity including the past few years, we extracted the data from the MAXI on-demand process (http://maxi.riken.jp/mxondem), which can efficiently exclude the background fluctuation mainly caused by the ISS orbital precession with the period of ~70 days. In this analysis, we used the default source/background region files provided by the process system. Our analysis used the 4-10 keV light curve with 4 days/bin. From this data, three possible outbursts with ~3 sigma significance were detected. Each outburst profile was approximated by the Gaussian function with a 16 days FWHM. The dates of the derived outburst peaks are summarized in the table below.

No.	peak date(MJD)
1	59744 ± 9
2	60004 ± 9
3	60267 ± 3

Intervals between outbursts are consistent with the claimed outburst period of 262.6±0.7 d (int' Zand et al. 2001). In addition, the outburst peak dates matched the prediction calculated by the ephemeris (T₀=50876(MJD), P_o=262.6±0.7d; int'Zand et al. 2000, 2001). Thus we reconfirmed the outburst periodicity of this system. Since the region of 4U 2238+60 (SAX J2239.3+6116) is in the out of FOV of MAXI/GSC from 2024-07-22 (MJD 60513), the expected outburst after the No.3 event around 2024-08-07 (MJD 60529) was not observed. According to the previous series of outbursts in 1996-2001 (int' Zand et al. 2000 and 2001), this source exhibited six consecutive outbursts. Considering that the source is in series of high activity, a few more outbursts synchronized with its period of 262.7 days might be seen in future. Follow-up observations are encouraged.