MAXI monitoring of Hercules X-1 entering the extended low state in 2025

ATel #17607; T. Mihara, K. Asai (RIKEN), M. Nakajima, H. Negoro, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto, M. Uenishi, T. Usuki, S. Yatsuzuka (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U), reports on behalf of the MAXI team:
on 17 Jan 2026; 01:26 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

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Her X-1 is a well-studied binary X-ray pulsar. It has the 35 day super orbital cycle besides 1.24 s spin period and 1.70 day orbital period (e.g. Du et al. 2026). MAXI/GSC has been monitoring the 35-day cycle since 2009. It had repeated regular 35-day cycle until the cycle number 212 in 2025 May. The cycle number is defined by Leahy and Wang (2020) based on the Swift/BAT data. The flux of the main-on is decreasing since then, as shown in
https://maxi.riken.jp/plot/cyc211_218.png
The normalization (relative flux with respect to the average) changed as 1.1, 0.63, 0.39 and 0.11 in cycle number 212, 214, 216 and 218, respectively. In the last cycle, the flux dropped to ∼1/10 of the regular flux and was barely detected with MAXI/GSC. Still some source flux remains as the periodical flux drop due to eclipses are seen at around 61012.0, 61013.5, 61015.0 MJD.
https://maxi.riken.jp/plot/herx1_2025.jpg
Note the observation pattern of MAXI repeats according to the 70 day precession cycle of the ISS orbit. Her X-1 is located in a region covered by the broken cameras. Therefore the observation of the main-on is once in two cycles. At present, the observations are done in the even-numbered cycle.
Her X-1 seems to be entering the extended low-state (LS). The extended LS was first reported by EXOSAT observations in Parmar et al. (1985). There were two extended LS in the RXTE/ASM era. The last one occurred around 53000 MJD (2003 December) (Leahy et al. 2010). The current one is the first one since then after 22 years. The decreasing trend of the flux is similar to the previous two extended LS with RXTE/ASM.
https://maxi.riken.jp/plot/herx1_lowstate_asm_gsc.png