MAXI/GSC observes GS 1354-64 entering the soft state

ATel #17724; H. Negoro, M. Nakajima, K. Takagi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, D. Iijima, 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, 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.), and S. Yamada (Tohoku U)
on 16 Mar 2026; 09:59 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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MAXI/GSC detected a new outburst from the black hole X-ray binary GS 1354-64 on December 25, 2025, MJD 61034 (ATel #17563). The source started the hard-to-soft state transition from around January 19, MJD 61059 (ATel #17618). The transition to the intermediate state were reported from a number of observations in radio (ATel #17582, #17611, #17638, #17710), infrared (ATel #17611, #17638), optical (ATel #17586), soft X-rays (ATel #17583, #17611, #17638, #17650) including two XRISM observations (ATel #17625, #17697), and hard X-rays (ATel #17612, #17673).

Recent MAXI/GSC data show that after the secondary flux peak around February 16, MJD 61087, the X-ray flux gradually decreased with the spectral softening. (Note that no GSC data are available from MJD 61090 to 61097 because the source was obscured by Space-X attached to the ISS.) The count ratio of 4-10 keV to 2-4 keV had decreased rapidly since March 9, MJD 61108, staying at 0.2-0.3 from March 12. Concurrently, the GSC does not detect 10-20 keV X-ray photons significantly, implying that the source has transitioned into a soft state.

Further followup multi-wavelength observations encouraged.

MAXI data for GS 1354-64 data