GS 1354-64 is likely undergoing a hard-to-soft transition

ATel #17618; 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, 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 21 Jan 2026; 14:45 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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Recent MAXI/GSC data show that the probably most distant black hole X-ray binary GS 1354-64 in outburst (ATel #17563, #17582, #17583, #17586, #17611, #17612) is likely undergoing a hard-to-soft transition. The 2-4 keV flux has been increasing rapidly since January 19 (MJD 61059), while the 10-20 keV flux has been declining since January 18.

The 2-20 keV X-ray count rate on January 17 just before the transition is 0.53 +/- 0.02 photons/cm^2/s (~0.14 Crab count rate). From NuSTAR observation on January 13, Adegoke et al. reported that the 1-100 keV flux was about 1.2e-8 erg/cm^2/s (ATel #17612). If we assume that the shape of energy spectra on January 13 and 17 did not change, the 1-100 keV flux on January 17 can be estimated to be about 1.4e-8 erg/cm^2/s from 2-20 keV count rates obtained with the GSC on the two days.
From this, we obtain the 1-100 keV luminosity of 1.1e39 erg/s for the 25 kpc distance (for an argument about possible distances, see Casares et al. 2009, ApJS, 181, 238). This luminosity is more than 80% of the Eddington luminosity for a 10 solar mass black hole, and much higher than a typical value of 10% or less during the hard-to-soft transition (Tetarenko et al., 2016, ApJS, 222, 15; Vahdat, et al., 2019, MNRAS, 485, 2744). This suggests that the source is much closer than 25 kpc, say 8.5 kpc if the estimated transition luminosity is 10% of the Eddington luminosity. Other possibilities that GS 1354-64 is very massive or anomalous, however, still remain.

To restrict the distance and the mass of the possibly most distant black hole in our galaxy, long-term multiwavelength follow-up observations are highly recommended.

MAXI data for GS 1354-64