A probable black hole X-ray binary MAXI J1750-327 is undergoing a hard-to-soft state transition

ATel #17974; H. Negoro, M. Nakajima, K. Saito, T. Yokoyama, M. Wada (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, A. Yoshida (AGU), Y. Tsuboi, Y. Ishihara, H. Ogaki (Chuo U.), M. Shidatsu, C. Kang, M. Uenishi, T. Usuki, S. Yatsuzuka, H. Aoe (Ehime U.), I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, 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), and S. Ogawa (TUS)
on 16 Aug 2026; 02:36 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Tweet

MAXI J1750-327 was discovered on August 13 (ATel #17963). Just before the detection, the GSC cameras had been disabled from 12:14 UT on August 12 to 01:09 UT on August 13 due to an ISS network issue, and the first alert from the Nova-Alert system for this transient was issued at 01:09 UT. At that time, the source was already as bright as ~100 mCrab in the 4-10 keV band. The EP-WXT also detected this new transient (designated EP260812c) about 5 hours earlier than the MAXI detection (Huang et al. ATel #17964), and the follow-up observation by the EP-FXT was performed from August 14.64 UT, revealing that the source is an uncatalogued new object (Huang et al. ATel #17969).

MAXI/GSC light curves show that the 2-4 keV count rate has been almost linearly increasing since August 13.04 UT (= MJD 61265.04) (at least until August 15.63 (MJD 61267.63)). On the other hand, the 10-20 keV count rate linearly increased to 0.33 photons/cm2/s (~830 mCrab) until around August 14.1 UT. After that, the count rate became almost constant (but still slightly increasing), suggesting that the source has been undergoing a hard-to-soft transition since August 14.1 UT. Accordingly, the 4-10 keV/2-4 keV count ratio has been slightly decreasing from approximately 1.1, which is a typical value of black hole X-ray binaries in the hard state, and the 10-20 keV/2-4 keV ratio decreased from approximately 0.55 to 0.40. We note that the EP-FXT detected a relatively low-frequency QPO (ATel #17969), often seen in the hard-to-intermediate state transition.

The latest count fluxes averaged over three scans from MJD 61267.50 to 61267.63 are 0.97 +/- 0.06, 0.95 +/- 0.04, and 0.39 +/- 0.03 photons/cm2/s in the 2-4, 4-10, and 10-20 keV bands, respectively. These correspond to 460, 760, and 980 mCrab, indicating the source spectrum is still very hard. Based on these recent properties, MAXI J1750-327 is thought to be a black hole X-ray binary.

The X-ray flux is expected to exceed 1 Crab in a few days, as seen in very bright black hole X-ray novae, such as MAXI J1535-571 (ATel #10699), MAXI J1348-630 (ATel #12425), MAXI J1820+070 (ATel #11399), and MAXI J1631-479 (ATel#12320). Multi-wavelength observations are highly recommended.

MAXI light curve of MAXI J1750-327