XRISM/Xtend Transient Search (XTS) detected spectral change of MAXI J1744-294: approaching soft-to-hard state transition?

ATel #17354; C. Kang, M. Shidatsu (Ehime U.), K. Fukushima, Y. Kanemaru, S. Ogawa, Y. Uchida, T. Yoneyama (JAXA), M. Audard (U. de Geneve), E. Behar (Technion), T. Hakamata (UOsaka), S. Inoue (Kyoto U.), Y. Ishihara (Chuo U.), T. Kiyomoto (Saitama U.), T. Kohmura (TUS), H. Kuramoto (UOsaka), J. Kurashima (U. of Miyazaki), Y. Maeda (JAXA), H. Matsumoto (UOsaka), T. Matsushima (U. of Miyazaki), A. Miyamoto (UOsaka), M. Mizumoto (UTEF), K. Mori (U. of Miyazaki), Y. Motogami (Saitama U.), N. Nagashima (Chuo U.), T. Narita (Kyoto U.), M. Nobukawa (NUE), H. Noda (Tohoku U.), Y. Sakamoto (Tohoku U.), H. Sugai (Chuo U.), H. Takahashi (Hiroshima U.), T. Takagi (Ehime U.), S. Takatuska (UOsaka), R. Takemoto (U. of Miyazaki), Y. Terada (Saitama U.), Y. Terashima (Ehime U.), Y. Tsuboi (Chuo U.), H. Uchida (Kyoto U.), M. Yoshimoto (Ehime U.)
on 23 Aug 2025; 03:25 UT
Credential Certification: Megumi Shidatsu (shidatsu.megumi.wr@ehime-u.ac.jp)

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XRISM/Xtend Transient Search (XTS) detected the black hole candidate MAXI J1744-294, located in the Galactic center region. The observation was conducted from 2025 August 20 to 2025 August 21 TT. The source was discovered with MAXI on 2025 January (ATel 16975). Follow-up X-ray observations were conducted at various epochs by many missions including NinjaSat, Swift, NICER, XRISM, NuSTAR, Chandra, and Einstein Probe (ATel 17009, 17070, 17031, 17040, 17063, 17068, 17087) at least until 2025 March.

The spectrum in the latest XRISM observation is well reproduced by an absorbed disk blackbody plus power-law model with the absorption of NH = 2.2 +/- 0.2 × 10²³cm^-2 (90% confidence level), an inner disk temperature of Tin = 0.32 +/- 0.02 keV, and a photon index of Γ = 3.0 +/- 0.2. The disk blackbody and power-law components dominate the flux below and above ~5 keV, respectively. We obtain an absorbed flux of 7.6 (+0.1/-0.2) × 10^-11 erg s^-1cm^-2 (2 – 10 keV). A systematic error of roughly 20% should be added to the statistical error. This flux is ~1 order of magnitude lower than that measured in the XRISM DDT observation in 2025 March, when the inner disk temperature was ~2 times higher (ATel 17063). The large flux decrease, together with the low inner disk temperature, suggests that the source is approaching the transition from the soft state to the hard state.

We derived the above systematic error for the flux by comparing our derived values for the sources detected with XTS in several observations with those for the corresponding X-ray counterparts.