Insight–HXMT observations of the 2025–2026 outburst of GS 1354 - 64: distinct disk flux–temperature tracks during the state evolution

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We report Insight–HXMT observations of the 2025–2026 outburst from the black hole X-ray binary GS 1354 - 64, complemented by MAXI monitoring. Unlike the previous hard-only outbursts of this source, the 2025–2026 event reached a much higher intensity, rose slowly, and was accompanied by multiple prominent X-ray flares.In the hardness–intensity diagram, the source evolved from the hard state to the intermediate state, and continued toward softer states. Broadband spectral analysis shows two distinct disk flux–temperature tracks: Track 1 follows 𝐹 d ⁢ 𝑖 ⁢ 𝑠 ⁢ 𝑘 ∝ 𝑇 0 . 6 1 ± 0 . 0 4 i ⁢ 𝑛 , whereas Track 2 follows 𝐹 d ⁢ 𝑖 ⁢ 𝑠 ⁢ 𝑘 ∝ 𝑇 4 . 6 9 ± 0 . 1 4 i ⁢ 𝑛 , suggesting a change in the radiative coupling between the disk and corona. The After Flares observations at MJD ≥ 61098 follow 𝐹 d ⁢ 𝑖 ⁢ 𝑠 ⁢ 𝑘 ∝ 𝑇 3 . 8 3 ± 0 . 2 7 i ⁢ 𝑛 , consistent within uncertainties with the standard F ∝ T 4 expectation, while the MJD ≥ 61098 diskbb normalizations are described by a characteristic normalization with measurable intrinsic scatter. Under the assumption that the apparent inner radius corresponding to this characteristic normalization is identified with the ISCO , we obtain tentative constraints of D ≳ 2.18 kpc and 47.7 ∘ ≲ i ≲ 79 ∘ . For a black hole mass of 10 M ⊙ , the peak unabsorbed 0.1–100 keV luminosity is approximately (1.3–4.7)% L Edd for distances of 2.18–4.1 kpc.

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