Spectral–timing evolution of low-frequency QPOs and the inner accretion flow in the black hole X-ray binary GS 1354 – 64 with NuSTAR

We present a broadband spectral–timing analysis of the black hole X-ray binary GS 1354 – 64 using five NuSTAR observations obtained during its 2026 outburst, together with long-term soft X-ray monitoring from MAXI . The rms-normalized NuSTAR cospectra reveal persistent type-C low-frequency quasi-periodic oscillations (QPOs) in all observations, with centroid frequencies evolving from ∼ 1.4 to ∼ 6.1 Hz. The QPO quality factor varies non-monotonically and reaches its maximum value at an intermediate frequency, indicating that the coherence of the oscillation is governed more by the changing inner accretion-flow geometry than by the total flux alone. The 3–79 keV spectra are well described by the model constant*TBabs*(diskbb + cutoffpl + relxill) , with the absorption column fixed at the Galactic value and the photon index and cutoff energy of the reflection component tied to the direct continuum. The source shows clear spectral softening, with the photon index increasing from Γ ∼ 1.74 to ∼ 2.53, while the inner disc temperature rises from ∼ 0.38 keV in the first two observations to ∼ 0.84–1.16 keV in the later observations. The cutoff power-law component dominates the broadband 0.01–100 keV flux throughout the outburst, whereas the reflection flux remains comparatively stable. The cutoff energy increases from ∼ 33–37 keV to values above 100 keV, indicating significant evolution of the high-energy continuum. We find a positive correlation between QPO frequency and photon index, while the QPO frequency does not follow the total flux monotonically. These results are consistent with a truncated-disc interpretation in which the optically thick disc may evolve inward during the outburst, enhancing coronal cooling and contributing to the observed increase in QPO frequency. Within the framework of Lense–Thirring precession, the QPO evolution can be interpreted as arising from changes in the geometry of the inner hot flow.