Abstract
The Galactic transient black hole GS 1354-64 recently showed a new outburst, which has been continuously monitored by MAXI, NuSTAR, and Insight-HXMT missions. The ongoing 2025-2026 outburst shows a slow rise with an unusually short period (similar to 3 days) of X-ray flare of peak flux similar to 1.4 Crab, followed by another relatively weak flare of intensity similar to 0.8 Crab. The source was observed to evolve through "canonical" spectral states in a hardness-intensity diagram (HID) during the rising phase of the outburst, but the subsequent outburst profile during this study, the source did not follow the reverse trend of the HID. A rapid evolution of quasiperiodic oscillation (QPO) frequencies (similar to 0.07-4 Hz) is observed during hard and intermediate spectral states without any signature of QPOs in the soft state. The evolution of the observed low-frequency QPOs shows a monotonically increasing (rising phase) signature as well as a decreasing (decay phase) one; these are further studied with the propagating oscillatory shock model to understand the nature of the evolution of the shock wave responsible for the origin of the observed QPOs. The broadband energy spectra from NuSTAR (3-70 keV) and Insight-HXMT (2-60 keV) are well described by thermal (diskbb) and reflection (relxill) model components, indicating a strong signature of a relativistic reflection feature. Using "canonical" observations of a softer state, we found the source to be maximally rotating with ak similar to 0.998 and the inclination angle to be i similar to 63 degrees-70 degrees, which are consistent with earlier reports.