Multi-mission Observations of GS 1354-64 during the 2025-26 Outburst: First Results

Abstract

The Galactic transient black hole GS 1354-64 recently showed a new outburst, which has been continuously monitored by {\it MAXI}, {\it NuSTAR} and {\it Insight-HXMT} missions. The ongoing $2025-26$ outburst shows a slow-rise with an unusual short period ($\sim 3$~days) of X-ray flare of peak flux $\sim 1.4$~Crab, followed by another relatively weak flare of intensity $\sim 0.8$ Crab. The source is observed to evolve through `canonical' spectral states in a Hardness-Intensity Diagram (HID) during rising phase of the outburst, however subsequent outburst profile did not follow the reverse trend of the HID. A rapid evolution of Quasi-periodic Oscillation (QPO) frequencies ($\sim 0.07-4$ Hz) is observed during hard/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) as well as decreasing (decay phase) signature, which 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 {\it NuSTAR} ($3-70$ keV) and {\it Insight-HXMT} ($2-60$ keV) are well described by thermal (\textit{diskbb}) and reflection (\textit{relxill}) model components, indicating a strong signature of a relativistic reflection feature. Using `canonical' softer state observations, we found the source to be maximally rotating with $a_k \sim 0.998$ and inclination angle to be as $i \sim 63^\circ - 70^\circ$, which are consistent with earlier reports.