Multimission Observations of GS 1354-64 during the 2025-2026 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 $2025$--$2026$ outburst shows a slow rise with an unusually 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 was observed to evolve through ``canonical'' spectral states in a hardness-intensity diagram (HID) during 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 quasi-periodic oscillation (QPO) frequencies ($\sim 0.07-4$ 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 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 {\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'' observations of a softer state, 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.

12 pages, 5 figures, 2 tables

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