We present a spectroscopic analysis of XRISM and NuSTAR observations of the black hole X-ray binary GS~1354--64 during its 2026 outburst. A total number of 3.5 million photons are collected by the microcalorimeter Resolve on board XRISM, providing an unprecedented high-resolution view of the iron line profile. A clean broad iron line is found in the data, without significant narrow features. Modeling the broad iron line with relativistic reflection from the inner accretion disk suggests a rapidly spinning black hole (a>0.98) in the system. Measurements of the disk inclination angle from the reflection method are model-dependent. This work demonstrates the power of X-ray microcalorimeters in studying the inner accretion flow and constraining black hole parameters.
@article{arxiv.2603.06883,
title = {A clean broad iron line in GS 1354--64 as seen by XRISM},
author = {Honghui Liu and Lingda Kong and Oluwashina K. Adegoke and Jiachen Jiang and Cosimo Bambi and Andrew C. Fabian and Adam Ingram and Swati Ravi and James F. Steiner and Qingcang Shui and Dominic J. Walton and Yerong Xu and Andrew J. Young and Yuexin Zhang and Zuobin Zhang and Andrea Santangelo},
journal= {arXiv preprint arXiv:2603.06883},
year = {2026}
}