A clean broad iron line in GS 1354--64 as seen by XRISM

TL;DR

This study uses high-resolution XRISM and NuSTAR data to analyze the broad iron line in the black hole binary GS 1354--64, revealing a rapidly spinning black hole and demonstrating the capabilities of X-ray microcalorimeters.

Contribution

First high-resolution spectroscopic analysis of GS 1354--64's iron line with XRISM, constraining black hole spin and demonstrating microcalorimeter effectiveness.

Findings

Detected a clean broad iron line without narrow features.

Indicated a rapidly spinning black hole (a>0.98).

Showed the potential of X-ray microcalorimeters for black hole studies.

Abstract

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.