A XRISM View of Relativistic Reflection in Cygnus X-1

TL;DR

This paper uses high-resolution XRISM data to analyze the relativistic Fe K line in Cygnus X-1, revealing a broad emission feature indicative of a rapidly rotating black hole and providing insights into the accretion disk geometry.

Contribution

First high-resolution XRISM observation of Cygnus X-1's Fe K line, clearly separating broad and narrow features, and constraining black hole spin and disk inclination.

Findings

Black hole spin estimated at ~0.98.

Inner disk inclination around 63 degrees.

Confirmation of relativistic reflection features.

Abstract

We present the first high-resolution XRISM/Resolve view of the relativistically broadened Fe K line in Cygnus X-1. The data clearly separate the relativistic broad line from the underlying continuum and from narrow emission and absorption features in the Fe band. The unprecedented spectral resolution in the Fe K band clearly demonstrates that the flux excess can be attributed to a single, broad feature, as opposed to a superposition of previously unresolved narrow features. This broad feature can be best interpreted as emission consistent with an origin near the innermost stable circular orbit around a rapidly rotating black hole. By modeling the shape of the broad line, we find a black hole spin of $a\simeq0.98$ and an inclination of the inner accretion disk of $\theta\simeq63^\circ$. The spin is consistent with prior reflection studies, reaffirming the robustness of past spin measurements using the relativistic reflection method. The measured inclination provides reinforcing evidence of a disk-orbit misalignment in Cygnus X-1. These results highlight the unique abilities of XRISM in separating overlapping spectral features and providing constraints on the geometry of accretion in X-ray binaries.