XRISM High-Resolution X-ray Spectroscopy of Cygnus X-1 -- Orbital and Short-Term Variability of Iron Absorption

TL;DR

This paper presents high-resolution XRISM spectroscopy of Cygnus X-1, revealing orbital and short-term variability in iron absorption lines, which provides new insights into the stellar wind and accretion processes in this black hole binary.

Contribution

First high-resolution spectral analysis of Cygnus X-1 with XRISM, showing orbital and rapid variability in Fe absorption features and wind structure.

Findings

Orbital-phase-dependent variability in Fe absorption lines.

Evidence of short-term variability in absorption features on second timescales.

Indications of wind clumps interacting with X-ray emission.

Abstract

We present the first high-resolution spectroscopy of the black hole high-mass X-ray binary Cygnus X-1 with XRISM, including orbital-phase-resolved analyses and tentative evidence of short-term variability in the Fe-K band on second timescales. Using data from the Performance Verification phase in April 2024, we analyzed spectral variability across orbital phases with the Resolve microcalorimeter and the Xtend CCD imager. The unprecedented resolution of Resolve reveals variability in highly ionized Fe absorption lines. The absorption features show orbital-phase-dependent variability in column density, ionization state, and blueshifted velocity, suggesting structural variations in the focused stellar wind along the line of sight. We also find indications of subtle broadening of the neutral Fe emission profile. In addition, intensity-sorted spectroscopy during dip phases suggests possible variability on timescales of a few seconds in the absorption features, consistent with cooler, denser and lower-ionized gas clumps. Although the statistical significance is limited, these results hint that the stellar wind and the X-rays from the accretion disk around the black hole may interact on timescales as short as a few seconds. These XRISM results constrain wind-fed accretion in Cyg X-1 and highlight Resolve's capability to probe plasma environments in high-mass X-ray binaries.