XRISM detection of the 6.4 keV Fe K$\alpha$ line in the radio galaxy Cygnus A

TL;DR

This study uses XRISM Resolve's high spectral resolution to analyze the Fe Kα line in Cygnus A, revealing two distinct components likely originating from different regions near the black hole.

Contribution

First detailed spectral analysis of Cygnus A's Fe Kα line with XRISM Resolve, identifying two Keplerian components and their likely origins in the broad line region and torus.

Findings

Detected two Keplerian broadened Fe Kα components.

Estimated their distances from the black hole: 0.1-0.17 pc and 6-10 pc.

Observed a redshifted Fe K edge indicating possible inflow or motion between components.

Abstract

We detail the spectral analysis of a 170 ks XRISM Resolve observation of the core of Cygnus A. The high spectral resolution of Resolve have enabled us to probe the inner accretion region of Cygnus A by analyzing the 6.4 keV Fe K$\alpha$ line complex. We find that it consists of two Keplerian broadened components. (1) A broad component with a velocity dispersion of $3400^{+800}_{-600}$ km s$^{-1}$ and (2) a narrow component of $440^{+60}_{-50}$ km s$^{-1}$. For an inclination of $50^{\circ}-85^{\circ}$, constrained by VLBI, we find that the broad component arises from a distance of $\sim 0.1-0.17$ pc ($800-1400$ gravitational radii) and the narrow component from $\sim 6-10$ pc ($50,000-80,000$ gravitational radii) from the central black hole depending on the inclination angle. Our result suggests that the origin of the broad component is consistent with the broad line region and the narrow component from the torus of Cygnus A. We also find a potential emission line possibly from intermediate ionized Fe XVII with a very low dispersion ($<80$ km s$^{-1}$) that originates from either the outer edge of the torus or the narrow line region. Finally, we find that the Fe K edge is redshifted compared to the Fe K$\alpha$ line components, suggesting a line of sight bulk velocity of $470 \pm 100$ km s$^{-1}$. Such a shift may be due to an inflowing wind or relative motion between the two components originating from the near and far side of an inflowing torus, respectively.