XRISM Detection of the 6.4 keV Fe Kα Line in the Radio Galaxy Cygnus A

We detail the spectral analysis of a 170 ks XRISM Resolve observation of the core of Cygnus A. The high spectral resolution of Resolve has enabled us to probe the inner accretion region of Cygnus A by analyzing the 6.4 keV Fe Kα line complex. We find that it consists of two Keplerian broadened components: a broad component with a velocity dispersion of 3400⁺⁸⁰⁰₋₆₀₀ km s⁻¹ and a narrow component of (Formula presented) 440⁺⁶⁰₋₅₀ km s⁻¹. For an inclination of 50°–85°, constrained by very long baseline interferometry, we find that the broad component arises from a distance of ∼0.1–0.17 pc (800–1400 gravitational radii) and the narrow component from ∼6 to 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 Kα line components, suggesting a line-of-sight bulk velocity of 470 ± 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.