Probing the Extent of Fe K$\alpha$ Emission in Nearby Active Galactic Nuclei using Multi-Order Analysis of Chandra High Energy Transmission Grating Data

TL;DR

This study investigates the spatial extent and velocity of Fe Kα emission in nearby AGN using multi-order Chandra HETG data, revealing extended emission regions and suggesting non-gravitational motions like outflows.

Contribution

It introduces a novel multi-order spectral analysis method to measure the spatial extent and velocity of Fe Kα emission in AGN, highlighting the presence of extended emission regions.

Findings

Fe Kα emission extends over 5-100 pc in AGN.

The spatial extent is larger than velocity-based estimates, indicating complex gas motions.

Gas mass estimates range from 10^5 to 10^8 solar masses.

Abstract

We present a study of the narrow Fe K$\alpha$ line in seven bright, nearby AGN that have been observed extensively with the Chandra High Energy Transmission Grating (HETG). The HETG data reveal a wider Fe K$\alpha$ line in the first order spectrum than in the second and third order spectra, which we interpret as the result of spatially extended Fe K$\alpha$ emission. We utilize these differences in narrow Fe K$\alpha$ line widths in the multi-order Chandra HETG spectra to determine the spatial extent and intrinsic velocity width of the emitting material in each object. We find that there is modest evidence for spatially extended emission in each object, corresponding to extension of $r\sim5-100$ pc. These distances are significantly larger than those inferred from velocity widths assuming gravitational motions, which give $r\sim0.01-1$ pc. This implies that either the gas is emitting at a range of radii, with smaller radii dominating the velocity width and larger radii dominating the spatial extent, or that the gas is exhibiting non-gravitational motions, which we suggest would be outflows due to slight excess redshift in the line and velocities that exceed the freefall velocity. We also use the spatial extent information to estimate the mass of the emitting gas by counting fluorescing iron atoms, finding masses on the order of $M_\mathrm{gas}\sim10^5-10^8\,M_\odot$. Future work with observatories like XRISM will be able to extend this study to a larger number of AGN and decrease uncertainties that arise due to the low signal-to-noise of the higher order HETG data.