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

In this work, we utilize multiple orders of the Chandra High Energy Transmission Grating (HETG) to probe the spatial extent of the X-ray emitting gas around the supermassive black holes in nearby AGN. We focus on the narrow core of the Fe Kα line, whose width is usually used to determine where the gas is coming from by assuming all of the width comes from the gravitational motion of the gas around the black hole. However, when using a dispersive spectrograph, any spatial extent of the narrow line emitting matieral can provide additional broadening of the line. This effect will be dependent on the order of the spectrum because the amount of dispersion depends on the order. After doing a bit of grating and dispersion math, this all boils down to the first order spectrum being broadened most by any spatial extent. Then, we can compare the line widths in different order spectra to determine the spatial extent! That’s exactly what we do in this paper, and we find that all of the sources in our sample show some evidence for extrended emission (much more extended than the intrinsic velocity width implies). This is really hard to do with current observations because the 2nd and 3rd order spectra from the HETG are much lower quality than the 1st order spectra, but with XRISM and other future microcalorimeters, we will be able to extend this study by using the non-dispersive microcalorimeter spectroscopy in combination with dispersed 1st order Chandra HETG data alone.