Unveiling BLR Structure in AGN with High Resolution X-Ray Spectra: An Analytic Approach to Wind Emission Line Profiles

TL;DR

This paper introduces a new analytic model for wind emission line profiles in AGN, specifically targeting the broad line region, to interpret high-resolution X-ray spectra like those from XRISM.

Contribution

It develops a physically motivated, computationally efficient analytic model for BLR-scale wind emission lines, filling a gap in current line-profile modeling techniques.

Findings

Model successfully fits XRISM data of NGC 4151

Provides physically reasonable parameters for BLR wind

Reproduces smooth line profiles observed in data

Abstract

XRISM has provided an unprecedented view of the emission and absorption lines in the X-ray. Notably, early results showed significant complexity to the Fe-K$\alpha$ line profile in AGN, with clear contributions from at least three emitting structures: an inner disc, intermediary broad line region (BLR) scale material, and an outer torus. This poses a new challenge for the modelling of the emission lines, as while fast sophisticated models exist for disc line-profiles, large scale-height material is typically much more complex. In this paper we aim to address this gap, by building a fully analytic model for the emission line profiles from a wind, aimed towards BLR scale material, motivated on previous reverberation studies suggesting a wind on the inner edge of the BLR. Our approach gives a physically motivated, yet computationally fast, model for the intermediary component to the Fe-K$\alpha$ complex seen in the XRISM data. We demonstrate our model on the XRISM observations of NGC 4151 from the performance verification phase, showing that it gives a good description of the data, with physically reasonable parameters for BLR scale material. We also show that our model naturally gives the smooth line profile seen in the data, due to the large spatial extent of a wind. Finally, we make our model code public to the community, and name it xwind.