Variability of the coronal line region in NGC 4151

TL;DR

This study investigates the variability of coronal lines in NGC 4151 over eight years, revealing weak variability and suggesting the coronal line region is a distinct, distant, high-ionisation zone beyond the dusty torus.

Contribution

First extensive analysis of coronal line variability in an active galaxy, providing estimates of physical conditions and spatial location of the coronal line region.

Findings

Coronal lines show weak or no variability over time.

Coronal line gas has low density (~10^3 cm^-3) and high ionisation parameter (~10).

Region is located about two light years from the ionising source.

Abstract

We present the first extensive study of the coronal line variability in an active galaxy. Our data set for the nearby source NGC 4151 consists of six epochs of quasi-simultaneous optical and near-infrared spectroscopy spanning a period of about eight years and five epochs of X-ray spectroscopy overlapping in time with it. None of the coronal lines showed the variability behaviour observed for the broad emission lines and hot dust emission. In general, the coronal lines varied only weakly, if at all. Using the optical [Fe VII] and X-ray O VII emission lines we estimate that the coronal line gas has a relatively low density of n~10^3 cm^-3 and a relatively high ionisation parameter of log U~1. The resultant distance of the coronal line gas from the ionising source is about two light years, which puts this region well beyond the hot inner face of the obscuring dusty torus. The high ionisation parameter implies that the coronal line region is an independent entity rather than part of a continuous gas distribution connecting the broad and narrow emission line regions. We present tentative evidence for the X-ray heated wind scenario of Pier & Voit. We find that the increased ionising radiation that heats the dusty torus also increases the cooling efficiency of the coronal line gas, most likely due to a stronger adiabatic expansion.