The X-ray spectral signatures from the complex circumnuclear regions in the Compton thick AGN NGC 424

TL;DR

This study analyzes the X-ray spectra of the Compton-thick AGN NGC 424, revealing complex circumnuclear gas with multiple ionisation phases and evidence of Ni/Fe overabundance, using phenomenological and photoionisation models.

Contribution

First detailed spectral analysis of NGC 424 combining phenomenological and CLOUDY-based photoionisation models revealing multiple gas phases.

Findings

Presence of neutral Compton reflection and Ni/Fe overabundance.

Detection of multiple photoionised gas phases with different ionisation parameters.

Soft X-ray emission dominated by lines from highly ionised C, N, O, and Ne.

Abstract

We present the XMM-Newton RGS and EPIC pn spectra of a long (\simeq 100 ks) observation of one of the soft X-ray brightest Compton-thick Seyfert 2 galaxies, NGC 424. As a first step, we performed a phenomenological analysis of the data to derive the properties of all the spectral components. On the basis of these results, we fitted the spectra with self-consistent photoionisation models, produced with CLOUDY. The high-energy part of the spectrum is dominated by a pure neutral Compton reflection component and a neutral iron K-alpha line, together with K-alpha emission from neutral Ni, suggesting a significant Ni/Fe overabundance. The soft X-ray RGS spectrum comes mostly from line emission from H-like and He-like C, N, O, and Ne, as well as from the Fe L-shell. The presence of narrow RRC from O VIII, O VII, and C VI, the last two with resolved widths corresponding to temperatures around 5-10 eV, is a strong indication of a gas in photoionisation equilibrium, as confirmed by the prevalence of the forbidden component in the O VII triplet. Two gas phases with different ionisation parameters are needed to reproduce the spectrum with a self-consistent photoionisation model, any contribution from a gas in collisional equilibrium being no more than 10% of the total flux in the 0.35-1.55 keV band. When this self-consistent model is applied to the 0.5-10 keV band of the EPIC pn spectrum, a third photoionised phase is needed to account for emission lines with higher ionisation potential, although K-alpha emission from S XV and Fe XXVI remains under-predicted.