A Photoionization Model For The Soft X-Ray Spectrum Of NGC 4151

TL;DR

This paper analyzes XMM-Newton data of NGC 4151, modeling its soft X-ray emission with photoionization models, and introduces a three-component model to better fit observed emission lines, especially He-like triplets.

Contribution

It presents a novel three-component photoionization model that improves fitting of soft X-ray emission lines in NGC 4151, addressing limitations of previous two-component models.

Findings

A three-component model better fits the observed spectrum.

Resonance lines require a high covering factor component.

Model faces challenges fitting line profiles perfectly.

Abstract

We present analysis of archival data from multiple XMM-Newton observations of the Seyfert 1 galaxy NGC 4151. Spectral data from the RGS instruments reveal several strong soft X-ray emission lines, chiefly from hydrogen-like and helium-like oxygen, nitrogen, neon and carbon. Radiative recombination continua (RRC) from oxygen and carbon are also detected. Our analysis suggests that the emission data are consistent with photoionization. Using the CLOUDY photoionization code, we found that, while a two-component, high column density model (10e23 cm-2) with low covering factor proved adequate in reproducing all detected Lyman series lines, it proved insufficient in modeling He-like triplets observed (neon, oxygen, and nitrogen). If resonance line data were ignored, the two-component model was sufficient to match flux from intercombination and forbidden lines. However, with the inclusion of resonance line data, He-like triplets could no longer be modeled with only two components. We found that observed oxygen G and R ratios especially were anomalous in parameter space investigated. We investigated, and were forced to dismiss, the possibility that a third purely collisional component could be responsible for enhanced resonance line contributions. We succeeded in modeling the observed spectrum with the addition of a third, lower column density (10e20.5 cm-2) component with non-zero microturbulence and high covering factor. While sufficient to reproduce observed soft X-ray flux, our model faces certain shortcomings, particularly in a less-than-ideal visual fit to the line profile. Two of the three emission model components bear similarities to components determined by Kraemer et al. (2005) in their study of NGC 4151 absorption spectra.