A Photoionization Model for the Soft X-Ray Spectrum of NGC 4151

We present an 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 (10²³ cm ^-2) with low covering factor proved adequate in reproducing all detected Lyman series lines, it proved insufficient in modeling the 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 in particular were anomalous in the 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 (10^20.5 cm^-2) component with nonzero microturbulence and high covering factor. While sufficient to reproduce observed soft X-ray flux, our model has 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. in their 2005 study of NGC 4151 absorption spectra.