Time-evolving diagnostic of the ionized absorbers in NGC 4051. I. High-resolution time-averaged spectroscopy

Abstract

We present a high-resolution X-ray spectroscopic study of the Narrow-Line Seyfert 1 galaxy NGC 4051 using two XMM-Newton high-resolution Reflection Grating Spectrometer (RGS) observations. The spectra reveal three distinct layers of photoionized gas flowing outward from the central black hole: a low-ionization phase (LIP), a higher-ionization phase (HIP), and a high-velocity and high ionization phase (HVIP). Each absorber leaves characteristic imprints on the soft X-ray spectrum. While the LIP and HVIP are fully consistent with being in ionization equilibrium with the central radiation field over the course of the $\sim$250 ks spanned by the two observations, the HIP shows a significant change in ionization ($3.8\sigma$), suggesting non-equilibrium. By modeling the two spectra with our time-dependent photoionization code (TEPID), we constrain the density of the HIP gas to $\log n_{\mathrm{H}}=7.7_{-0.9}^{+0.2}$ and estimate its distance to be about $R=0.45_{-0.09}^{+0.80}$ light-days from the black hole, corresponding to $R=4000_{-800}^{+7000}$ gravitational radii. In contrast, the narrow soft X-ray emission lines remain constant, consistent with an origin in the more extended narrow-line region. Our results show the value of combining high-resolution and time-resolved spectroscopy to probe the structure, physical conditions, and variability of AGN outflows.

Accepted for publication in The Astrophysical Journal. 11 pages, 6 figures, 4 tables