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

TL;DR

This study uses high-resolution, time-averaged X-ray spectroscopy of NGC 4051 to identify three ionized absorber phases (LIP, HIP, HVIP) with distinct ionization, column density, and velocities. By applying time-evolving photoionization modeling via TEPID to two ≈80 ks XMM-Newton RGS epochs, the authors demonstrate that the HIP is out of ionization equilibrium, enabling a robust density and distance estimate of $igl\log(n_{ m H}/{ m cm}^{-3}) = 7.7^{+0.2}_{-0.9}$ and $R = 0.45^{+0.80}_{-0.09}$ light-days, i.e., $\sim 4000^{+7000}_{-800}R_g$. The HVIP component is consistent with a persistent fast, highly ionized outflow seen in previous studies, while the soft X-ray emission lines remain stable and likely originate in the extended NLR. Overall, the work highlights the power of combining high-resolution and time-resolved spectroscopy to probe AGN outflow structure and variability, with future XRISM and Athena-era observations poised to further resolve absorber densities and locations.

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 ($3.8σ$), suggesting non-equilibrium. By modeling the two spectra with our time-dependent photoionization code (TEPID), we constrain the density of the HIP gas to $\log n_{\rm H}=7.7^{+0.2}_{-0.9}$ and estimate its distance to be about $R=0.45^{+0.80}_{-0.09}$ light-days from the black hole, corresponding to $R=4000^{+7000}_{-800}$ 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.

Figures (6)

• Figure 1: Upper panel. NuSTAR combined FPMA+B background-subtracted light curve in the $3-79$ keV energy band. Lower panel. EPIC-pn light curves of both observations in the $2-10$ keV energy band. All light curves are binned at $1$ ks per time bin.
• Figure 2: The $\lambda=6-35$ Å $\,$ RGS spectra analyzed in this paper. The grey spectrum is Epoch 1, while the light red spectrum is the one taken in Epoch 2. The black line is the best-fit model that includes all the absorbers detected in this paper. We also include the model without the absorbers, in blue, that also includes Gaussian emission lines, showing that there are significant residuals that strongly point at the presence of several ionized absorbers. The sharp vertical dips are instrumental gaps. For visual purposes only, the spectral data points have been rebinned at a minimum of 5 counts per wavelength bin.
• Figure 3: Data-to-model ratios that highlight the contributions to the fit from the three ionized absorbers. The horizontal line in red represents the best-fit model described in Table \ref{['tab:rgs_2abs']}. We removed one absorber at a time to highlight the difference between the best-fit model and the model that does not include the absorber. The green shaded area represents residuals obtained by removing the LIP, the lavender area is obtained when we remove the HIP, while the brown area is obtained by removing the HVIP. The upper panel represents Epoch 1 and the lower panel represents Epoch 2. We stress that the data are not refit after removing each absorber and are just plotted for illustrative purposes. The data is also visually binned at a minimum of 20 counts per wavelength bin.
• Figure 4: Examples of absorption lines for the contribution of the three absorbers. The grey points are the Epoch 1 observation, while the light red points are the Epoch 2 one. The black line represents the best-fit model listed in Table \ref{['tab:rgs_2abs']}, while the blue one represents the model without one of the three absorbers, without refitting the model. Left panel. Spectral region in $\lambda=21-23$ Å, the blue line was drawn by removing the LIP to highlight some of its contribution, namely O IV K$\alpha$ at $\lambda=22.76$ Å and O VI K$\alpha$ at $\lambda=22.03$ Å. Middle panel. Spectral region in $\lambda=15.3-16.8$ Å, the blue line is obtained by removing the HIP, highlighting the strong contribution for O VIII Ly$\beta$ at $\lambda=16.01$ Å. Right panel. Spectral region in $\lambda=10-12.5$ Å, the blue line is obtained by subtracting the HVIP component, contributing for the Ne X Ly$\alpha$ at $\lambda=12.14$ Å and for the Fe XXII L-transition at $\lambda=11.57$ Å.
• Figure 5: Ionization versus power law normalization of the three absorbers identified in this paper, each data point represents one Epoch. The red line is obtained assuming that the lowest flux state is in equilibrium.