Density constraint of the warm absorber in NGC 5548

TL;DR

The study constrains the density and location of warm absorbers in NGC 5548 by analyzing archival Chandra HETGS/LETGS data from January 2002. By tracking variability in MgXII Lyα and OVIII Lyβ absorption lines on timescales of about 144–162 ks and applying the SPEX/PION framework, the authors derive electron-density lower limits of $n_e \gtrsim (7.2-9.0)\times10^{11}\ \mathrm{m}^{-3}$. These densities yield upper limits on absorber distance via the ionization parameter relation $\xi = L_{\mathrm{ion}}/(n_{\mathrm{H}} r^{2})$, placing the variable WA components within $\sim$0.2–0.5 pc of the central source. The results significantly tighten previous density constraints and enhance our understanding of the proximity of AGN outflows, with implications for feedback processes. The work demonstrates the power of time-resolved X-ray spectroscopy and ionization-recombination modeling to map the circumnuclear environment of AGNs.

Abstract

Context. Ionized outflows in active galactic nuclei (AGNs) are thought to influence the evolution of their host galaxies and super-massive black holes (SMBHs). Distance is important to understand the kinetic power of the outflows as a cosmic feedback channel. However, the distance of the outflows with respect to the central engine is poorly constrained. The density of the outflows is an essential parameter for estimating the distance of the outflows. NGC 5548 exhibits a variety of spectroscopic features in its archival spectra, which can be used for density analysis. Aims. We aim to use the variability of the absorption lines from the archival spectra to obtain a density constraint and then estimate the distance of the outflows. Methods. We used the archival observations of NGC 5548 taken with Chandra in January 2002 to search for variations of the absorption lines. Results. We found that the Mg XII Ly$α$ and the O VIII Ly$β$ absorption lines have significant variation on the 144 ks time scale and the 162 ks time scale during the different observation periods. Based on the variability timescales and the physical properties of the variable components that dominated these two absorption lines, we derive a lower limit on the density of the variable warm absorber components in the range of $7.2-9.0{\times}10^{11} m^{-3}$, and an upper limit on their distance from the central source in the range of 0.2-0.5 pc.

Figures (4)

• Figure 1: The equivalent width variation over time. The light curve during the MEG and LETGS observations from Jellelightcurve2004AA is plotted in the background as orange points. The vertical gray dotted lines and blue lowercase letters mark the split LETGS spectra discussed in the text. The blue points are the equivalent width values. The dashed lines represent the average values from Table \ref{['tab:EWfitting']}: light blue lines for the MEG and LETGS observations, purple lines for the LETGS observation only.
• Figure 2: The line profiles of the MgXII Ly$\alpha$ and OVIII Ly$\beta$ lines with the best-fit Gaussian profile for the MEG and LETGS spectra.
• Figure 3: The equivalent width of MgXII Ly$\alpha$ and OVIII Ly$\beta$ lines, plotted as a function of the continuum flux in 1.5-24 $\AA$.
• Figure 4: The relationship between density and charge change time calculated by the PION models for MgXII Ly$\alpha$ and OVIII Ly$\beta$ in their dominant components. The dotted line marked the variability timescale (144 ks for MgXII Ly$\alpha$ and 162 ks for OVIII Ly$\beta$).