A constant pressure model for the warm absorber in Mrk 509

TL;DR

This study tests a constant total pressure (CTP) warm-absorber model against a rich X-ray dataset for Mrk 509, using Titan photoionization calculations and a self-consistent treatment of line and continuum transfer. The authors fit a forward-folded, self-consistent CTP absorber to 900 ks of XMM-Newton RGS data with the Mrk 509 SED, finding a best-fit $\log \xi_0 \approx 1.98$ and $\log N_{\rm H} \approx 20.46$, an outflow velocity of $\sim 195$ km s$^{-1}$, and a cloud located at $\sim 0.02$ pc from the nucleus; the absorber is dense ($n_{\rm H,0} \sim 10^{10}\ \mathrm{cm^{-3}}$) and optically thin. While the CTP model captures many observed lines and remains more physically self-consistent than constant-density fits, a single static CTP component cannot reproduce the Ne X absorption and the full absorption measure distribution, suggesting the coexistence of an additional high-ionization/dynamic component or velocity structure. The work demonstrates the viability of CTP modeling for AGN WA studies and highlights the need for more complex, possibly time-dependent or multi-component descriptions to fully capture the observed spectra.

Abstract

We present the analysis of 900 ks high-resolution RGS XMM-Newton observations of the nearby Seyfert galaxy Mrk 509 with the use of a self-consistent warm absorber (WA) model. We perform a physically motivated approach to the modeling of WA as a stratified medium in constant total pressure (CTP) regime. Powerful outflows are fundamental ingredients of any active galactic nuclei (AGN) structure. They may significantly affect the cosmological environment of their host galaxy. High-resolution X-ray data are best suited for outflow's studies, and the observed absorption lines on heavy elements are evidence of the physical properties of an absorbing gas. Our models allow us to fit continuum shapes bounded together with the line profiles, which gives additional constraints on the gas structure of WA in this source. In this work, we benchmark and test the CTP model on the soft X-ray spectrum of Mrk 509. A grid of synthetic absorbed spectra was computed with the photoionization code Titan assuming that the system was under CTP. As an illuminating spectral energy distribution (SED), we used the most actual multiwavelength observations available for Mrk 509. We apply these models to the high-resolution spectrum of the WA in the Mrk 509, taking into account cold/warm/hot Galactic absorption on the way to the observer. CTP gas with $\log ξ_{0} \sim 1.9$, defined on the cloud surface, fits the data well. A higher ionization component is needed for Ne X absorption. The best-fit model is optically thin with $\log N_{\rm H }= 20.456 \pm 0.016$. The lines are non-saturated, and the CTP spectral fit aligns with previous analyses of Mrk 509 with a constant density WA. The model constrains the gas density, placing the WA cloud at 0.02 pc, consistent with the inner broad line region and the thickening region of the accretion disk.

Figures (13)

• Figure 1: Spectral energy distribution of Mrk 509 used as an input to the photoionization code titan marked as black solid line. The red circles are the observational points reported by kaastra2011a and normalised to the incident flux.
• Figure 2: Continuum model and Galactic absorption fitted to Mrk 509 RGS spectrum. RGS 1 and RGS 2 spectra are plotted in orange and cyan. RG 2 is arbitrarily shifted by 0.02 counts s$^{-1}$ Å$^{-1}$. The continuum model of Mrk 509 SED absorbed by the Milky Way is marked with red and blue continuous lines. The lower panel shows residuals against the best-fit model. The wavelengths are in the observed frame of reference. For the purpose of this plot the data are binned by 5 channels.
• Figure 3: The dependence of O vii profile on the turbulent velocities used in photoionization computations by titan code for a given model.
• Figure 4: Warm absorber on top of continuum modified by Galactic absorption with Mrk 509 RGS data points. RGS 1 and RGS 2 spectra are plotted in orange and cyan. RGS 2 is arbitrarily shifted by 0.02 counts s$^{-1}$ Å$^{-1}$. The total model: continuum together with a single CTP WA is plotted in red and blue. The wavelengths are in the observed frame of reference. The lower panel shows residua. For the purpose of this plot the data are binned by 5 channels.
• Figure 5: BXA contour plots for fitted parameters of the total model to the Mrk 509 RGS 1 and RGS 2 spectra. Cross normalization factor shows difference between two RGS specra where normalization gives the position of the total model. Best fit log $\xi$ = 1.9829 $^{+0.0093}_{-0.0089}$ and log $N_{\rm H}$ = 20.456 $\pm$ 0.016 with z = 0.03374 $\pm$ 0.00005 corresponds to the WA outflowing with $v=195\pm16$ km s$^{-1}$. The diagonal panels show marginalized posterior probability distributions while off-diagonal panels indicate conditional probability distribution functions among each parameter.