Warm Absorbers in the Radiation-driven Fountain Model of Low-mass Active Galactic Nuclei

TL;DR

This study models warm absorbers in low-mass AGNs using a radiation-driven fountain approach, predicting ionization structures and spectra that match some observed features but not the fastest outflows, indicating additional components are needed.

Contribution

It introduces a radiation-driven fountain model to simulate warm absorbers in low-mass AGNs and compares the predictions with high-resolution X-ray spectra, highlighting the model's successes and limitations.

Findings

Reproduces slow outflow velocities (~hundreds km/s) in NGC 4051.

Shows multi-phase ionization structure consistent with observations.

Underproduces fast outflows (~thousands km/s) and certain spectral features.

Abstract

To investigate the origins of the warm absorbers in active galactic nuclei (AGNs), we study the ionization-state structure of the radiation-driven fountain model in a low-mass AGN (Wada et al. 2016) and calculate the predicted X-ray spectra, utilizing the spectral synthesis code Cloudy (Ferland et al. 2017). The spectra show many absorption and emission line features originated in the outflowing ionized gas. The O VIII 0.654 keV lines are produced mainly in the polar region much closer to the SMBH than the optical narrow line regions. The absorption measure distribution of the ionization parameter ($\xi$) at a low inclination spreads over 4 orders of magnitude in $\xi$, indicating multi-phase ionization structure of the outflow, as actually observed in many type-1 AGNs. We compare our simulated spectra with the high energy-resolution spectrum of the narrow line Seyfert 1 galaxy, NGC 4051. The model reproduces slowly outflowing (a few hundreds km s$^{-1}$) warm absorbers. However, the faster components with a few thousands km s$^{-1}$ observed in NGC 4051 are not reproduced. The simulation also underproduces the intensity and width of the O VIII 0.654 keV line. These results suggest that the ionized gas launched from sub-parsec or smaller regions inside the torus, which are not included in the current fountain model, must be important ingredients of the warm absorbers with a few thousands km s$^{-1}$. The model also consistently explains the Chandra/HETG spectrum of the Seyfert 2 galaxy, the Circinus galaxy.