Dynamics of Warm Absorbing Gas in Seyfert Galaxies: NGC 5548

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Abstract
A magnetohydromagnetic (MHD) wind from a clumpy molecular accretion disk is invoked to explain observations of warm absorbing gas at UV and X-ray energies in Seyfert 1 galaxies. This paper focuses on two important issues: (1) compatibility of kinematics and dynamics of an MHD wind with the observed properties of warm absorbers, and (2) the relationship between the UV and X-ray absorbing gases. We provide an in-depth comparison between the MHD wind model and the well-studied Seyfert 1 galaxy NGC 5548, which at high spectral resolution exhibits a number of discrete UV absorption components. Model parameters used in this study have been fixed by our previous work on the variability of broad-line region in this object. The detailed UV-X-ray (UVX) absorption structure in NGC 5548 is modeled in order to infer the position, size, density and temperature of each component, as well as the overall dynamics of the absorbing gas. We find that for NGC 5548 (1) the total column densities of O VII, O VIII, and H inferred from X-ray observations are reproduced by constraining the UV ion column densities of C IV and N V in each component to lie within a factor of 2 of their observed values and optimizing over the possible sets of component ionization states and C IV column densities; (2) the warm absorbing gas exists in the outer part of the wind and is not a continuation of the flow in the broad emission-line region; and (3) the warm absorber extends both in radial and polar directions and is ionization stratified. X-ray absorption is found to be heavily biased toward smaller radii, and UV absorption originates at larger distances from the central continuum source. Our analysis shows that the discrete absorption components along the line of sight are intrinsically clumpy. Density differences between kinematic components result in a range of ionization and recombination timescales. We further test the applicability of the MHD wind to warm absorbers in general by constructing a quasi-continuous flow model and extending it to arbitrary aspect angles. Constraining the ionization and volume filling factor for a generic case, we estimate the fraction of Seyfert 1 galaxies having detectable warm absorbers with larger O VII than O VIII column densities, and the range of total hydrogen column densities. We also find that the ratio of O VII to O VIII optical depths can serve as a new diagnostic of active galactic nucleus aspect angle. Finally, the thermal stability of the UVX absorption model is tested. We find that all kinematic components in NGC 5548 are thermally stable to isobaric perturbations. In a more general case, we show that the magnetic field is crucial in order to stabilize the warm absorber gas over a wide range of incident continuum spectral energy distribution and gas metallicity.
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