The shocked outflow in NGC 4051 - momentum-driven feedback, UFO's and warm absorbers

Abstract

An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009 revealed an unusually rich absorption spectrum with outflow velocities, in both RGS and EPIC spectra, up to ~ 9000 km/s (Pounds and Vaughan 2011). Evidence was again seen for a fast ionised wind with velocity ~ 0.12c (Tombesi 2010, Pounds and Vaughan 2012). Detailed modelling with the XSTAR photoionisation code now confirms the general correlation of velocity and ionisation predicted by mass conservation in a Compton-cooled shocked wind (King 2010). We attribute the strong column density gradient in the model to the addition of strong two-body cooling in the later stages of the flow, causing the ionisation (and velocity) to fall more quickly, and confining the lower ionisation gas to a narrower region. The column density and recombination timescale of the highly ionised flow component, seen mainly in Fe K lines, determine the primary shell thickness which, when compared with the theoretical Compton cooling length, determines a shock radius of ~ 10^17 cm. Variable radiative recombination continua (RRC) provide a key to scaling the lower ionisation gas, with the RRC flux then allowing a consistency check on the overall flow geometry. We conclude that the 2009 observation of NGC 4051 gives strong support to the idea that a fast, highly ionised wind, launched from the vicinity of the supermassive black hole, will lose much of its mechanical energy after shocking against the ISM at a sufficiently small radius for strong Compton cooling. However, the total flow momentum will be conserved, retaining the potential for a powerful AGN wind to support momentum-driven feedback (King 2003; 2005). We speculate that the `warm absorber' components often seen in AGN spectra result from accumulation of shocked wind and ejected ISM.

accepted for publication in Mon.Not.R.Astr.Soc

References in corpus (4)

• Unification of X-ray winds in Seyfert galaxies: from ultra-fast outflows to warm absorbers
• High-resolution X-ray spectroscopy of the low and high states of the Seyfert 1 galaxy NGC 4051 with Chandra LETGS
• The XMM-Newton RGS spectrum of the high luminosity Seyfert 1 galaxy Markarian 509
• Ultraviolet and X-ray variability of NGC 4051 over 45 days with XMM-Newton and Swift

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