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 Reflection Grating Spectrometers and EPIC spectra, up to ∼9000 km s−1. Evidence was again seen for a fast ionized wind with velocity ∼0.12c. Detailed modelling with the xstar photoionization code now confirms the general correlation of velocity and ionization predicted by mass conservation in a Compton-cooled shocked wind. 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 ionization (and velocity) to fall more quickly, and confining the lower ionization gas to a narrower region. The column density and recombination time-scale of the highly ionized 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 ∼1017 cm. Variable radiative recombination continua (RRC) provide a key to scaling the lower ionization 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 ionized wind, launched from the vicinity of the supermassive black hole, will lose much of its mechanical energy after shocking against the interstellar medium (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. We speculate that the ‘warm absorber’ components often seen in AGN spectra result from the accumulation of shocked wind and ejected ISM.
Highlights
High-resolution spectra of the bright Seyfert 1 galaxy NGC 4051 obtained by Chandra, XMM–Newton and Suzaku over the past decade have detected soft X-ray absorption lines indicating a ubiquitous outflow with velocities in the range ∼200– 600 km s−1 (Collinge et al 2001; Ogle et al 2004; Pounds et al 2004; Steenbrugge et al 2009), with occasional reports of higher velocities of ∼2340 km s−1 (Collinge et al 2001) and ∼4600 km s−1 (Steenbrugge et al 2009)
To quantify the overall photoionized absorption in the complex outflow in NGC 4051 the Reflection Grating Spectrometers (RGS) and EPIC spectra for the sum of the four high-flux orbits 5–8 were modelled in XSPEC (Arnaud 1996) with alternative grids based on the XSTAR photoionization code (Kallman et al 1996)
Fitting the EPIC pn absorption spectra, again for the sum of the four high-flux orbits (5–8), allows an extension of the XSTAR modelling to heavier ions whose K-shell wavelengths fall outside the sensitive range of the RGS
Summary
High-resolution spectra of the bright Seyfert 1 galaxy NGC 4051 obtained by Chandra, XMM–Newton and Suzaku over the past decade have detected soft X-ray absorption lines indicating a ubiquitous outflow with velocities in the range ∼200– 600 km s−1 (Collinge et al 2001; Ogle et al 2004; Pounds et al 2004; Steenbrugge et al 2009), with occasional reports of higher velocities of ∼2340 km s−1 (Collinge et al 2001) and ∼4600 km s−1 (Steenbrugge et al 2009). In an initial analysis of the 2009 XMM–Newton observation of NGC 4051, Pounds & Vaughan (2011a; hereafter Paper I) considered an apparent correlation of outflow velocity and ionization parameter in terms of a mass-conserved decelerating flow, perhaps resulting from strong Compton cooling after shocking of the highspeed primary wind with the interstellar medium (ISM) or slower moving ejecta (King 2010; Zubovas & King 2012). Paper II outlined an alternative origin of the broad soft X-ray emission lines in NGC 4051, arising in a limb-brightened shell of shocked gas, and noted that self-absorption in the near-orthogonal flow could explain the low-velocity absorption component seen across a wide range of ionization states. The subsequent radial structure of the decelerating post-shock flow is determined by the competing cooling processes, which provide a physical basis on which to understand the complex X-ray absorption and emission spectra in the 2009 XMM–Newton observation of NGC 4051
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