Notes on ‘Towards a Unified AGN Structure’

Notes are based on the (submitted) paper ‘Towards a Unified AGN Structure,’ by Kazanas et al.


‘The notion of AGN as an astronomical object of solar system dimensions and luminosity surpassing that of a gaalxy has been with us for about half a century……..accretion onto a black hole as the source of the observed radiation…’

Spectroscopically inferred components (BLR, molecular torus, radio jets) led to well known Urry & Padovani (1995) structure of the AGN, which is simply an arrangement of the components, but with no physical motivation to support it:

Urry & Padovani, 1995, PASP, 107, 803

For instance, statistical analyses indicate the torus to be roughly h/R=1, but this is not supported by hydrostatic equilibrium. The Urry & Padovani picture also does not include UV and Xray outflows. The above components are independent with physical properties assigned as needed to understand the observations. It is of note that, simultaneous presence of Xray and UV absorption outflowing at high velocities implies they belong to the same outflowing plasma, but this ALSO has lacked a physical explanation.

Murray et al. (1995) say AGN outflows were proposed to be driven off the inner regions of the QSO accretion disks by UV and optical line radiation pressure to achieve outflow velocities observed. Along with Proga et al., these works show that efficient wind driving by line pressure requires X-ray shielding, otherwise overionization occurs, and line driving no longer works. A ‘failed wind’ from the innermost regions could provide this shielding, and the fact that BALs are X-ray weak advocates this.

The obvious conclusion being that AGN outflows MUST be included in our idea of the AGN structure, but the broad range of observed velocities and ionization parameters makes this very difficult without some kind of underlying physical principle/structure/physics/etc. This paper proposes a wind dynamical model that explains simultaneously the outflows in UV and Xray, and extends to all accretion powered objects: Seyferts, BALs, XRBs.

Xray Spectroscopy, Warm Absorbers

Modern Xray spectroscopy confirms and details absorption from many ionization states of N, Ne, Mg, Na, Si, Ni, Fe, and O, all absorption blueshifted, indicating a warm absorber and outflowing. In quasars specifically, most absorption in this regime is the Fe-K features [Fe-K features are: iron transitions involving the K orbital]. Xray outflowing absorption shows up in both BAL and non-BAL. Note the range of velocities, upwards of 0.8c! George Chartas has papers on these.

The ionization parameter in a chunk of gas is defined as:

    \[ \xi = {L \over n r^2} \]

where L is the ionizing luminosity, n the local gas density, r distance from source.

Absorption Measure Distribution

The fact that species such as FeXXV, MgV, OI are detected in Xray spectra indicates there MUST be a diverse set of ionization parameters throughout the wind. i.e., this wind has a density distribution that produces ionic column densities sufficiently large to be detected. Originally was considered as multiple components with static \xi values, however, studies fit the absorption data in the Xray with a continuous distribution of \xi. This has been called the Absorption Measure Distribution (AMD). In formula:

\[ AMD(\xi) = {dN_H \over dlog \xi}

Definition: The hydrogen equivalent column density of specific ions, N_H, per decade of ionization paramets \xi, as a function of \xi.

Aside: How to calculate AMD? (from Holczer et al. 2007)

‘The total hydrogen column along the line of sight can be expressed as an integral over its distribution in log \xi.’ A given object will have multiple, highly ionized species, from which to gleen an ionic column density. This is done through fitting routines, not topic here. Assume you have the ionic columns for all species in your Xray spectrum. You must fit to a distribution of dN_H/dlog \xi, that recreates the observed column densities in all ions.

‘For a monotonic distribution of the wind density n(r) with radius r, determination of AMD is the same as determining the ionized wind’s density dependence on r. So, determine AMD, and you can determine how the density of the wind changes with radius. All outflows where AMD can be established imply a flat to modestly increasing AMD with \xi, which says that n(r) \propto 1/r.

The AMD approach suggests that there is a powerlaw dependence for the wind density on r, i.e., n \propto r^{-s}, where s= (2\alpha+1)/(\alpha+1), which is n \propto 1/r. There are some physics I don’t quite get here, but the result is: ‘for radiatively driven winds, the column DECREASES with INCREASING ionization and INCREASING velocity, in significant disagreement with the dependence found by Holcser and Behar.’

So, while it hard to make radiation pressure work with AMD (match the density profile implied by AMD), MHD winds off accretion disks CAN work with AMD.

From the paper, the AMD slopes in their data says the density profile should be n(r) \propto r^{\alpha} where 1<\alpha<1.3, ruling out the standard assumption of n(r) \propto r^{-2}. An MHD wind CAN produce this density relationship. The AMD discussion here comes from Behar (2009), ApJ, 703, 1346. ALSO note that a paper was able to show that an XRB could not have been driven by radiation or Xray heating, thus magnetism must be invoked….MHD will solve all our problems.

MHD wind model

MHD winds are launched by poloidal magnetic fields under the combined action of rotation, gravity, and magnetic stresses (see Blandford & Payne 1982). The Grad Safranov equation is the force balance in the \theta-direction, the solution of this equation provides the angular dependence of all the fluid and magnetic field variables with their initial values at (r_not,90). The Grad Safranov equation has the form of a wind equation with several critical points, in our case the Alfven point is most important.

Winds, in general, are known to be self-similar when:

1. The radius is normalized to the Schwarzschild radius r_s (x=r/r_s, r_s=3M km, where M is the mass)

2. The mass flux Mdot is expressed in units of the eddington accretion rate, mdot = Mdot/M_Edot = Mdot/M

3. Their velocities are Keplerian, v=x^-1/2c

This paper is able to show that equations for accretion, winds, photoionization, and the AMD are all independent of the object’s mass. **what about density in accretion and winds?

The scalings for ionization, velocity, and AMD of the winds are independent of mass, but AGN, GBHC, or XRBs all have different X-ray absorber properties. The ‘self-similarity’ is broken by the dependence of the ionizing flux on the mass of the accreting object, and therefore leads to different wind ioinization propoerties.


The authors presented a broad strokes picture of the 2D AGN structure, which covers many decades of radius, frequency, and supplements UP95 with an outflow launched across the entire disk area, and velocity roughly equal to local Keplerian velocity at each launch radius. Here is an image showing their model.

The outflows have the property that their ionization, velocity, and AMD scale mainly with  the accretion rate \dot{m}. They should be applicable to all accretion powered sources from galactic accreting black holes to quasars. The mass, M, is what scales the overall scale of the the luminosity and size. The authors note the ionization structure depends on the spectrum of the ionizing radiation, which breaks the scale invariance on M with the wind flow.

The crucial and fundamental aaspect of the underlying MHD wind model, is their ability to produce density profiles that decrease as 1/r. This property allows the ionization parameter to decrease with distance, while still providing sufficient column to allow the detection of both high and low ionization in the AGN Xray spectra. This specific density scaling also allows the incorporation of Torii physics.

The price to pay for the density distribution we seek is the need to invoke winds whose mass flux increases with distance from the source (as r^{1/2}).

A radiation driven wind, while 2D in the region of launch, it will appear radial at sufficiently large distance producing density profiles that act as 1/r^2. As appealing as radiative line driving is, there is little evidence for them. Magnetic fields, such as those in MHD, appear to have the right amount of momentum needed to drive a wind with the required \dot(m)\proptor^{1/2}.