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Astron. Astrophys. 363, 455-475 (2000)

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2. AGN wind properties

In this section we explicitly describe and discuss the main assumptions, approximations and requirements we want a wind-type flow to fulfill to be descriptive, even though schematically, of our astrophysical problem.

As for the geometry of the wind, we might expect a wind that does not cover the whole [FORMULA] solid angle, at least not on every scale of distance (for example in the region in which an "obscuring" torus is present, we can imagine the wind surviving only in the "open" solid angle region, thus acquiring a cone-like geometry). However, as for symmetry properties, it is reasonable to simplify the wind description by assuming a spherical geometry and symmetry in any case, thus implying that all the relevant physical quantities only depend on the radial coordinate r, identifying the distance from the central black hole, and that the wind velocity is essentially radial.

There are some general properties and requisites that we ask to be verified for a wind-type outflow in an Active Galactic Nucleus to be modeled in a simple way.

1) The wind gas must be such that its total optical depth to photon scattering is [FORMULA]: this is a necessary condition so as not to have to account for modifications in the spectrum and energy content of the central radiation field (illuminating the wind plasma) through Compton thick interactions; also, and substantially in other words, the power exchanged through Compton interaction must be negligible with respect to the total luminosity of the radiation field, so that we can neglect any significant effect on the central radiation field itself due to Compton interactions along the wind flow.

In fact, what we assume is that the radiation field that illuminates the wind is basically unchanged, along the wind and emerging from the nuclear region, by the presence of the wind itself, neither because of its interaction with the plasma electrons, nor because of the hot wind plasma emission by bremsstrahlung, that, consistently, turns out to be negligible with respect to the central source luminosity.

The issue above refer to requests that are due not only to our choice of a schematic treatment of the problem, but also rely on the idea that substantial contribution to the high energy radiation field preferentially comes from the inner region of the AGN. Moreover, we stress that these requirements refer to the tenuous wind as a background medium, and not to the denser phenomenological components identified as BLR and outflowing UV absorbers.

2) The flow velocity must be in sub-relativistic regime ([FORMULA]), since we are referring to radio-quiet AGNs (Seyfert 1s), in which no extreme phenomenology, suggestive of relativistic flows, is observed.

3) We expect the wind total mass flux, [FORMULA], to be much lower than the critical (Eddington) accretion rate, [FORMULA] [where [FORMULA] is the black hole accretion efficiency and it is [FORMULA] (Krolik 1999)], for a given central black hole mass: [FORMULA]. Actually [FORMULA] should be [FORMULA], the accretion mass flux, and, therefore, we can reasonably suppose it to be much lower than the critical value of the accretion mass flux itself.

The requirements above both characterize AGN environment and guarantee the consistency of the chosen treatment and astrophysical description of the AGN wind problem; on the other hand, their fulfillment ends up to strongly characterize the outcoming model wind as a hot and very tenuous one, especially in the external regions, as it will be discussed in the following sections.

We also have to require that the wind solutions we obtain show a "regular" behaviour all along the range of distances involved, i.e. , that the various physical quantities turn out to be non-diverging, so as to properly represent a physical solution for the outflow. In fact, diverging physical quantities would imply that the chosen starting hypotheses do not lead to a physical condition existing in nature for the AGN wind; on the contrary, if a range of parameters is found, corresponding to which a regular solution is obtained, the assumed framework can be analysed as possibly representative of the physics of the AGN wind.

Especially important is then the identification and the detailed treatment of relevant radiative losses, and of energy deposition and loss mechanisms for the wind plasma, including the interaction of the plasma itself with the central source radiation field, through Compton processes. Thus, heating sources for the thermal wind plasma and their radial distance dependence have to be included in the wind description. Our present choice is to account for the known sources of heating and momentum deposition, and of cooling mechanisms with a physically consistent description and, on the other hand, to try to suitably parameterize an additional heating function contribution, so as to obtain a physically reasonable solution for the wind equations (see Sect. 4).

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000