X-ray and UV line emission in X-ray binaries can be accounted for by a hot corona. Such a corona forms through irradiation of the outer disk by radiation produced in the inner accretion flow. The same irradiation can produce a strong outflow from the
disk at sufficiently large radii. Outflowing gas has been recently detected in several X-ray binaries via blue-shifted absorption lines. However, the causal connection between winds produced by irradiation and the blue-shifted absorption lines is problematic, particularly in the case of GRO J1655-40. Observations of this source imply wind densities about two orders of magnitude higher than theoretically predicted. This discrepancy does not mean that these `thermal disk-winds cannot explain blue-shifted absorption in other systems, nor that they are unimportant as a sink of matter. Motivated by the inevitability of thermal disk-winds and wealth of data taken with current observatories such as Chandra, XMM-Newton and Suzaku, as well as the future AstroH mission, we decided to investigate the requirements to produce very dense winds. Using physical arguments, hydrodynamical simulations and absorption line calculations, we found that modification of the heating and cooling rates by a factor of a few results in an increase of the wind density of up to an order of magnitude and the wind velocity by a factor of about two. Therefore, the mass loss rate from the disk can be one, if not even two orders of magnitude higher than the accretion rate onto the central object. Such a high mass loss rate is expected to destabilize the disk and perhaps provides a mechanism for state change.
Accretion disk winds are thought to produce many of the characteristic features seen in the spectra of active galactic nuclei (AGN) and quasi-stellar objects (QSOs). These outflows also represent a natural form of feedback between the central superma
ssive black hole and its host galaxy. The mechanism for driving this mass loss remains unknown, although radiation pressure mediated by spectral lines is a leading candidate. Here, we calculate the ionization state of, and emergent spectra for, the hydrodynamic simulation of a line-driven disk wind previously presented by Proga & Kallman (2004). To achieve this, we carry out a comprehensive Monte Carlo simulation of the radiative transfer through, and energy exchange within, the predicted outflow. We find that the wind is much more ionized than originally estimated. This is in part because it is much more difficult to shield any wind regions effectively when the outflow itself is allowed to reprocess and redirect ionizing photons. As a result, the calculated spectrum that would be observed from this particular outflow solution would not contain the ultraviolet spectral lines that are observed in many AGN/QSOs. Furthermore, the wind is so highly ionized that line-driving would not actually be efficient. This does not necessarily mean that line-driven winds are not viable. However, our work does illustrate that in order to arrive at a self-consistent model of line-driven disk winds in AGN/QSO, it will be critical to include a more detailed treatment of radiative transfer and ionization in the next generation of hydrodynamic simulations.
We report on the first phase of our study of cloud irradiation. We study irradiation by means of numerical, two-dimensional time-dependent radiation-hydrodynamic simulations of a cloud irradiated by a strong radiation. We adopt a very simple treatmen
t of the opacity, neglect photoionization and gravity, and instead focus on assessing the role of the type and magnitude of the opacity on the cloud evolution. Our main result is that even relatively dense clouds that are radiatively heated (i.e., with significant absorption opacity) do not move as a whole instead they undergo a very rapid and major evolution in its shape, size and physical properties. In particular, the cloud and its remnants become optical thin within less than one sound crossing time and before they can travel over a significant distance (a distance of a few radii of the initial cloud). We also found that a cloud can be accelerated as a whole under quite extreme conditions, e.g., the opacity must be dominated by scattering. However, the acceleration due to the radiation force is relatively small and unless the cloud is optically thin the cloud quickly changes its size and shape. We discuss implications for the modelling and interpetation broad line regions of active galactic nuclei.
A simple 1D dynamical model of thermally driven disc winds is proposed, based on the results of recent, 2.5D axi-symmetric simulations. Our formulation of the disc wind problem is in the spirit of the original Parker (1958) and Bondi (1952) problems,
namely we assume an elementary flow configuration consisting of an outflow following pre-defined trajectories in the presence of a central gravitating point mass. Viscosity and heat conduction are neglected. We consider two different streamline geometries, both comprised of straight lines in the (x,z)-plane: (i) streamlines that converge to a geometric point located at (x,z)=(0,-d) and (ii) streamlines that emerge at a constant inclination angle from the disc midplane (the x-axis, as we consider geometrically thin accretion discs). The former geometry is commonly used in kinematic models to compute synthetic spectra, while the latter, which exhibits self-similarity, is likely unused for this purpose, although it easily can be with existing kinematic models. We make the case that it should be, i.e. that geometry (ii) leads to transonic wind solutions with substantially different properties owing to its lack of streamline divergence. Both geometries can be used to complement recent efforts to estimate photoevaporative mass loss rates from protoplanetary discs. Pertinent to understanding our disc wind results, which are also applicable to X-ray binaries and active galactic nuclei, is a focused discussion on lesser known properties of classic Parker wind solutions. We find that the parameter space corresponding to decelerating Parker wind solutions is made larger due to rotation and leads instead to disc wind solutions that always accelerate after the bulk velocity is slowed to a minimum value. Surprisingly, Keplerian rotation may allow for two different transonic wind solutions for the same physical conditions.
We review the main results from axisymmetric, time-dependent hydrodynamical simulations of radiation driven disk winds in AGN. We illustrate the capability of such simulations to provide useful insights into the three domains of observational astrono
my: spectroscopy, time-variability, and imaging. Specifically, the synthetic line profiles predicted by the simulations resemble the broad absorption lines observed in quasars. The intrinsically time dependent nature of radiation driven disk winds that have been predicted by the simulations can be supported by a growing number of the observed dramatic variability in the UV absorption lines. And finally, the intensity maps predicted by the simulations give physical and geometrical justification to the phenomenologically deduced fact that a proper interpretation of the observed line absorption requires the wind covering factor to be considered as being partial, inhomogeneous, and velocity dependent.
We summarize the results from numerical simulations of mass outflows from AGN. We focus on simulations of outflows driven by radiation from large-scale inflows. We discuss the properties of these outflows in the context of the so-called AGN feedback
problem. Our main conclusion is that this type of outflows are efficient in removing matter but inefficient in removing energy.
We present a brief summary of the main results from our multi-dimensional, time-dependent simulations of gas dynamics in AGN. We focus on two types of outflows powered by radiation emitted from the AGN: disk winds and winds driven from large-scale in
flows. We show spectra predicted by the simulations and discuss their relevance to observations of broad- and narrow-line regions of the AGN. We finish with a few remarks on whether these outflows can have a significant impact on their environment and host galaxy.
We study the axisymmetric, time-dependent hydrodynamics of rotating flows that are under the influence of supermassive black hole gravity and radiation from an accretion disk surrounding the black hole. This work is an extension of the earlier work p
resented by Proga, where nonrotating flows were studied. Here, we consider effects of rotation, a position-dependent radiation temperature, density at large radii, and uniform X-ray background radiation. As in the non-rotating case, the rotating flow settles into a configuration with two components (1) an equatorial inflow and (2) a bipolar inflow/outflow with the outflow leaving the system along the pole. However, with rotation the flow does not always reach a steady state. In addition, rotation reduces the outflow collimation and the outward flux of mass and kinetic energy. Moreover rotation increases the outward flux of the thermal energy and can lead to fragmentation and time-variability of the outflow. We also show that a position-dependent radiation temperature can significantly change the flow solution. In particular, the inflow in the equatorial region can be replaced by a thermally driven outflow. Generally, as it have been discussed and shown in the past, we find that self-consistently determined preheating/cooling from the quasar radiation can significantly reduce the rate at which the central BH is fed with matter. However, our results emphasize also a little appreciated feature. Namely, quasar radiation drives a non-spherical, multi-temperature and very dynamic flow. These effects become dominant for luminosities in excess of 0.01 of the Eddington luminosity.
