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Register a new userAccretion disk winds in active galactic nuclei: X-ray observations, models, and feedback
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Added by
Francesco Tombesi PhD
Publication date
2016
fields
Physics
and research's language is
English
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Powerful winds driven by active galactic nuclei (AGN) are often invoked to play a fundamental role in the evolution of both supermassive black holes (SMBHs) and their host galaxies, quenching star formation and explaining the tight SMBH-galaxy relations. A strong support of this quasar mode feedback came from the recent X-ray observation of a mildly relativistic accretion disk wind in a ultraluminous infrared galaxy (ULIRG) and its connection with a large-scale molecular outflow, providing a direct link between the SMBH and the gas out of which stars form. Spectroscopic observations, especially in the X-ray band, show that such accretion disk winds may be common in local AGN and quasars. However, their origin and characteristics are still not fully understood. Detailed theoretical models and simulations focused on radiation, magnetohydrodynamic (MHD) or a combination of these two processes to investigate the possible acceleration mechanisms and the dynamics of these winds. Some of these models have been directly compared to X-ray spectra, providing important insights into the wind physics. However, fundamental improvements on these studies will come only from the unprecedented energy resolution and sensitivity of the upcoming X-ray observatories, namely ASTRO-H (launch date early 2016) and Athena (2028).
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This letter presents a revised radiative transfer model for the infrared (IR) emission of active galactic nuclei (AGN). While current models assume that the IR is emitted from a dusty torus in the equatorial plane of the AGN, spatially resolved observations indicate that the majority of the IR emission from 100 pc in many AGN originates from the polar region, contradicting classical torus models. The new model CAT3D-WIND builds upon the suggestion that the dusty gas around the AGN consists of an inflowing disk and an outflowing wind. Here, it is demonstrated that (1) such disk+wind models cover overall a similar parameter range of observed spectral features in the IR as classical clumpy torus models, e.g. the silicate feature strengths and mid-IR spectral slopes, (2) they reproduce the 3-5{mu}m bump observed in many type 1 AGN unlike torus models, and (3) they are able to explain polar emission features seen in IR interferometry, even for type 1 AGN at relatively low inclination, as demonstrated for NGC3783. These characteristics make it possible to reconcile radiative transfer models with observations and provide further evidence of a two-component parsec-scaled dusty medium around AGN: the disk gives rise to the 3-5{mu}m near-IR component, while the wind produces the mid-IR emission. The model SEDs will be made available for download.
To explain X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave observations of nearby Seyferts establish the existence of weak non-thermal coronal activity. Besides, the IceCube collaboration reported NGC 1068, a nearby Seyfert, as the hottest spot in their 10-yr survey. These pieces of evidence are enough to investigate the non-thermal perspective of AGN coronae in depth again. This article summarizes our current observational understandings of AGN coronae and describes how AGN coronae generate high-energy particles. We also provide ways to test the AGN corona model with radio, X-ray, MeV gamma-ray, and high-energy neutrino observations.
We briefly review the synergy between X-ray and infrared observations for Active Galactic Nuclei (AGNs) detected in cosmic X-ray surveys, primarily with XMM-Newton, Chandra, and NuSTAR. We focus on two complementary aspects of this X-ray-infrared synergy (1) the identification of the most heavily obscured AGNs and (2) the connection between star formation and AGN activity. We also briefly discuss future prospects for X-ray-infrared studies over the next decade.
X-ray variation is a ubiquitous feature of active galactic nuclei (AGNs), however, its origin is not well understood. In this paper, we show that the X-ray flux variations in some AGNs, and correspondingly the power spectral densities (PSDs) of the variations, may be interpreted as being caused by absorptions of eclipsing clouds or clumps in the broad line region (BLR) and the dusty torus. By performing Monte-Carlo simulations for a number of plausible cloud models, we systematically investigate the statistics of the X-ray variations resulting from the cloud eclipsing and the PSDs of the variations. For these models, we show that the number of eclipsing events can be significant and the absorption column densities due to those eclipsing clouds can be in the range from 10^{21} to 10^{24} cm^{-2}, leading to significant X-ray variations. We find that the PSDs obtained from the mock observations for the X-ray flux and the absorption column density resulting from these models can be described by a broken double power law, similar to those directly measured from observations of some AGNs. The shape of the PSDs depend strongly on the kinematic structures and the intrinsic properties of the clouds in AGNs. We demonstrate that the X-ray eclipsing model can naturally lead to a strong correlation between the break frequencies (and correspondingly the break timescales) of the PSDs and the masses of the massive black holes (MBHs) in the model AGNs, which can be well consistent with the one obtained from observations. Future studies of the PSDs of the AGN X-ray (and possibly also the optical-UV) flux and column density variations may provide a powerful tool to constrain the structure of the BLR and the torus and to estimate the MBH masses in AGNs.
We consider a sample of type-I active galactic nuclei (AGN) that were observed by Chandra/HETG and resulted in high signal-to-noise grating spectra, which we study in detail. All objects show signatures for very high ionization outflows. Using a novel scheme to model the physics and spectral signatures of gaseous winds from these objects, we are able to estimate the mass loss rates and kinetic luminosities associated with the highly ionized gas and investigate its physical properties. Our conclusions are as follows: 1) There is a strong indication that the outflowing gas in those objects is multi-phase with similar kinematics for the different phases. 2) The X-ray spectrum is consistent with such flows being thermally driven from ~pc scales, and are therefore unlikely to be associated with the inner accretion disk. 3) The underlying X-ray spectrum consists of a hard X-ray powerlaw which is similar for all objects shining below their Eddington rate and a soft excess whose contribution becomes more prominent for objects shining close to their Eddington limit. 4) The physical properties of the outflow are similar in all cases and a coherent picture emerges concerning its physical properties. 5) The deduced mass loss rates are, roughly, of the order of the mass accretion rate in those objects so that the kinetic luminosity carried by such winds is only a tiny fraction (<<1%) of the bolometric luminosity. We discuss the implications of our results for AGN structure and AGN interaction with the environment.
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