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Physics of Warm Absorber in AGN
Abstracts and presentations (6 Dec 2005)

The abstracts and presentations (in various formats) are available here

Darrin Casebeer et al. (The University of Oklahoma) (pdf file: 2.5mb)
Spectral Synthesis Models for AGN Winds and the Warm Absorber Using PHOENIX
I will describe the general purpose radiative transfer code PHOENIX and its advantages for synthesis of AGN spectra. I will present current model results for optical-FUV spectra of FeLoBAL quasars. I will describe work in progress to obtain accurate accelerations for general AGN wind models. I will also briefly discuss the prospects for using PHOENIX to model the warm absorber.
Loïc Chevalier et al. (LUTH, Observatoire de Paris) (pdf file: 528kb)
The puzzle of the soft X-ray excess in AGN: absorption or reflection?
The 2-10 keV continuum of AGN is generally well represented by a single power law. However, at smaller energies the continuum displays an excess with respect to the extrapolation of this power law, called the `soft X-ray excess´. Until now this soft X-ray excess was attributed, either to reflection of the hard X-ray source by the accretion disk, or to the presence of an additional comptonizing medium, giving steep spectrum. An alternative solution proposed by Gierlinski and Done (2004) is that a single power law well represents both the soft and the hard X-ray emission and the impression of the soft X-ray excess is due to absorption of a primary power law by a relativistic wind. We examine the advantages and drawbacks of the reflection versus absorption models, and we conclude that the observed spectra can be well modeled, either by absorption (for a strong excess), or by reflection (for a weak excess). However the physical conditions required by the absorption models do not seem very realistic.
Suzy Collin (LUTH, Observatoire de Paris) (ppt file: 689kb) HTML
The inner region of AGN
I will recall some basic properties of the different constituants of the inner parts of an AGN inferred from the observations: the accretion disc, the BLR, the torus, the wind and the Warm Absorber, the NLR, and I will present my own vision of the inter-relations between these constituants.
Elisa Constantini et al. (High Energy Astrophysics Division, SRON) (ppt file: 628kb)
The black hole environment of the Seyfert 1 Mrk 279 seen in X-rays
Thanks to the high resolution spectrometer on board of Chandra and XMM-Newton, many new features, likely to arise from the innermost part of the AGN, are now detected in a number of objects. We present the results of a long (350 ks) exposure observation of the bright Seyfert 1 Mrk 279, performed using Chandra-LETGS. The spectrum of Mrk~279 displays absorption by ionized gas and a variety of emission lines. In particular, broad emission structures are seen in particular around OVIII Ly-alpha and OVII triplet.
Using simultaneous HST and FUSE measurement, we model these features as arising from a Broad Line Region, located ~1016 cm from the source. We detect at least two main components for the out-flowing ionized gas. They are found to be well distinct in terms of gas velocity and ionization parameter. We discuss the possible response of these components to the primary continuum changes, and show different scenarios for the location and structure of the warm absorber.
Chris Done (University of Durham) (ppt file: 4.8mb) (pdf file: 9.2mb) HTML
Ionised absorption in X-ray binaries
The ´warm absorber´ is a well known component in AGN spectra. Here I will look at the evidence for ionised absorption in X-ray binaries, and the effect that narrow absorption line systems can have on the derived properties of the relativistic iron line.
Marian Fenovcik et al. (High Energy Astrophysics Division, SRON) (pdf file: 480kb)
High resolution X-ray spectroscopy of the narrow line Seyfert I NGC 4051 with Chandra
We present two data sets of the narrow line Seyfert 1 galaxy NGC 4051, one of the most variable AGN, observed with Chandra-LETGS. During the first observation, on December 31, 2002, the source was in a low flux state and at about halfway of the exposure time, it increased to a somewhat higher level. On July 23, 2003 the source was rapidly variable from a high flux level to a quiescent, low flux, level. In both observations the spectrum is rich in absorption and emission features. In the earlier observation we detect two X-ray absorption systems of photo-ionized gas with different ionization states and outflow velocities. In the second observation, thanks to the higher S/N ratio, we can resolve the lower ionization component, seen in the first observation, in two components with different outflow velocities. After a rapid (< 2000 s) transition from high to low flux level we detect only one absorption component, however other absorption systems cannot be ruled out. In this quiescent part the emission spectrum shows strong Radiative Recombination Continua (RRCs). The fast response of the RRCs to the continuum flux change indicates that these features must originate very close to the central source. However, the lack of significant velocity broadening of the RRCs requires a very narrow opening angle for the emitting gas.
Anabela C. Goncalves et al. (LUTH, Observatoire de Paris) (ppt file: 1.7mb)
Modelling the Warm Absorber in NGC 3783 under pression equilibrium conditions
Many AGN exhibit X-ray absorption features caused by the presence of highly ionized gas located on the line of sight of the central continuum. Such a material is called "Warm Absorber" (WA) and should be stratified, displaying zones of different density, temperature and ionization. Our approach to the study of the WA relies on the assumption of pressure equilibrium, resulting in the natural stratification of the medium, which allows to explain the presence of lines from different ionization states in many AGN observed by Chandra and XMM-Newton. Among the best WA observations available are those of NGC 3783, which we have analyzed. We have used the TITAN code, developed by our team, to calculate a constant pressure grid of models dedicated to fit the WA in NGC 3783. Our study shows that the WA can be modeled under constant pressure conditions. Finally, this work provides a good example of the application of the TITAN code to the study of the Warm Absorber in AGN and opens perspectives for the future use of the code by a larger community, through a larger grid of models to be made available.
Jelle Kaastra (High Energy Astrophysics Division, SRON) (ppt file: 956kb)
The warm absorber in NGC 5548
I will summarize the results that we obtained from our analysis of the warm absorber in NGC 5548 using both Chandra and XMM-Newton data. A particular focus will be put on the ionisation distribution and the constraints on time variability. I will also present preliminary results from a recent Chandra LETGS observation that was obtained when the source is in a very low state.
Tim Kallman (NASA, Greenbelt) (ppt file: 5.4mb)
Constraints on Continuous AGN Warm Absorber Flows
Chandra and XMM grating spectra provide the best evidence and the strongest constraints so faron the nature of AGN warm absorber flows. Detailed work has been done on fitting and interpreting these spectra using photoionization models consisting of a small number of discrete velocity components, providing important insight into the nature of the absorbing gas. In this talk I will discuss an alternate scenario, which has received less attention so far, in which the absorption occurs in a gas flow which is intrinsically continuous in velocity space. I will show tests of this scenario by fitting synthetic spectra to observed data such as the 900 ksec Chandra observation of NGC 3783, and will derive constraints on the parameters describing the flow.
Shai Kaspi (Physics Department, Technion) (ppt file: 8.2mb) (pdf file: 6.2mb) HTML
X-ray absorption and high-velocity outflows in AGNs - a second look
The phenomenon of outflowing mass from the cores of AGNs has been well established by now with typical velocities of few hundreds to few thousands km/s. Recent X-ray studies suggested that high-velocity outflows, reaching a significant fraction of the speed of light, are seen in several objects. We reanalyze the the high resolution X-ray spectrum of the quasar PG 1211+143 as observed by the CCD and grating spectrometers on board XMM-Newton and suggest an alternative interpretation. Using an ion by ion fitting method we find an outflow component of about 3000 km/s identified by absorption lines of H- and He-like ions of C, N, O, Ne, Mg, and S as well as lines of lower ionization states of O and Mg, and L-shell lines of Si, S, Ar, and Fe. This result is in contrast with the ultra-high velocity of about 24000 km/s reported in an earlier study for this source based on the same data set, and has implication for the mass outflow rate. We will also explore the possibility of an alternative interpretation to other X-ray observations of high-velocity absorbers and the evidence for several outflowing WA velocity components in AGNs.
Karen Leighly (The University of Oklahoma)
AGN Winds: Causes and Consequences
Absorption lines are the most commonly cited evidence for winds from AGN. Blueshifted emission lines indicate the presence of winds as well. In this talk I will describe my work on optical and UV spectra of Narrow-line Seyfert 1s. I find that the blueshifted emission lines are associated with steep alpha_ox, a fact that supports resonance-line driving as the acceleration mechanism. Enhanced low-ionization line emission (such as SiII and FeII) is frequently observed in objects with blueshifted high-ionization lines; this may be explained if the continuum is filtered through the wind before it illuminates the low-ionization line-emitting gas. Current work and future prospects will be discussed.
Fabrizio Nicastro et al. (Harvard-Smithsonian Center for Astrophysics) (ppt file: 1.3mb)
Warm Absorbers in Active Galactic Nuclei are Compact and Thin Accretion-Disk Winds Playing an Important Role in the IGM-Galaxy Feedback Processes
Daniel Proga (University of Nevada) (ppt file: 1.7mb) (pdf file: 6.1mb)
Warm Absorbers in AGN: are they disk outflows?
Warm absorbers are likely associated with mass outflows in AGN. Magnetic fields, the radiation force, and thermal expansion have been suggested as mechanisms that can drive AGN outflows. These three outflow mechanisms have been studied extensively using analytic as well as numerical methods.I briefly review these studies. In particular, I present results from numerical hydrodynamic and magnetohydrodynamic simulations of outflows from accretion disks. The simulations allow us to gain interesting insights into many outflow properties, e.g., dynamics, geometry, and ionization structure. I conclude that it is unlikely that one model can account for all warm absorbers because they are observed in AGN with so different properties. In particular, outflows driven from an accretion disk by radiation pressure can account for warm absorbers observed in very luminous systems but a different kind of outflows such as torus winds are required for less luminous systems.
Agata Różańska (CAMK) (pdf file: 1.2mb)
Stratified warm absorbers in AGN
The new grid of models for a structure and transmission through the warm cloud under constant pressure will be described and compared with observations. Unlikely to the clouds being under constant density, we assume the warm absorber under constant pressure. So, the absorbing gas is strongly stratified in density and temperature. We calculate full radiative transfer of continuum and lines including Compton scattering. Therefore, equivalent widths of both saturated and unsaturated lines are properly determined. In all models we note considerable absorption around 6.4 keV which modifies intrinsic relativistically broadened iron line profile originating in accretion disk illuminated atmosphere. Our models can be applied to fitting the spectroscopic data from XMM and Chandra satellites.
Nick Schurch (University of Durham) (ppt file: 13mb)
AGN; the answer is blowing in the wind
For any individual AGN, a detailed understanding of its absorption properties is essential if we are to understand the broad-band X-ray spectrum and, in particular, the underlying X-ray continuum emission mechanism. Focussing on ionised outflowing gas, I will argue that careful simulations of this material, and detailed calculations of its effects on the X-ray spectrum, are both needed to correctly characterize this complex region. Following this, I will discuss the details of the X-ray spectrum that results from the best available theoretical simulations of the messy environment of an AGN and how these calculations and simulations can be improved.
Aneta Siemiginowska (Harvard-Smithsonian Center for Astrophysics) (ppt file: 3.6mb) HTML
Nature of absorbers in Young Radio Sources
Compact radio sources with a convex radio spectrum are thought to be the precursors of large radio galaxies. Their morphology is very similar to the typical one for a radio galaxy, with double structures and jets, but on much smaller scale. The entire radio source is usually contained within the host galaxy. The evidence for a young age comes from measurements of the expansion velocity of the radio components in several Compact Symmetric Objects that indicate the ages from a few hundred to a few thousand years. While recent X-ray observations indicate that the compact sources are highly obscured we still do not understand the nature of the absorbing gas and the role it plays in the evolution of a radio source. Here, we present results of the X-ray observations of a sample of compact, young radio sources at redshift < 1 performed by Chandra and XMM-Newton. These observations more than double the number of members of this class with measured spectra in hard (E > 2keV) X-rays. Our sample contains both quasars and galaxies. We measure large absorbing columns in all galaxies. Comparison between the HI 21 cm absorption columns with the measured X-ray columns indicate that the absorber is ionized. Comparison between the measured columns in the other radio loud galaxies leads to locating the absorbing gas in an obscuring ``torus´´, which prevents us from observing the nuclear region along lines-of-sight perpendicular to the radio axis. The quasars do not show significant absorption. We discuss the implication of detected absorption on our understanding of the evolution of a radio source.
Martin Ward (University of Durham) (ppt file: 1.2mb) (pdf file: 1.6mb) HTML
The Warm Absorber, High Excitation Emission, Lines and Outflows
It is been proposed that the warm absorber, which is detected in a high fraction of AGN, and the high excitation so-called coronal emission lines are intimately related. If so, then the properties of these lines eg. their ratios, profiles and velocity shifts can provide us with important addition information about this zone. I will discuss the current state of our knowledge of the coronal lines, and the relationship to outflows.
Tahir Yaqoob (Johns Hopkins University) (ppt file: 4.3mb)
Chandra Grating Observations of AGN
Observations of active galactic nuclei (AGN) with Chandra and XMM, combined with UV studies and improved theoretical models have resulted in new insights into the nature of the photoionzed, apparently outflowing, absorbers in many AGN. We review our understanding of the so-called warm absorbers from the observations thus far, with particular reference to high-resolution X-ray spectroscopy using grating data. We also describe a public archive of reduced and reprocessed data products from Chandra grating data (HotGAS: Home of the Grating Archive Spectra). The database is set up to easily enable even non-experts in X-ray spectroscopy to use, model, and interpret the data.

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See also:

» Abstract of the workshop (Physics of Warm Absorber in AGN)
» The list of participants (Physics of Warm Absorber in AGN)
» Workshop programme (Physics of Warm Absorber in AGN)

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