High Energy Astrophysics

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High Energy Astrophysics

• Active Galactic Nuclei

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Introduction

• Active Galactic Nuclei (AGN) are powerful
  sources of radiation which exist in the centre of
  1-10 of all galaxies
• Galaxies which host an AGN are known as active
  galaxies
• The span of observed AGN luminosities is huge
  L1040 1047 erg/s
• The more luminous AGN outshine their host
  galaxies by factors of 1000 or more
• AGN are the most luminous long-lived objects in
  the universe

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AGN Zoology

• Radio galaxies
• Radio Quasars
• BL Lac Objects
• Optically Violent Variables (OVVs)
• Radio Quiet Quasars (QSOs)
• Seyfert I galaxies (SyI)
• Seyfert II galaxies (SyII)
• Low Ionization Nuclear Emission-Line Regions
  (LINERS)

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AGN classifiction
Radio-quiet AGN classification
Seyfert I galaxies/ radio-quiet quasars
Seyfert II galaxies
Narrow line Seyfert I galaxies (NLS1)
Narrow Emission Line galaxies (NELGs, LINERS)
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AGN classifiction
Radio-Loud AGN classification
FR-I (Fanaroff-Riley-I) radio galaxies
FR-II radio galaxies
Radio-loud quasars
Blazars
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The supermassive black hole model

• The current paradigm is that the fundamental
  power source of all AGN is accretion onto a
  supermassive black hole
• Some theoretical models predict that there is a
  massive black hole at the center of most galaxies
  due to processes probably related to galaxy
  formation
• In the standard model, the accretion disk is
  geometrically thin and radiatively efficient
• In some instances, there may be advection
  dominated disks that radiatively inefficient

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The supermassive black hole model

• Data supporting the existence of supermassive
  black holes

• The width of the broad optical/UV emission lines
  
• The stellar kinematics in the central regions of
  many galaxies
• The large amplitude variability of X-ray
  emission
• The efficiency in which matter is converted into
  energy
• Balance between accretion flow and radiation
  pressure
• Masers
• The X-ray fluorescent Ka emission line

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AGN unification

• The central feature of the unification schemes
  is that the observed properties, and thus the
  classification, of a given AGN depend upon its
  orientation
• The main ingredients of this scheme are

• A supermassive black hole 106-10 Msolar
• An accretion disk and corona, heated by magnetic
  and/or viscous processes so that it radiates at
  optical through soft X-ray energies
• high velocity gas, often referred as the BLR
• lower velocity gas in the NLR

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AGN unification

• The main ingredients of this scheme are

• an obscuring torus (or other geometrical form)
  of gas and dust, hiding the BLR from some
  directions
• a relativistic jet, formed within 100 Rsch of
  the black hole, and extending outwards for tens
  of kpc, and in some cases as much as a Mpc

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Type 1 AGN SED
X-rays
mm
far-IR
near-IR
Optical-UV
Manners, 2002
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Type 2 AGN SED
X-rays
Radio
far-IR
optical-UV
Norman et al, 2002
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History of X-ray Observations of AGN

• The first successful X-ray observations of AGN
  were performed in the 1960s with rocket and
  balloon experiments. They detected emission from
  the bright QSO 3C273
• The third Uhuru catalog (Giacconi et al 1974)
  identified X-ray emission from the 3 brightest
  AGN in the sky 3C273, NGC 4151, Cen-A
• The Ariel-V sky survey established the ubiquity
  of X-ray emission from Seyfert I galaxies
• The first X-ray spectral result from OSO-7 and
  Uhuru were published in the mid 1970s

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History of X-ray Observations of AGN

• The first measurements of X-ray variability came
  from OSO-7 and Copernicus in the mid 1970s
• Ariel-V showed that the X-ray variability on
  timescales greater than one day was a common
  property of AGN
• Improved X-ray spectra of NGC 4151 and Cen-A
  from OSO-8 and Ariel-V established the power-law
  shape of the X-ray continuum, measured a
  significant low energy roll-over due to
  photoelectric absorption (NHgt1022 atoms/cm2) and
  detected the 6.4 keV Fe line in emission in both
  objects

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History of X-ray Observations of AGN

• The first large spectral samples of AGN,
  obtained with the HEAO-1 satellite (early
  1980s), were well modeled by a power law in the
  2-20 keV band, with little intrinsic absorption,
  and the observed range of photon spectral indexes
  were narrow and centered on G1.7

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History of X-ray Observations of AGN

• The HEAO-1 mission also performed the first
  extended and sensitive search for variability,
  finding very few objects that exhibited rapid
  (shorter than a few hours) large amplitude (a
  factor of 2) changes

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History of X-ray Observations of AGN

• The Einstein observatory (HEAO-2) increased the
  known number of X-ray AGN by more than an order
  of magnitude
• The imaging proportional counter showed that AGN
  of all types were powerful X-ray sources
  detecting 1000
• The solid state spectrometer gave the first
  higher quality spectra of AGN and confirmed the
  narrow distribution of spectral slopes around
  G1.7 in the 0.7-4 keV band
• However, some of the lower luminosity Seyferts
  were not well described by a single power law
  with absorption by cold material

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History of X-ray Observations of AGN

• Einstein low resolution spectra with its imager
  (IPC) showed that at lower energies 0.2-4 keV the
  spectral slope presented higher variations and
  was not so uniform as at higher enegies 2-20 keV

Wilkes Elvis 1987
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History of X-ray Observations of AGN

• EXOSAT, launched in 1983, provided the first
  detailed temporal studies of large samples of
  Seyfert galaxies. The data showed that variations
  were stochastic, with no characteristic time
  scale
• There was a large amplitude variability in
  Seyfert galaxies (in contrast with the HEAO-1
  results)
• The broad energy coverage resulted in the
  discovery that a sharp rise at low energy, the
  so-called soft excess, occurs in the spectra of
  50 of Seyfert galaxies

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Arnaud et al 1985
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History of X-ray Observations of AGN

• EXOSAT, launched in 1983, provided the first
  detailed temporal studies of large samples of
  Seyfert galaxies. The data showed that variations
  were stochastic, with no characteristic time
  scale
• There was a large amplitude variability in
  Seyfert galaxies (in contrast with the HEAO-1
  results)
• The broad energy coverage resulted in the
  discovery that a sharp rise at low energy, the
  so-called soft excess, occurs in the spectra of
  50 of Seyfert galaxies
• The power-law index distribution in the energy
  range 2-20 keV is found uniform as previously
  found

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Turner Pounds 1989
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History of X-ray Observations of AGN

• Ginga, launched in 1987, data allowed detailed
  spectral decomposition to be performed for the
  first time, showing that a significant fraction
  of all Seyfert galaxies exhibit Fe 6.4 keV lines
  and at a level that is much more intense than can
  be explained by the general small columns of
  absorbing material in the line of sight

Pounds et al 89
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History of X-ray Observations of AGN

• Ginga data also showed a spectral flattening at
  Egt8 keV which could be most readily attributed to
  the reprocessing of a (considerable) fraction of
  the incident X-ray flux by cold material near the
  central engine, either totally or partially out
  of the line of sight

Lightman White 88
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History of X-ray Observations of AGN

• Ginga obtained the first crude spectra of a
  significant sample of quasars in the 2-20 keV
  band (Williams et al 1992)

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History of X-ray Observations of AGN

• ROSAT, launched in 1990 and with good soft
  energy sensitivity carried out the ROSAT All Sky
  Survey in which tens of thousands of AGN were
  detected. At low fluxes the fraction of NELG
  increses compared to type I AGN
• ROSAT provide detailed studies of the soft X-ray
  excess and made a clear connection of this excess
  to the width of the permitted lines (NLS1)

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History of X-ray Observations of AGN

• ASCA with its superior spectral resolution
  resolved the Fe K line into components. In
  particular it showed a broad component in SyIs
  which was a dynamical signature of the innermost,
  relativistic accretion disk around the
  supermassive black hole
• ASCA measured narrower and stronger Fe K line
  from Sy IIs as expected from the unified model
  predictions

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Nandra et al 1996
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History of X-ray Observations of AGN

• ASCA also provided spectral evidence for
  intrinsic warm absorption

Otani et al 96
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History of X-ray Observations of AGN

• RXTE, launched in 1995, has provided the best
  temporal resolution of both the line and
  continuum variability
• The Fe K line variability is not universal and
  does not correlate with the continuum
• BeppoSAX, launched in 1996, have the best hard
  energy response and have been fundamental in the
  study of blazars and the hard X-ray-soft gamma
  ray connection
• Chandra XMM

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History of X-ray Observations of AGN

• Chandra XMM have brought the best spatial and
  spectral resolution and collecting area
• They have acquired the deepest X-ray images with
  which the X-ray background is resolved
• They are also providing detailed data to
  constrain the AGN structure

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X-ray Spectra of AGN
Overview

• To first order the 2-50 keV spectrum of the
  great majority of Seyfert I galaxies can be
  characterized by a simple power law of the form
  F(E) A E-G ph/cm2/s/keV
• The X-ray spectra are continuum dominated
• The strongest broad spectral feature in the
  range 0.1-100 keV is low energy absorption due to
  photoelectric absorption of cold or partly
  ionized material in the line of sight to the
  nucleus

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X-ray Spectra of AGN
Overview

• Fe is the most abundant heavy element and its
  large absorption cross section at high energies,
  combine with a high fluorescent yield, make that
  the strongest spectral features at Egt6 keV are
  due to Fe. These include the Fe fluorescent
  emission at 6.4 keV (the only strong spectral
  line from near-neutral (or cold) material),
  He-like and H-like lines at 6.7 and 6.97 keV from
  ionized material

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X-ray Spectra of AGN
Overview

• In the 0.3-3 keV band, the spectral features are
  due to K shell transitions of O, Mg, Si, and S,
  and L shell transition of Fe
• However, since these elements have low
  fluorescent yields, the strongest spectral
  features at low energy from cold material are due
  to absorption. Highly ionized material have both
  emission and absorption features across the
  entire X-ray spectral band. The strongest
  features in both emission and absorption at Elt2
  keV are due to O K and Fe L shell transitions

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X-ray Spectra of AGN
Seyfert I
Continuum

• Simple fits to power-law spectra over the 2-20
  keV band give photon number indices that are
  narrowly distributed around G1.7 with a 1 s
  width of 0.13 which is represent a true
  dispersion in spectral slopes around the mean.
  This spectral slope appears to be an average of a
  intrinsic steeper spectrum G2.0 and an
  additional flatter component which is produced as
  a result of reprocessing in material close to the
  central source

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X-ray Spectra of AGN
Seyfert I
Continuum

• The spectrum of the X-ray background provides an
  integral constraint on the total flux from AGN.
  Since AGN make most of the XRB, the spectrum of
  the average AGN must steepen at energies above 60
  keV in the observers frame in order not to
  exceed the XRB

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X-ray Spectra of AGN
Seyfert I
Fe K emission lines

• The 6.4 keV Fe emission line due to fluorescence
  of cold material is present on most Seyfert I
  galaxies
• The distribution of EW is quite broad 50ltEWlt350
  eV
• There is no apparent correlation between the
  line strength and the line-of-sight column
  density
• The line at 6.4 keV comes from fluorescence from
  cold material with low velocities
• There is no sign of a 6.7 keV thermal Fe line

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X-ray Spectra of AGN
Seyfert I
Fe K emission lines

• BLRG show also Fe K lines

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X-ray Spectra of AGN
Seyfert I
Features at high energies

• Apart from the 6.4 Fe line there is a decrement
  between 7-8 keV and a flattening of the spectrum
  at higher energies
• The mean flattening between 2-10 keV and 10-18
  keV is ?G0.5

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X-ray Spectra of AGN
Seyfert I
Interpretation

• The Fe K line and hard tail features are most
  probably due to the reprocessing (or reflection)
  of primary X-rays in optically thick material
  subtending a substantial solid angle to the X-ray
  source, possibly the accretion disk
• The reflection albedo is energy dependent, at
  Elt2 keV, there are many elements (C,N,O) that
  will photoelectrically absorb any incident flux,
  so few photons are reflected

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X-ray Spectra of AGN
Seyfert I
Interpretation

• However, the albedo increases at higher enegies
  due to the decreasing abundance of the higher Z
  elements required for photoelectric absorption
• Fe is the last abundant element that can
  significantly affect the probability of
  reflection, leading to a marked
  pseudo-absorption feature in the reflected flux
  from 7.1-9 keV, together with the associated Fe
  Ka fluorencent line with a predicted equivalent
  width of 150 keV

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X-ray Spectra of AGN
Seyfert I
Interpretation

• At higher energies, Compton down-scattering and
  the reduction of the scattering cross-section
  deplete the number of photons reflected,
  resulting in a broad band spectral bump, peaking
  at 20-30 keV

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X-ray Spectra of AGN
Seyfert I
Interpretation

• This Compton reflection model introduces four
  new free parameters into a spectral fit the
  fraction of all photons that are reflected by the
  scatterer (essentially the solid angle covered by
  the scatterer), the inclination of the scatterer
  to the line of sight, the element abundance, and
  the ionization state of the material

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X-ray Spectra of AGN
Seyfert I
Interpretation

• Observationally, the covering fractions of the
  material are large and there is an intrinsic
  slope of G1.9 which translates into an observed
  G1.7 when the Compton reflection component is
  taken into account. The distribution is narrow
  implying a strong coupling between the spectral
  index and the fraction of solid angle occupied by
  the reflector. This could be a selection effect
  where most of SyI are observed with small
  inclination angles, consistent with the unified
  models

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X-ray Spectra of AGN
Seyfert I
Interpretation

• The Fe K line can provide information about
  these selection effects. If the material is in
  the form of an accretion disk, Doppler and
  gravitational broadening will combine to produce
  a characteristic skewed line profile, where the
  detailed line shape depends on the inclination
  and the emissivity law of the disk
• Typically such a line will be broad, with a
  Gaussian width of at least several hundred eV

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Fabian et al 1989
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X-ray Spectra of AGN
Seyfert I
Interpretation

• In principle measurements of the Fe K line
  profile can determine the distance of the line
  emitting region from the central object, in units
  of the Schwartzschild radius, and thus prove
  the existence of a compact massive object
• Similarly, an absorption edge will be distorted
  by these effects

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X-ray Spectra of AGN
Seyfert I
Interpretation

• An earlier competing explanation to a reflection
  disk invoked partial covering of the X-ray source
  by cold, dense material. Only a fraction of the
  photons seen by the observer are absorbed by
  quite thick material while the rest is little
  affected by the absorption.

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X-ray Spectra of AGN
Seyfert I
Excess emission below 2 keV

• There is a soft excess in at least 30 of
  hard-X-ray-selected AGN
• The data indicates that the observed spectrum
  frequently steepens below 2 keV
• This steepening can be rather abrupt
• The steepening normally does not contaminate
  the higher energy X-ray spectrum

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X-ray Spectra of AGN
Seyfert I
Excess emission below 2 keV

• The soft excess has been typically modeled with
  a black body of Tlt150 eV, a steep powerlaw of
  photon index gt 3, or by a thermal bremmstrahlung
  spectrum of kTlt500 eV
• The soft excess does not show a uniform pattern
  and several mechanisms can be at play. In
  particular, some objects are better explained by
  line-like features than a steep continuum

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X-ray Spectra of AGN
Seyfert I
Excess emission below 2 keV

• ROSAT has shown that the narrow line Seyfert I
  galaxies (NLS1) have generally strong soft excess
  components compared to Sy1 with broader optical
  permitted lines. When simple power law models are
  fit to the data, photon values reach values up to
  about 5, much higher than the photon index of
  around 2 seen in Sy1
• A clear anticorrelation is found between the
  ROSAT spectral softness and the line width of the
  optical permitted lines

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X-ray Spectra of AGN
Seyfert I
Excess emission below 2 keV

• There is a lack of significant soft X-ray
  absorption in NLS1
• NLS1 are highly variable

Boller 00
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X-ray Spectra of AGN
Seyfert I
Absorption

• Although absorption by cold material is typical
  in Sy2 and narrow emission line galaxies, it is
  rare in Sy1
• In order to fit the X-ray spectra with the
  presence of some ionized Fe edges and absorption
  by partially-ionized Fe and O, it is necessary to
  invoke the presence of ionized absorbing
  material, the warm absorber model

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Blustin et al 01
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X-ray Spectra of AGN
Quasars
Continuum

• Quasars present steeper power-law fits G2.0
  than Seyfert Is in the 2-20 keV energy range and
  the spread in G values is higher.
• Radio loud quasar show shallower slopes dG0.2
  than the majority of the radio quiet quasars
• At lower energies the slope steepens somewhat
  G2.5 for radio quiet, but these measurements are
  more difficult (soft excess and galactic column
  density complex spectral structures)

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X-ray Spectra of AGN
Quasars
Fe K emission line

• The Fe K lines appear to be weaker in powerful
  quasars compared to Sy1s

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X-ray Spectra of AGN
Quasars
Reflection

• There is no conclusive evidence for the
  existence of a Compton reflection component in
  the X-ray spectra of radio-quiet quasars

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X-ray Spectra of AGN
Quasars
Soft Excess

• Quasars show a similar soft excess component as
  Seyfert 1s which starts to depart from the
  higherenergy power-law at E0.3-0.8 keV
• This component is fitted with either a power-law
  G3.5-5.0, a high energy tail of a 40-80 eV
  blackbody
• In ROSAT samples the power-law photon index G
  decreases as the redshift of the sources increase
  as the rest-frame soft energy is redshifted out
  of the bandpass

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X-ray Spectra of AGN
Seyfert II
Continuum photoelectric absorption

• Seyfert IIs present a similar continuum to
  Seyfert Is, but there are differences in the
  amount of X-ray absorbing material
• All Sy2 show strong absorption by cold material,
  unlike Sy1s
• Given their high column densities Sy2s are
  fainter than Sy1s, although their unabsorbed
  luminosities are similar to the low luminosity
  Sy1s

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X-ray Spectra of AGN
Seyfert II
Fe K emission

• Seyfert IIs present strong Fe K emission
• The line structure is complex with fluorescent
  and recombination lines
• The distribution of EW is broad

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X-ray Spectra of AGN
Seyfert II
Low energy component

• The low energy component is different from the
  heavily absorbed hard spectrum. The column
  densities implied at high energies would obscure
  the low energy soft excess seen in Sy1s and
  therefore the soft component is not a
  continuation of the hard one
• The soft photons are likely scattered into the
  line of sight, while the hard (Egt4 keV) come
  directly to the observer and that would explain
  the lack of variability of the soft component

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X-ray Spectra of AGN
Seyfert II
Interpretation

• In the standard model the central parts of Sy2s
  are obscured from our direct view by cold
  material (the torus)
• In most Sy2s the torus becomes optically thin
  at E2-4 keV
• The range of observed column densities is too
  large to be produced by different viewing angles
  through identical tori, so there must be a range
  of torus columns in Sy2s

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X-ray Spectra of AGN
Seyfert II
Interpretation

• There has to be a scattering cloud of electrons
  to see the broad optical lines in polarized light
• This scattering region is illuminated by the
  nuclear flux and becomes strongly ionized.
  Therefore it produces little low energy
  absorption but can efficiently scatter low energy
  X-rays and UV radiation. This scatter flux is
  thought to be the origin of the excess soft
  emission seen in Sy2s
• This photoionized scattering region will produce
  emission lines by flourescence and recombination

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X-ray Variability of AGN

• X-ray emission shows the shortest timescale for
  AGN variability in any wavelength band
• A substantial variability cannot be observed on
  timescales shorter than the light crossing time
  of the source. This gives an upper limit to the
  size R lt c?t
• Assuming a source size, this can be translated
  into a mass limit M lt 2 104 ?t Msolar

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X-ray Variability of AGN

• The variability can also be used to derive the
  efficiency to convert matter to luminosity
• A change in luminosity ?L in time ?t implies an
  efficiency of ? 50 ?L/?t (?L 1044 erg/s, ?t s)
  
• Nuclear processes have a maximum efficiency of
  ?0.007 gravitational energy release on
  accretion gives ?0.057 for Schwarzschild or 0.32
  for extreme Kerr metrics. Several AGN require
  efficiencies larger than those produced by
  nuclear processes, providing strong evidence that
  AGN are powered by accretion

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X-ray Variability of AGN

• Rapid, large (?I/I gt1) variations are common in
  Seyfert galaxies
• Short term variability is generally weaker (?I/I
  lt1)
• The light curves are varied and the variability
  seems to be stochastic
• The power spectrum of variability a f-1 in the
  10-3-10-5 Hz range
• Variability tends to be anticorrelated with
  luminosity
• Spectral variability is observed in
  approximately half of the Seyferts observed

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X-ray Variability of AGN

• Variability in absorption features is seldom
  observed as expected from the unified model
• In Seyfert IIs the hard component generally
  varies while the soft one hardly does

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X-ray the unified model of AGN

• At the center there is a massive black hole
• The nonthermal continuum is thought to originate
  from the innermost regions (Rlt20Rsch)
• Close in and surrounding the MBH there is an
  optically thick, geometrically thin, accretion
  disk identified with the Compton reflector and
  the origin of the cold Fe K line in many
  objects
• In a quasi-spherical volume surrounding the MBH
  there is a cloud of hot electrons and ions,
  perhaps the warm absorber responsible for the low
  energy spectral features and high ionization Fe
  features

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X-ray the unified model of AGN

• This material is also the probable origin of the
  6.7 keV line in some SyIIs as well as being the
  innermost part of the electron scattering region
  responsible for the reflection of the SyII
  continuum
• Further out, there are the clouds responsible
  for the broad optical lines and perhaps the
  narrow Fe K lines
• Further out there is a region of thick cold
  material in the shape of a torus which obscures
  the line of sight to the central engine of SyII
  and is thought to be responsible for the high
  X-ray column densities

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X-ray the unified model of AGN

• Still further out lies the narrow emission line
  region responsible for the narrow optical and UV
  lines and the furthest extensions of the electron
  scattering cloud

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Current X-ray AGN research
The nature of the soft X-ray spectral complexity

• The new XMM-Newton Chandra high resolution
  spectra allow to test models thoroughly testing
  our current knowledge of high energy transitions
• At soft energies there are plenty of transitions
• New spectral features have been discovered in
  the spectra of Seyfert galaxies including the
  Unresolved Transition Array (UTA) absorption of
  M-shell iron and relativistically-broadened soft
  X-ray emission lines

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Kahn et al 01
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Current X-ray AGN research
The nature of the soft X-ray spectral complexity

• Some characteristics found in the spectra
  narrow forbidden lines from a distant
  photoionized region strong resonance absorption
  lines and inner shell absorption lines from an
  outflowing, photoionized wind broad emission
  lines consistent with being produced in the BLR
  weak relativistic emission lines from an inner
  accretion disk
• Models consistent with relativistic boadened
  lines from inner disk and warm absorber narrow
  absorption lines

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Current X-ray AGN research
Fe K line profiles

• Lines are composites Chandra shows narrow
  components but broad expected from reflection
• Not all line profiles are the same different
  narrow contribution different geometry
  ionization differences
• The form of the continuum (and the reflection
  component) is very important to understand the
  lines
• Lines vary in most cases but not all however it
  does not in general follow the continuum
• No strong lines in luminous quasars

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Current X-ray AGN research
Warm absorbers

• There are indications that the apparent changes
  in the soft X-ray continuum slope may be due to
  variations in the ionization of the obscuring gas
• The reflection component is important when
  studying the spectrum
• At least in one case the warm absorber is
  located well within the assumed torus and perhaps
  close to the center as the BLR

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Blustin et al 01
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Blustin et al 01