Active Galactic Nuclei : I

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Active Galactic Nuclei I
Keith Arnaud NASA Goddard University of Maryland
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AGN Overview

• First identified as bright (blue) point-like
  emission from the centers of some galaxies. Now
  characterized in most cases by strong optical
  emission lines from photoionized material.
• Come in a bewildering number of types - Quasars,
  Seyfert 1, Seyfert 2, Bl Lac, Liner, NLAGN,
  NLSy1, BLRG,
• Powered by accretion onto a supermassive
  (106-108 Mo) black hole (other processes may also
  be significant).
• Seen both near (our Galactic Center) and far (z
  gt 6).
• Excellent background light sources - Ly alpha
  forest, gravitational lenses,

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Spectrum of Mkn 421
Takahashi et al. 1998
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Fundamental Questions

• Where does the emission come from and how is
  accretion energy converted to radiation.
• Why are there so many different types of AGN and
  how are they related. Is there a unified model ?
  Can we draw an H-R diagram for AGN ?
• What is the relationship between the massive
  black hole (MBH) and the host galaxy ? Which
  forms first and what causes the excellent
  correlation between black hole mass and bulge
  velocity dispersion.
• Do all galaxies have MBH ? If so, why are they
  not all AGN ? How long does AGN activity last ?
  What is the connection with starbursts ?

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• Is G.R. correct in the vicinity of a MBH ? The
  strong gravity limit.
• Why do some AGN have jets ? What are jets made
  of ? What powers and collimates them ?

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The importance of X-ray observations

• AGN are easy to find in X-rays. Away from the
  Galactic plane most X-ray sources are AGN. Many
  X-ray selected AGN show weak or no optical
  signatures.
• X-rays come from very close to the MBH. The most
  rapid variability is seen in X-rays.
• The only spectral lines observed that come from
  close to the MBH are in the X-ray band. The
  strongest line is from Fe at 6.4 keV but other
  lines have been observed.
• All types of AGN are strong X-ray sources.
• We can X-ray the material around AGN using the
  emission from close to the MBH as a background
  source.

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Schematic view of AGN central engine
Blazar
Sy 2
Torus
Disk
Narrow line region
Broad line region
Jet
Padovani Urry 1995
Sy 1
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X-ray emission from around the MBH
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Reflection and Fluorescence

• The MBH is surrounded by an accretion disk.
  Suppose that X-rays are generated above the disk.
  
• We observe some photons directly.
• Others hit the accretion disk. Some are
  reflected. Some eject an inner shell electron
  from an atom to give fluorescent line emission.

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NGC 4945
direct
fluorescence
reflected
Madejski et al. 2000
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Reflected X-ray Spectra
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Reflection from neutral slab
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Reflection from an ionized slab
Increasing ionization
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Reflection and Fluorescence

• The MBH is surrounded by an accretion disk.
  Suppose that X-rays are generated above the disk.
  
• We observe some photons directly.
• Others hit the accretion disk. Some are
  reflected. Some eject an inner shell electron
  from an atom to give fluorescent line emission.
• X-rays from parts of the disk moving towards us
  are blue-shifted due to Doppler and red-shifted
  due to gravity. Emission from regions moving away
  from us is red-shifted by both effects.
• We see a line with a red wing. The shape depends
  on the disk inclination and distribution of X-ray
  emission over the disk.

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ASCA observation of MCG 6-30-15
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Fluorescence line from disk around Kerr black hole
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Effect of changing emission profile of disk
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Effect of changing black hole spin
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The effect of MBH spin
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ASCA 1994 and 1997 observations
Time-averaged
Snapshot
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Chandra observation of NGC 5548
Yaqoob et al. 2001
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Location of Fe K line in NGC 5548
Line origin is outer BLR or molecular torus.
Yaqoob et al. 2001 BLR results from Peterson
Wandel 1999
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Comparison of ASCA and Chandra
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Narrow Fe-K lines with Chandra
Padmanabhan Yaqoob 2002
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Complex Fe line in NGC 5506
Neutral line
Ionized line
Matt et al. 2001
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XMM observations of Sy 1
Reeves 2002
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More Sy 1s from XMM
Reeves 2002
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Mkn 841 narrow line variability
15 hours later
Petrucci et al. 2002
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XMM observation of MCG 6-30-15
Requires emission peaked near MBH
Wilms et al. 2001
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Mean profile from XMM MCG 6-30-15 long look
Fabian et al. 2002
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Difference between bright and faint spectra of
MCG 6-30-15
Line varies with continuum.
Fabian et al. 2002
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Chandra and XMM observation of NGC 3516
Turner et al. 2002
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Model for NGC 3516
Turner et al. 2002
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Flares above the accretion disk
Reynolds Young
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Fe K line results from Chandra and XMM

• The Chandra HETG can resolve narrow (few 1000
  km/s) lines.
• A narrow line is seen in many objects. This
  must be subtracted from the broad line when using
  the line shape to estimate disk parameters.
• NGC 5548 line width gt origin in either BLR or
  the molecular torus.
• A systematic analysis (in progress) finds broad
  lines consistent with earlier results using ASCA.

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• XMM-Newton has observed emission from highly
  ionized iron in several sources.
• Probably from photo-ionized gas (BLR?). It is
  not clear how common this is.
• XMM-Newton observations of MCG-6-30-15 show a
  very relativistically broadened line.
• Wilms et al. claim that most of the emission
  must come from close to the MBH and this is not
  possible with standard accretion disk models.
• The line may be powered by magnetic extraction
  of MBH spin energy (Penrose effect).

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• In MCG-60-30-15 (at least) the iron line does
  not lag the continuum as would be predicted by
  simple reflection models.
• If the emission comes from very close to the MBH
  then we do not expect a simple relation between
  line and continuum.
• Joint Chandra and XMM-Newton observations of NGC
  3516 find evidence for sharp line-like features
  within the broad line.
• Lines may be due to flares covering small
  sections of the disk.

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What is required next

• Systematic studies of Fe lines from many objects
  with both Chandra HETG and XMM-Newton EPIC.
• Longer observations to study time variability.
• High resolution spectroscopy at Fe K energies
  with higher sensitivity than available with the
  Chandra HETG (Astro-E2).
• Observations extending to higher energies - we
  need to accurately measure the continuum and
  determine the amount of reflected emission
  (ChandraRXTE, Astrosat).

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