active galactic nuclei : i
DESCRIPTION
Active Galactic Nuclei : I. Keith Arnaud NASA Goddard University of Maryland. 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. - PowerPoint PPT PresentationTRANSCRIPT
COSPAR Workshop, Udaipur 2003
Active Galactic Nuclei : I
Keith ArnaudNASA Goddard
University of Maryland
COSPAR Workshop, Udaipur 2003
AGN Overview
o 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.
o Come in a bewildering number of types - Quasars, Seyfert 1, Seyfert 2, Bl Lac, Liner, NLAGN, NLSy1, BLRG,…
o Powered by accretion onto a supermassive (106-108 Mo) black hole (other processes may also be significant).
o Seen both near (our Galactic Center) and far (z > 6).
o Excellent background light sources - Ly alpha forest, gravitational lenses,…
COSPAR Workshop, Udaipur 2003
Spectrum of Mkn 421
Takahashi et al. 1998
COSPAR Workshop, Udaipur 2003
Fundamental Questions
o Where does the emission come from and how is accretion energy converted to radiation.
o 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 ?
o 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.
o 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 ?
COSPAR Workshop, Udaipur 2003
o Is G.R. correct in the vicinity of a MBH ? The strong gravity limit.
o Why do some AGN have jets ? What are jets made of ? What powers and collimates them ?
COSPAR Workshop, Udaipur 2003
The importance of X-ray observations
o 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.
o X-rays come from very close to the MBH. The most rapid variability is seen in X-rays.
o 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.
o All types of AGN are strong X-ray sources.
o We can “X-ray” the material around AGN using the emission from close to the MBH as a background source.
COSPAR Workshop, Udaipur 2003
Schematic view of AGN central
engine
Padovani & Urry 1995
Sy 2
Sy 1
Blazar
Jet
Narrow line regionBroad line region
Disk Torus
COSPAR Workshop, Udaipur 2003
X-ray emission from around
the MBH
COSPAR Workshop, Udaipur 2003
Reflection and Fluorescence
o 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.
COSPAR Workshop, Udaipur 2003
NGC 4945
Madejski et al. 2000
direct
reflected
fluorescence
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Reflected X-ray Spectra
COSPAR Workshop, Udaipur 2003
Reflection from neutral
slab
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Reflection from an ionized slab
Increasing ionization
COSPAR Workshop, Udaipur 2003
Reflection and Fluorescence
o 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.
o 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.
COSPAR Workshop, Udaipur 2003
ASCA observation of MCG 6-30-15
COSPAR Workshop, Udaipur 2003
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
COSPAR Workshop, Udaipur 2003
ASCA 1994 and 1997 observations
Time-averaged
Snapshot
COSPAR Workshop, Udaipur 2003
Chandra observation of NGC 5548
Yaqoob et al. 2001
COSPAR Workshop, Udaipur 2003
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
COSPAR Workshop, Udaipur 2003
Comparison of ASCA and Chandra
COSPAR Workshop, Udaipur 2003
Narrow Fe-K lines
with Chandra
Padmanabhan & Yaqoob 2002
COSPAR Workshop, Udaipur 2003
Complex Fe line in NGC 5506
Matt et al. 2001
Neutral line
Ionized line
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XMM observations of Sy 1
Reeves 2002
COSPAR Workshop, Udaipur 2003
More Sy 1s from XMM
Reeves 2002
COSPAR Workshop, Udaipur 2003
Mkn 841 narrow line variability
Petrucci et al. 2002
15 hours later
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XMM observation of MCG 6-
30-15
Wilms et al. 2001
Requires emission peaked near MBH
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Mean profile from XMM MCG 6-30-15 “long look”
Fabian et al. 2002
COSPAR Workshop, Udaipur 2003
Difference between bright
and faint spectra of MCG 6-30-15
Fabian et al. 2002
Line varies with continuum.
COSPAR Workshop, Udaipur 2003
Chandra and XMM observation of NGC
3516
Turner et al. 2002
COSPAR Workshop, Udaipur 2003
Model for NGC 3516
Turner et al. 2002
COSPAR Workshop, Udaipur 2003
Flares above the accretion disk
Reynolds & Young
COSPAR Workshop, Udaipur 2003
Fe K line results from Chandra and XMM
o 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 => origin in either BLR or the molecular torus.
• A systematic analysis (in progress) finds broad lines consistent with earlier results using ASCA.
COSPAR Workshop, Udaipur 2003
oXMM-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.
o 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).
COSPAR Workshop, Udaipur 2003
o 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.
o 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.
COSPAR Workshop, Udaipur 2003
What is required next
o Systematic studies of Fe lines from many objects with both Chandra HETG and XMM-Newton EPIC.
o Longer observations to study time variability.
o High resolution spectroscopy at Fe K energies with higher sensitivity than available with the Chandra HETG (Astro-E2).
o Observations extending to higher energies - we need to accurately measure the continuum and determine the amount of reflected emission (Chandra+RXTE, Astrosat).
COSPAR Workshop, Udaipur 2003