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

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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

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X-ray emission from around

the MBH

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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.

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NGC 4945

Madejski et al. 2000

direct

reflected

fluorescence

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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

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.

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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

Matt et al. 2001

Neutral line

Ionized line

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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

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

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Difference between bright

and faint spectra of MCG 6-30-15

Fabian et al. 2002

Line varies with continuum.

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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

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