Active Galactic Nuclei

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Active Galactic Nuclei

• 4C15 - High Energy Astrophysics
• jlc_at_mssl.ucl.ac.uk
• http//www.mssl.ucl.ac.uk/

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Introduction

• Apparently stellar
• Non-thermal spectra
• High redshifts
• Seyferts (usually found in spiral galaxies)
• BL Lacs (normally found in ellipticals)
• Quasars (nucleus outshines its host galaxy)

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Quasars - Monsters of the Universe

• Artists
  impression

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AGN Accretion

• Believed to be powered by accretion onto
  supermassive black hole

high luminosities
highly variable
Eddington limit gt large mass
small source size
Accretion onto supermassive black hole
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Quasars - finding their mass

• The Eddington Limit

Where inward force of gravity balances the
outward push of radiation on the surrounding
gas.
L
mass
Edd
So a measurement of quasar luminosity gives the
minimum mass assuming radiation at the
Eddington Limit
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Measuring a Quasars Black Hole

• Light travel time effects

If photons leave A and B at the same time, A
arrives at the observer a time t ( d / c )
later.
A
B
If an event happens at A and takes a time dt,
then we see a change over a timescale tdt. This
gives a maximum value for the diameter, d,
because we know that our measured timescale must
be larger than the light crossing time.
d c x t
c speed of light
d diameter
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Accretion Disk and Black Hole

• In the very inner regions, gas is believed to
  form a disk to rid itself of angular momentum

• Disk is about the size of our Solar System

• Geometrically thin, optically-thick
• and radiates like a collection of
• blackbodies

• Very hot towards the centre
• (emitting soft X-rays) and
• cool at the edges (emitting
• optical/IR).

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Quasars

• Animation of a quasar

This animation takes you on a tour of a quasar
from beyond the galaxy, right up to the edge of
the black hole.
It covers ten orders of magnitude, ie the last
frame covers a distance 10 billion times smaller
than the first.
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Accretion Rates

• Calculation of required accretion rate

.
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More about Accretion Disks

• Disk self-gravitation is negligible so material
  in differential or
• Keplerian rotation with angular velocity WK(R)
  (GM/R3)1/2

• If n is the kinematic viscosity
• for rings of gas rotating,
• the viscous torque
• exerted by the outer
• ring on the inner will be
• Q(R) 2pR nS R2 (dW/dR) (1)
• where the viscous force per unit length is acting
  on 2pR and
• Hr is the surface density with H (scale height)
  measured
• in the z direction.

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More about Accretion Disks (Cont.)

The viscous torques cause energy dissipation of Q
W dR/ring Each ring has two plane faces of area
4pRdR, so the radiative dissipation from the disc
per unit area is from (1) D(R) Q(R) W/4pR
½ n S (RW)2 (2) and since W WK (G
M/R3)1/2 differentiate and then D(R) 9/8
n S Q(R) M/R3 (3)


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More about Accretion Disks (Cont.)
From a consideration of radial mass and angular
momentum flow in the disk, it can be shown
(Frank, King Raine, 3rd ed., sec 5.3/p 202,
2002) that n S (M/3p) 1
(R/R)1/2 where M is the accretion rate and
from (2) and (3) we then have D(R) (3G M
M/8pR3) 1 (R/R)1/2 and hence the radiation
energy flux through the disk faces is
independent of viscosity



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Accretion Disk Structure

• The accretion disk (AD) can be considered as
• rings or annuli of blackbody emission.

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Disk Temperature

• Thus temperature as a function of radius T(R)

and if
then for
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Disk Spectrum

• Flux as a function of frequency, n -

Total disk spectrum
Log nFn
Annular BB emission
Log n
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Black Hole and Accretion Disk
For a non-rotating spherically symetrical BH, the
innermost stable orbit occurs at 3rg or
and when
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High Energy Spectra of AGN

• Spectrum from the optical to medium X-rays

Low-energy disk tail
Comptonized disk
Balmer cont, FeII lines
high-energy disk tail
Log (nFn)
optical UV EUV soft X-rays X-rays
14 15 16
17 18
Log n
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Fe Ka Line

• Fluorescence line observed in Seyferts from gas
  with temp of at least a million degrees.

FeKa
X-ray
e-
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Source of Fuel

• Interstellar gas
• Infalling stars
• Remnant of gas cloud which originally
  formed black hole
• High accretion rate necessary if z cosmological -
  not required if nearby

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The Big Bang and Redshift

• All galaxies are moving
• away from us.
• This is consistent with
• an expanding Universe,
• following its creation
• in the Big Bang.

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Cosmological Redshift

• Continuity in luminosity from Seyferts to quasars
• Absorption lines in optical spectra of quasars
  with

flux
l
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Alternative Models

• Supermassive star
  - 10 solar mass star radiating at 10
  J/s or less does not violate Eddington limit. It
  would be unstable however on a timescale of
  approx 10 million years.
• May be stabilized by rapid rotation gt
  spinar - like a scaled-up pulsar

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• Also, general relativity predicts additional
  instability and star evolves into black hole.
• Starburst nuclei
  - a dense cluster of massive, rapidly
  evolving stars lies in the nucleus, undergoing
  many SN explosions.
• Explains luminosity and spectra of low-luminosity
  AGN

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• BUT SN phase will be short (about 1 million
  years) then evolves to black hole
• radio observations demonstrate well-ordered
  motions (i.e. jets!) which are hard to explain in
  a model involving random outbursts

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Radio Sources

• Only few of galaxies contain AGN
• At low luminosities gt radio galaxies
• Radio galaxies have powerful radio emission -
  usually found in ellipticals
• RG 10 - 10 erg/s 10 - 10 J/s
• Quasars 10 - 10 erg/s 10 - 10 J/s

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Radio Galaxies and Jets

• Cygnus-A ?
• VLA radio image at
• n 1.4.109 Hz
• the closest powerful
• radio galaxy
• (d 190 MPc)

? 3C 236 Westerbork radio image at n
6.08.108 Hz a radio galaxy of very
large extent (d 490 MPc)
Jets, emanating from a central highly active
galaxy, are due to relativistic electrons that
fill the lobes
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Jets Focussed Streams of Ionized Gas
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Electron lifetimes
For Synchrotron radiation by electrons

• Calculating the lifetimes in AGN radio jets.
• If nm 10 Hz (radio) 4.17x10 E B
• E B 2.5x10 J Tesla
• tsyn 5x10 B E sec
• For B 10 Tesla, t 3x10 sec, 1 month
• For B 10 Tesla, t 10 sec, 3x10 yrs

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2
8
2
-29
2
-13
-2
-1
-3
6
syn
-8
14
6
syn
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Shock waves in jets

• Lifetimes short compared to extent of jets
  gt additional acceleration required.
  Most jet energy is ordered kinetic energy.
• Gas flow in jet is supersonic near hot spot gas
  decelerates suddenly gt shock wave forms. Energy
  now in relativistic e- and mag field.

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Equipartition of energy

• Relative contributions of energy
• What are relative contributions for minimum
  energy content of the source?

Energy in source
particles
magnetic field
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• Assume electrons distributed in energy according
  to power-law

Total energy density in electrons,
Must express k and E as functions of B.
max
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• Assume electrons distributed in energy according
  to power-law

Total energy density in electrons,
Must express k and E as functions of B.
max
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• We observe synchrotron luminosity density
• And we know that

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• Hence

So
and the total energy density in electrons then
becomes
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Finding Emax

• Find E by looking for n

max
max
So
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• The energy density in the magnetic field is
• Thus total energy density in source is
• For T to be minimum with respect to B

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• Thus
• So

particle
magnetic field
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• And finally,
• This corresponds to saying that the minimum
  energy requirement implies approximate equality
  of magnetic and relativistic particle energy or
  equipartition.

energy density in particles
energy density in magnetic field
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Equipartition in Radio Sources
For Cygnus A ? Lradio 5.1037 J/s

• If dlobe 75 kPc 2.3.1021 m and vjet 103
  km/s, then
• tlife 2.3.1021/106 2.3.1015 s 7.107
  years
• Rlobe 35 kPc 1021 m and hence Vlobe 4/3 p
  Rlobe3
• 5.1063 m3
• Total energy requirement 5.1037 x 2.3.1015
  1053 J
• and energy density 1053/1064 10-11
  J/m3
• So from equipartition ? B2/2mo 10-11 or B
  5.10-9 Tesla

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• Maximum frequency observed is 10 Hz.

Thus electron acceleration is required in the
lobes.
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Relativistic Beaming

• Plasma appears to radiate preferentially along
  its direction of motion
• Thus observer sees only jet pointing towards her
  - other jet is invisible.

Photons emitted in a cone of radiation and
Doppler boosted towards observer.
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Jet collimation

• Nozzle mechanism
  hot gas inside large, cooler cloud which is
  spinning hot gas escapes along route of least
  resistance rotation axis
  gt collimated jet
• But VLBI implies cloud small and dense and
  overpredicts X-ray emission

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Supermassive Black Hole

• Black hole surrounded by accretion disk
• Disk feeds jets and powers them by releasing
  gravitational energy
• Black hole is spinning gt jets are formed
  parallel to the spin axis, perhaps confined by
  magnetic field

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Geometrically-thick disk

• Black hole disk acc rate gt Eddington
• Disk puffs up due to radiation pressure
• Torus forms in inner region which powers and
  collimates jets
• Predicted optical/UV too high however, but still
  viable

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ACTIVE GALACTIC NUCLEI

• END OF TOPIC

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Q 4.d) If the high energy electron spectrum in
the galaxy is of the formN(E) ? E-3/2, express
the ratio of Inverse Compton-produced to
Synchrotron-produced X-ray intensities in terms
of gIC and gSynch.

• Ratio (no of electrons with )
• (no of electrons with )
• But

Hence IIC/ISynch gIC/gSynch2-3/2
gIC/gSynch1/2