Active Galactic Nuclei

1
Active Galactic Nuclei

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

2
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)

3
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.
4
Quasars - Monsters of the Universe

• Artists
  impression

5
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
6
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
7
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
8
Accretion disk and black hole

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

The disk is about the size of our Solar System.
It is geometrically thin and 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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Accretion rates

• Calculation of required accretion rate

.
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Accretion disk structure

• The accretion disk (AD) around a star can be
  considered as rings or annuli of blackbody
  emission. R is the stars radius.

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Disk temperature

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

We define the boundary condition T at radius R

gt
12
Disk spectrum

• Flux as a function of frequency, n -

Total disk spectrum
Log nFn
Annular BB emission
Log n
13
Black hole and accretion disk
The innermost stable orbit occurs at
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
15
FeKa line

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

FeKa
X-ray
e-
16
Source of fuel

• interstellar gas
• infalling stars
• remnant of gas cloud which originally
  formed black hole
• high acc rate necessary if z cosmological -
  otherwise not required if nearby

17
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.