Transcript Active Galactic Nuclei - Mullard Space Science Laboratory

Active Galactic Nuclei
4C15 - High Energy Astrophysics
[email protected]
http://www.mssl.ucl.ac.uk/
6.
Active Galactic Nuclei (AGN): AGN
accretion; Sources of energy; Radio
galaxies and jets;
[2]
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)
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Quasars - Monsters of the Universe
Artist’s impression
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AGN Accretion
Believed to be powered by accretion onto
supermassive black hole
high luminosities
highly variable
Eddington limit
=> 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.
LEdd
mass
So a measurement of quasar luminosity gives the minimum mass
– assuming radiation at the Eddington Limit
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Measuring a Quasar’s 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
d=cxt
c = speed of light
d = diameter
a time dt, then we see a change over
a timescale t+dt. This gives a
maximum value for the diameter, d,
because we know that our measured
timescale must be larger than the
light crossing time.
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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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Accretion Rates
Calculation of required accretion rate:
L  10 J / s
40
40
L
10
M 2 
8
c
0.1 3 10
.


2
 10 kg / s  310 kg / yr  10M Sun / yr
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Active Galactic Nuclei (AGN)
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Model of an AGN
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.
1.
2.
3.
4.
5.
6.
Enter galaxy – see spiral arms and stars
Blue and white blobs are “narrow line” clouds
Red/yellow disc is molecular torus
Purple/green/yellow blobs are “broad line” clouds
Blue/white disc is the accretion disc
Note the jets perpendicular to accretion disc plane
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Accretion Disk Structure
The accretion disk (AD) can be considered as
rings or annuli of blackbody emission.
R
Dissipation rate, D(R) is
0.5

3GMM   R*  

 
1  
3
8R   R  
= blackbody flux
 T ( R)
4
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Disk Temperature
Thus temperature as a function of radius T(R):
1/ 4
0.5




 R*  
 3GMM
  

T ( R)  
1

3


8

R

R








 3GMM
and if T*  
3
 8R* 
then for R  R*
1/ 4



T  T* R / R* 
3 / 4
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Disk Spectrum
Flux as a function of frequency, n -
Log n*Fn
Total disk spectrum
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 :
rmin
6GM
 2
c
and when R  R*
T  T* R / R* 
3 / 4
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High Energy Spectra of AGN
Log (nFn
Spectrum from the optical to medium X-rays
Low-energy
disk tail
Balmer cont,
FeII lines
optical
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UV
15
Comptonized
disk
high-energy
disk tail
EUV soft X-rays X-rays
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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
zem
zab 2
z ab 3
flux
• Continuity in luminosity from Seyferts to
quasars
• Absorption lines in optical spectra of
quasars with zabs  zem
zem
z ab1 z z z l
ab1 ab 2
ab 3
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Alternative Models
• Supermassive star
- 108 solar mass star radiating at 10 39 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
=> ‘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 lowluminosity AGN
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• BUT SN phase will be short (about 1
million years) then evolves to black hole
• radio observations demonstrate wellordered 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 => radio galaxies
• Radio galaxies have powerful radio
emission - usually found in ellipticals
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43
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• RG
10 - 10 erg/s = 10 - 10 J/s
• Quasars 1043 - 10 47erg/s = 10 36- 1040J/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)
150 kPc
Radio Lobes
← 3C 236 Westerbork radio image
Radio Lobes
5.7 MPc
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
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electrons that fill the lobes
Jets: Focussed Streams of Ionized Gas
lobe
jet
energy carried out
along channels
material
flows back
towards
galaxy
hot
spot
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Electron lifetimes
For Synchrotron radiation by electrons:
Calculating the lifetimes in AGN radio jets.
36 2
8
If nm = 10 Hz (radio) ~ 4.17x10 E B
2
-29 2
E B = 2.5x10 (J Tesla)
-13 -2 -1
tsyn = 5x10 B E sec
-3
For B = 10 Tesla, t syn ~3x10 6 sec, ~ 1 month
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14
6
For B = 10 Tesla, tsyn ~ 10 sec, ~ 3x10 yrs
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Shock waves in jets
Lifetimes short compared to extent of jets
=> additional acceleration required.
Most jet energy is ordered kinetic energy.
Gas flow in jet is supersonic; near hot spot gas
decelerates suddenly => shock wave forms.
Energy now in relativistic e- and mag field.
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Equipartition of energy
Relative contributions of energy
Energy in source
particles
magnetic field
What are relative contributions for minimum
energy content of the source?
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• Assume electrons distributed in energy
according to power-law:
N ( E)  kE
a
Total energy density in electrons,
E max
ETot 

0
k
2 a
N ( E ) EdE 
Emax
2 a
Must express k and E max as functions of B.
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We observe synchrotron luminosity density:
E max
L
 N (E)P
syn
dE
0
And we know that:
Psyn  k ' E B
2
2
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Hence:
E max
2
kk ' B 3a
L   kE k ' E B dE 
Emax
3 a
0
a
2
2
(
3

a
)
L
So: k 
2 3a
k ' B Emax
and the total energy
density in electrons
then becomes:
ETot
(3  a )
L

(2  a ) k ' B 2 Emax
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Finding Emax
Find E max by looking for nmax :
n max  const BE
2
max
So:
ETot
Emax  k ' ' B

1/ 2
1/ 2
max
(3  a )
L
3 / 2


aB
2
1/ 2 1/ 2
(2  a ) k ' B k ' ' B n max
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The energy density in the magnetic field is:
B2
2
 bB
2 0
Thus total energy density in source is:
3 / 2
T  aB
 bB
2
For T to be minimum with respect to B:
T
0
B
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Thus:
T
3 5 / 2
  aB
 2bB  0
B
2
3 7 / 2
b  aB
4
So:
T  aB
particle
3 / 2
3 3 / 2
 aB
4
magnetic field
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And finally,
4
energy density in particles
 1
energy density in magnetic field 3
This corresponds to saying that the minimum
energy requirement implies approximate
equality of magnetic and relativistic particle
energy or equipartition.
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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  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 → B2/2o ~ 10-11 or B ~ 5.10-9 Tesla
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Maximum frequency observed is 10 Hz.
n m  4.2 10 E B
2
26
E B  2.5 10
36
2
E  5 10 J  E  10 eV    10
2
18
2
10
t syn  5 10 B E
13
2
5
1
 10 sec  310 yrs
13
5
Thus electron acceleration is required in the lobes.
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Relativistic Beaming
Plasma appears to radiate preferentially along
its direction of motion:
Photons emitted in a
cone of radiation and
Doppler boosted
towards observer.
Thus observer sees only jet pointing towards
her - other jet is invisible.
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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
=> 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 => 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 > 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 form
N(E)  E-3/2, express the ratio of Inverse Compton-produced to Synchrotronproduced X-ray intensities in terms of IC and Synch.
Ratio = (no of electrons with  IC )
(no of electrons with  S )



But:
2
IC
2
S
N IC
NS
N IC  EIC 

 
N S  ES 
3

2
  IC
 
 S





2
IC
2
S
3

2
Hence IIC/ISynch = [IC/Synch]2-3/2 = [IC/Synch]1/2
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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
Q
Q(R) = 2R nS R2 (dW/dR) (1)
where the viscous force per unit length is acting on 2R and
S= 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 4RdR, so the radiative
dissipation from the disc per unit area is from (1):
•
•
D(R) = Q(R) W/4R = 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 85, 2002) that
•
n S = (M/3
[1 – (R*/R)1/2]
•
where M is the accretion rate and from (2) and (3) we then
have
•
D(R) = (3G M M/8R3) [1 – (R*/R)1/2]
and hence the radiation energy flux through the disk faces is
independent of viscosity
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