Active Galactic Nuclei - Mullard Space Science Laboratory

Report
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)
3
Quasars - Monsters of the Universe
Artist’s impression
4
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
5
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
6
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.
7
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).
8
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
24
31
9
Active Galactic Nuclei (AGN)
10
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
11
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
12
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
13
Disk Spectrum
Flux as a function of frequency, n -
Log n*Fn
Total disk spectrum
Annular BB emission
Log n
14
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
15
High Energy Spectra of AGN
Log (nFn
Spectrum from the optical to medium X-rays
Low-energy
disk tail
Balmer cont,
FeII lines
optical
14
UV
15
Comptonized
disk
high-energy
disk tail
EUV soft X-rays X-rays
16
Log n
17
18
16
Fe Ka Line
Fluorescence line observed in Seyferts – from
gas with temp of at least a million degrees.
FeKa
X-ray
e-
17
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
18
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.
19
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
20
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
21
• 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
22
• 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
23
Radio Sources
• Only few % of galaxies contain AGN
• At low luminosities => radio galaxies
• Radio galaxies have powerful radio
emission - usually found in ellipticals
38
43
31
36
• RG
10 - 10 erg/s = 10 - 10 J/s
• Quasars 1043 - 10 47erg/s = 10 36- 1040J/s
24
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
25
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
26
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
-8
14
6
For B = 10 Tesla, tsyn ~ 10 sec, ~ 3x10 yrs
27
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.
28
Equipartition of energy
Relative contributions of energy
Energy in source
particles
magnetic field
What are relative contributions for minimum
energy content of the source?
29
• 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.
30
We observe synchrotron luminosity density:
E max
L
 N (E)P
syn
dE
0
And we know that:
Psyn  k ' E B
2
2
31
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
32
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
33
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
34
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
35
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.
36
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
37
11
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.
38
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.
39
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
40
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
41
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
42
ACTIVE GALACTIC NUCLEI
END OF TOPIC
43
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
44
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.
45
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)
46
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
47

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