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; . Introduction. Apparently stellar Non-thermal spectra High redshifts
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6. Active Galactic Nuclei (AGN): AGN accretion; Sources of energy; Radio galaxies and jets; 
Believed to be powered by accretion onto supermassive black hole
Eddington limit => large mass
small source size
Accretion onto supermassive black hole
The Eddington Limit
Where inward force of gravity balances the outward ‘push’ of
radiation on the surrounding gas.
So a measurement of quasar luminosity gives the minimum mass
– assuming radiation at the Eddington Limit
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.
If an event happens at A and takes 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.
d = c x t
c = speed of light
d = diameter
Calculation of required accretion rate:
Model of an AGN
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.
Dissipation rate, D(R) is of energy; Radio galaxies and jets;
= blackbody flux
RAccretion Disk Structure
The accretion disk (AD) can be considered as
rings or annuli of blackbody emission.
Thus temperature as a function of radius T(R):
Flux as a function of frequency, n -
Total disk spectrum
Annular BB emission
For a non-rotating spherically symetrical BH, the
innermost stable orbit occurs at 3rg or :
Spectrum from the optical to medium X-rays
Low-energy disk tail
Balmer cont, FeII lines
high-energy disk tail
optical UV EUV soft X-rays X-rays
14 15 16 17 18
Fluorescence line observed in Seyferts – from gas with temp of at least a million degrees.
away from us.
an expanding Universe,
following its creation
in the Big Bang.
150 kPc star evolves into black hole.
5.7 MPcRadio Galaxies and Jets
← 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
lobe star evolves into black hole.
energy carried out along channels
material flows back towards galaxy
hot spotJets: Focussed Streams of Ionized Gas
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
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.
Relative contributions of energy
What are relative contributions for minimum energy content of the source?
Energy in source
Total energy density in electrons,
Must express k and E as functions of B.
We observe synchrotron luminosity density: power-law:
And we know that:
and the total energy density in electrons
Find E by looking for n :
The energy density in the magnetic field is: power-law:
Thus total energy density in source is:
For T to be minimum with respect to B:
And finally, power-law:
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
For Cygnus A → Lradio ~ 5.1037 J/s
tlife ~ 2.3.1021/106 = 2.3.1015 s ~ 7.107 years
= 5.1063 m3
and energy density ~ 1053/1064 = 10-11 J/m3
Maximum frequency observed is 10 Hz.
Thus electron acceleration is required in the lobes.
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
END OF TOPIC
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 )
Hence IIC/ISynch = [gIC/gSynch]2-3/2 = [gIC/gSynch]1/2
Q of the form
QMore about Accretion Disks
in the z direction.
Disk self-gravitation is negligible so material in differential or
Keplerian rotation with angular velocity WK(R) = (GM/R3)1/2
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)
W = WK = (G M/R3)1/2
differentiate and then
D(R) = 9/8 n S Q(R) M/R3 (3)
flow in the disk, it can be shown (Frank, King & Raine, 3rd
ed., sec 5.3/p 85, 2002) that
nS = (M/3p) [1 – (R*/R)1/2]
where M is the accretion rate and from (2) and (3) we then
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
•More about Accretion Disks (Cont.)