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Active Galactic Nuclei. 4C15 - High Energy Astrophysics 6. Active Galactic Nuclei (AGN): AGN accretion; Sources of energy; Radio galaxies and jets; [2]. Introduction. Apparently stellar Non-thermal spectra High redshifts

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Active galactic nuclei l.jpg

Active Galactic Nuclei

4C15 - High Energy Astrophysics

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6. Active Galactic Nuclei (AGN): AGN accretion; Sources of energy; Radio galaxies and jets; [2]

Introduction l.jpg
Introduction of energy; Radio galaxies and jets;

  • 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)

Quasars monsters of the universe l.jpg
Quasars - Monsters of the Universe of energy; Radio galaxies and jets;

Artist’s impression

Agn accretion l.jpg
AGN Accretion of energy; Radio galaxies and jets;

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

Quasars finding their mass l.jpg
Quasars - finding their mass of energy; Radio galaxies and jets;

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

Measuring a quasar s black hole l.jpg
Measuring a Quasar’s Black Hole of energy; Radio galaxies and jets;

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

Accretion disk and black hole l.jpg
Accretion Disk and Black Hole of energy; Radio galaxies and jets;

  • 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).

Accretion rates l.jpg
Accretion Rates of energy; Radio galaxies and jets;

Calculation of required accretion rate:


Active galactic nuclei agn l.jpg
Active Galactic Nuclei (AGN) of energy; Radio galaxies and jets;

Model of an AGN

Quasars l.jpg
Quasars of energy; Radio galaxies and jets;

This animation takes you on a tour of a quasar from beyond the galaxy, right up to the edge of the black hole.

  • Animation of a quasar

It covers ten orders of magnitude, ie the last frame covers a

distance 10 billion times smaller than the first.

  • 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

Accretion disk structure l.jpg

Dissipation rate, D(R) is of energy; Radio galaxies and jets;

= blackbody flux


Accretion Disk Structure

The accretion disk (AD) can be considered as

rings or annuli of blackbody emission.

Disk temperature l.jpg
Disk Temperature of energy; Radio galaxies and jets;

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

and if

then for

Disk spectrum l.jpg
Disk Spectrum of energy; Radio galaxies and jets;

Flux as a function of frequency, n -

Total disk spectrum

Log n*Fn

Annular BB emission

Log n

Black hole and accretion disk l.jpg
Black Hole and Accretion Disk of energy; Radio galaxies and jets;

For a non-rotating spherically symetrical BH, the

innermost stable orbit occurs at 3rg or :

and when

High energy spectra of agn l.jpg
High Energy Spectra of AGN of energy; Radio galaxies and jets;

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

Fe k a line l.jpg
Fe K of energy; Radio galaxies and jets; a Line

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




Source of fuel l.jpg
Source of Fuel of energy; Radio galaxies and jets;

  • Interstellar gas

  • Infalling stars

  • Remnant of gas cloud which originally formed black hole

  • High accretion rate necessary if z cosmological - not required if nearby

The big bang and redshift l.jpg
The Big Bang and Redshift of energy; Radio galaxies and jets;

  • All galaxies are moving

    away from us.

  • This is consistent with

    an expanding Universe,

    following its creation

    in the Big Bang.

Cosmological redshift l.jpg
Cosmological Redshift of energy; Radio galaxies and jets;

  • Continuity in luminosity from Seyferts to quasars

  • Absorption lines in optical spectra of quasars with



Alternative models l.jpg
Alternative Models of energy; Radio galaxies and jets;

  • 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 => ‘spinar’ - like a scaled-up pulsar



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  • BUT star evolves into black hole. 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

Radio sources l.jpg
Radio Sources star evolves into black hole.

  • Only few % of galaxies contain AGN

  • At low luminosities => 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









Radio galaxies and jets l.jpg

150 kPc star evolves into black hole.

Radio Lobes

Radio Lobes

5.7 MPc

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

Jets focussed streams of ionized gas l.jpg

lobe star evolves into black hole.


energy carried out along channels

material flows back towards galaxy

hot spot

Jets: Focussed Streams of Ionized Gas

Electron lifetimes l.jpg
Electron lifetimes star evolves into black hole.

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

















Shock waves in jets l.jpg
Shock waves in jets star evolves into black hole.

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.

Equipartition of energy l.jpg
Equipartition of energy star evolves into black hole.

Relative contributions of energy

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

Energy in source


magnetic field

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Total energy density in electrons,

Must express k and E as functions of B.


Slide31 l.jpg

We observe synchrotron luminosity density: power-law:

And we know that:

Slide32 l.jpg

Hence: power-law:


and the total energy density in electrons

then becomes:

Finding e max l.jpg
Finding power-law:Emax

Find E by looking for n :




Slide34 l.jpg

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:

Slide35 l.jpg

Thus: power-law:



magnetic field

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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

Equipartition in radio sources l.jpg
Equipartition in Radio Sources power-law:

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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11 power-law:

Maximum frequency observed is 10 Hz.

Thus electron acceleration is required in the lobes.

Relativistic beaming l.jpg
Relativistic Beaming power-law:

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.

Jet collimation l.jpg
Jet collimation power-law:

  • 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

Supermassive black hole l.jpg
Supermassive Black Hole power-law:

  • 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

Geometrically thick disk l.jpg
Geometrically-thick disk power-law:

  • 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

Active galactic nuclei43 l.jpg


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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 )


Hence IIC/ISynch = [gIC/gSynch]2-3/2 = [gIC/gSynch]1/2

More about accretion disks l.jpg

Q of the form


More about Accretion Disks

  • 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.

Disk self-gravitation is negligible so material in differential or

Keplerian rotation with angular velocity WK(R) = (GM/R3)1/2

More about accretion disks cont l.jpg
More about Accretion Disks (Cont.) of the form

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)

More about accretion disks cont47 l.jpg

From a consideration of radial mass and angular momentum of the form

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.)