1. Active Galactic Nuclei
4C15 - High Energy Astrophysics
For further information see:
Quasar Astronomy by Dan Weedman
Active Galactic Nuclei by Ian Robson
Accretion Power in Astrophysics (2nd edition) by Frank, King and Raine

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)
Active Galactic Nuclei are the central engines of distant galaxies. They are apparently stellar on the sky (although deeper observations often reveals evidence of the underlying host galaxy), their spectra extend from the radio up to X-rays and gamma-rays and seem to be dominated by a flat, non-thermal component, and they have high redshifts, up to z~5 when the Universe was one tenth of its present age.
There are three main types of objects:
Seyfert galaxies - normally found in spiral galaxies
BL Lacs - usually observed in ellipticals
Quasars - highly luminous AGN which outshine the underlying 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
Artist’s impression

5. Accretion onto supermassive black hole
AGN Accretion
Believed to be powered by accretion onto supermassive black hole
high luminosities
highly variable
Eddington limit => large mass
small source size
It is commonly accepted that AGN are all manifestations of accretion onto a supermassive black hole. This is deduced from:
1. The very high luminosities involved, up to ~1e47 erg/s or more
2. Fast variability on timescales of only days or even shorter for BL Lacs and optically-violent variables.
The fast variability implies a very small source size for the nucleus. If the time taken to vary significantly is 10,000 seconds (for example), then the size of the source must be no more than 1e4 times the speed of light, which is 3e12m. This corresponds to the Schwarzchild radius of a 1e9 solar mass black hole.
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 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 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

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).

9. Calculation of required accretion rate:
Accretion rates
Calculation of required accretion rate: .

10. Accretion disk structure
The accretion disk (AD) around a star can be considered as rings or annuli of blackbody emission. R* is the star’s radius.
Dissipation rate, D(R) M . R
It is assumed that the disk is geometrically-thin and optically-thick in the z-direction. Thus each annular element of the disk radiates roughly as a blackbody with a temperature T(R) , where :
Sigma x T^4(R) = D(R)
Where D(R) is the dissipation rate and sigma x T^4 is the blackbody flux.
R_* is the radius of the compact object.
Dissipation through the disk is independent of the viscosity in the disk – and the dissipation rate is the energy flux through the faces of the thin disk. Thus if the disk is optically-thick in the z-direction, we are justified in assuming that the dissipation rate is equivalent to the blackbody emission.
= blackbody flux

11. . . Disk temperature M =>
Thus temperature as a function of radius T(R): * M . M .
We define the boundary condition T* at radius R* :
Substituting the blackbody flux equation into the dissipation equation gives the temperature of the disk as a function of radius. At radii larger than the radius of the compact body, the temperature is given by the equation shown. Note that the temperature decreases with radius with a power –0.75. (The trick in this derivation is to multiply T* by (R/R)3/4 ).
Then when
=>

12. Disk spectrum Flux as a function of frequency, n - Total disk spectrum
Log n*Fn
The total disk spectrum is the sum of the emission from each of the annuli that make up the disk. The emission is dominated by the hottest regions ie from the annuli closest to the black hole. At long wavelengths therefore, the spectrum has the form of the Rayleigh-Jeans tail where the flux rises with the square of the frequency. At short wavelengths, the Wien form dominates and the flux falls with e^-nu.
Annular BB emission
Log n

13. Black hole and accretion disk
The innermost stable orbit occurs at :
When
The distance to the inner edge of the accretion disk is proportional to the mass of the central black hole. The temperature, on the other hand, decreases as the radius increases. Thus the inner edges of large mass black holes are relatively cool, while those of low mass black holes are relatively hot. This means that disks around black holes in AGN are much hotter than those around Galactic black hole candidates.

14. 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 n*Fn
Moving from low frequencies up to X-rays, these features are known as:
The small blue bump – emission from the Balmer series which forms an excess above the underlying continuum at the Balmer limit.
The big blue bump – a rise towards high frequencies above an extrapolation of the lower energy spectrum, believed to be due to the outer edges of the accretion disk.
The soft X-ray excess – an excess of flux above an extrapolation of the medium/hard X-ray spectrum (2-50keV). This has a mean slope of about -2 (ie Flux, F_nu is proportional to nu^-2) as measured by the ROSAT observatory in the 0.1-2keV range.
The medium to hard X-ray spectrum has a mean slope of about –1 (ie Fnu is proportional to 1/nu) as measured by EXOSAT and ASCA. It is thought to be due to the inverse Compton scattering of photons from the accretion disk in a hot, 100 million degree corona which surrounds the disk.
There is also a strong FeKalpha fluorescence line observed at about 6.7keV in Seyfert galaxies (not seen yet in quasars) which is believed to emitted from the very inner regions of the accretion disk, close to the black hole itself.
optical UV EUV soft X-rays X-rays
Log n

15. FeKa line
Fluorescence line observed in Seyferts – from gas with temp of at least a million degrees.
X-ray
FeKa e-
An X-ray photon collides with an Fe ion, removing an electron from an inner K or L shell. The ion may return to a lower energy state by emitting an electron from a higher shell (this is known as the ‘Auger effect’) – or by a radiative transition. The relative probability of a radiative transition is known as the fluorescence yield.
The energy of the Kalpha line depends on the number of electrons present and it increases as the line becomes more highly ionized, reaching 6.9keV for FeXXVI.
The ionization state observed indicates gas temperatures which are relatively cool (about a million degrees) and the strength (ie the equivalent width) is quite high, indicating that the gas producing the Fe line has a high covering factor.
FeKalpha emission from an accretion disk fits these observations very well, providing a high covering factor to the source of X-rays (probably the accretion disk corona) without obscuring our line of sight. It also has the right temperature in the inner regions, where the line is thought to originate.

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
Black holes could accrete this much material from the interstellar gas, or in the form of stars (disrupted by the gravitational field of the black hole) or from the remnant of the original rotating gas cloud from which the black hole is thought to have formed.
Large amounts of accreting matter are required to explain the observed luminosities mainly due to the assumption that their measured redshifts indicate the cosmological distances of the AGN. The emitted power would be less however if AGN are actually nearby objects but their spectra are redshifted by some other mechanism.

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.