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Active Galactic Nuclei. 4C15 - High Energy Astrophysics emp@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/. Introduction. Apparently stellar Non-thermal spectra High redshifts Seyferts (usually found in spiral galaxies) BL Lacs (normally found in ellipticals)
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Active Galactic Nuclei 4C15 - High Energy Astrophysics emp@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/
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)
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.
Quasars - Monsters of the Universe Artist’s impression
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
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
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
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).
Accretion rates Calculation of required accretion rate: .
. Dissipation rate, D(R) M = blackbody flux R 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.
. M * . M Then when Disk temperature Thus temperature as a function of radius T(R): We define the boundary condition T* at radius R* : =>
Disk spectrum Flux as a function of frequency, n - Total disk spectrum Log n*Fn Annular BB emission Log n
Black hole and accretion disk The innermost stable orbit occurs at : When
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 optical UV EUV soft X-rays X-rays 14 15 16 17 18 Log n
FeKa line Fluorescence line observed in Seyferts – from gas with temp of at least a million degrees. FeKa X-ray e-
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
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.
