Active Galactic Nuclei 4C15 - High Energy Astrophysics

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 - Monsters of the Universe Artist’s impression

AGN Accretion Believed to be powered by accretion onto supermassive black hole high luminosities Eddington limit => large mass highly variable 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 Edd mass So a measurement of quasar luminosity gives the minimum mass – assuming radiation at the Eddington Limit

Measuring a Quasar’s Black Hole Light travel time effects AB d = c x t 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  t, then we see a change over a timescale t+  t. This gives a maximum value for the diameter, d, because we know that our measured timescale must be larger than the light crossing time. 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 Very hot towards the centre (emitting soft X-rays) and cool at the edges (emitting optical/IR). Disk is about the size of our Solar System Geometrically thin, optically-thick and radiates like a collection of blackbodies

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.

Accretion Rates Calculation of required accretion rate:.

More about Accretion Disks Disk self-gravitation is negligible so material in differential or Keplerian rotation with angular velocity  K (R) = (GM/R 3 ) 1/2 Q Q If is the kinematic viscosity for rings of gas rotating, the viscous torque exerted by the outer ring on the inner will be Q(R) = 2  R  R 2 (d  /dR) (1) where the viscous force per unit length is acting on 2  R and  = H  is the surface density with H (scale height) measured in the z direction.

More about Accretion Disks (Cont.) The viscous torques cause energy dissipation of Q  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)  /4  R = ½ R  ) 2 (2) and since  K = (G M/R 3 ) 1/2 differentiate and then D(R) = 9/8 Q(R) M/R 3 (3)

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, 3 rd ed., sec 5.3/p 202, 2002) that  = (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  R 3 ) [1 – (R * /R) 1/2 ] and hence the radiation energy flux through the disk faces is independent of viscosity

Accretion Disk Structure The accretion disk (AD) can be considered as rings or annuli of blackbody emission. R Dissipation rate, D(R) is = blackbody flux

Disk Temperature Thus temperature as a function of radius T(R): then for and if

Disk Spectrum Flux as a function of frequency, - Log Log *F Total disk spectrum Annular BB emission

Black Hole and Accretion Disk For a non-rotating spherically symetrical BH, the innermost stable orbit occurs at 3r g or : and when

High Energy Spectra of AGN Spectrum from the optical to medium X-rays Log Log ( F  optical UV EUV soft X-rays X-rays high-energy disk tail Low-energy disk tail Comptonized disk Balmer cont, FeII lines

Fe K  Line Fluorescence line observed in Seyferts – from gas with temp of at least a million degrees. X-ray e- FeK 

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

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.

Cosmological Redshift Continuity in luminosity from Seyferts to quasars Absorption lines in optical spectra of quasars with flux

Alternative Models 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 839

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 low- luminosity AGN

BUT 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 Only few % of galaxies contain AGN At low luminosities => radio galaxies Radio galaxies have powerful radio emission - usually found in ellipticals RG erg/s = J/s Quasars erg/s = J/s

Radio Galaxies and Jets Cygnus-A → VLA radio image at  = Hz - the closest powerful radio galaxy (d = 190 MPc) ← 3C 236 Westerbork radio image at = 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 150 kPc Radio Lobes 5.7 MPc Radio Lobes

Jets: Focussed Streams of Ionized Gas energy carried out along channels lobe hot spot material flows back towards galaxy jet

Electron lifetimes Calculating the lifetimes in AGN radio jets. If m = 10 Hz (radio) ~ 4.17x10 E B E B = 2.5x10 J Tesla  syn = 5x10 B E sec For B = 10 Tesla,  ~3x10 sec, ~ 1 month For B = 10 Tesla,  ~ 10 sec, ~ 3x10 yrs syn For Synchrotron radiation by electrons:

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.

Equipartition of energy Relative contributions of energy What are relative contributions for minimum energy content of the source? Energy in source particlesmagnetic field

Assume electrons distributed in energy according to power-law: Total energy density in electrons, Must express k and E as functions of B. max

Assume electrons distributed in energy according to power-law: Total energy density in electrons, Must express k and E as functions of B. max

We observe synchrotron luminosity density: And we know that:

Hence: So: and the total energy density in electrons then becomes:

Finding Emax Find E by looking for  : max So:

The energy density in the magnetic field is: Thus total energy density in source is: For T to be minimum with respect to B:

Thus: So: particlemagnetic field

And finally, 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 If d lobe ~ 75 kPc = m and v jet ~ 10 3 km/s, then t life ~ /10 6 = s ~ years R lobe ~ 35 kPc = m and hence V lobe = 4/3  R lobe 3 = m 3 Total energy requirement ~ x ~ J and energy density ~ /10 64 = J/m 3 So from equipartition → B 2 /2  o ~ or B ~ Tesla For Cygnus A → L radio ~ J/s

Maximum frequency observed is 10 Hz. 11 Thus electron acceleration is required in the lobes.

Relativistic Beaming 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 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 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 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 NUCLEI END OF TOPIC

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 Synchrotron- produced X-ray intensities in terms of  IC and  Synch. Ratio = (no of electrons with ) (no of electrons with ) But: Hence I IC /I Synch = [  IC /  Synch ] 2-3/2 = [  IC /  Synch ] 1/2