Active Galactic Nuclei

1. Active Galactic Nuclei 4C15 - High Energy Astrophysics jlc@mssl.ucl.ac.uk http://www.mssl.ucl.ac.uk/

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

3. 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)

4. Quasars - Monsters of the Universe Artist’s impression

5. 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

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 – assuming radiation at the Eddington Limit

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 • 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).

9. Accretion Rates Calculation of required accretion rate: .

10. Active Galactic Nuclei (AGN) Model of an AGN

11. Quasars 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

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

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

14. Disk Spectrum Flux as a function of frequency, n - Total disk spectrum Log n*Fn Annular BB emission Log n

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

16. 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 (nFn) optical UV EUV soft X-rays X-rays 14 15 16 17 18 Log n

17. Fe Ka Line Fluorescence line observed in Seyferts – from gas with temp of at least a million degrees. FeKa X-ray e-

18. 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

19. 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.

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

21. 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 8 39

22. 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

23. 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

24. Radio Sources • 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 38 43 31 36 43 47 36 40

25. 150 kPc 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

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

27. Electron lifetimes 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 36 2 8 2 2 -29 -13 -2 -1 -3 6 syn -8 14 6 syn

28. 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.

29. Equipartition of energy Relative contributions of energy What are relative contributions for minimum energy content of the source? Energy in source particles magnetic field

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

31. We observe synchrotron luminosity density: And we know that:

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

33. Finding Emax Find E by looking for n : max max So:

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

35. Thus: So: particle magnetic field

36. 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

37. Equipartition in Radio Sources 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

38. 11 Maximum frequency observed is 10 Hz. Thus electron acceleration is required in the lobes.

39. 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.

40. 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

41. 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

42. 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

43. ACTIVE GALACTIC NUCLEI END OF TOPIC

44. 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 ) But: Hence IIC/ISynch = [gIC/gSynch]2-3/2 = [gIC/gSynch]1/2

45. Q Q 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

46. More about Accretion Disks (Cont.) • 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) • •

47. From a consideration of radial mass and angular momentum 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 have 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.) • •