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Lecture 7

Lecture 7. Continuum Emission in AGN. UV-Optical Continuum. Infrared Continuum. High Energy Continuum. Radio Continuum - Jets and superluminal motion.

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Lecture 7

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  1. Lecture 7 • Continuum Emission in AGN • UV-Optical Continuum • Infrared Continuum • High Energy Continuum • Radio Continuum - Jets and superluminal motion

  2. Goal: The foundation of all astrophysical observations is the photon. All morphological and spectral information about astrophysical sources is derived from the emitted radiation. We learned about the power of line emission (spectroscopy) Continuum radiation is a natural consequence of the principle that accelerating charges radiate. Can have : thermal or nonthermal emission

  3. Spectral Energy Distribution AGN show emission lines in all astrophysically relevant wavelength regimes

  4. Power Law Continuum • Emission observed from 108 Hz to 1027Hz: α=energy index now know to differ in different bands Actual SED is a function of the AGN Class

  5. From last class:AGN Taxonomy • Seyfert galaxies 1 and 2 • Quasars (QSOs and QSRs) • Radio Galaxies • LINERs • Blazars • Related phenomena

  6. Definition: radio-loud if is larger than 10 (Kellermann et al. 1989) • RL AGN have prominent radio features 10% of AGN population • RL: BLRGs, NLRGs, QSRs, Blazars RQ: Seyferts, most QSOs • Deep radio surveys show intermediate sources

  7. The Continuum A phenomenological approach: • Power law continuum • Thermal features • Spectral Energy Distributions of Radio-loud and Radio-quiet AGN

  8. Observing the SEDs of AGN

  9. Types of Continuum Spectra • Blazars: non-thermal emission from radio to gamma-rays (2 components) • Seyferts, QSOs, BLRGs: IR and UV bumps (thermal) radio, X-rays (non-thermal) Spectral Energy Distributions (SEDs): plots of power per decade versus frequency (log-log)

  10. Spectral Energy Distributions Big Blue Bump EUV gap IR bump Sanders et al. 1989

  11. The radio and IR bands • Radio emission is two orders of magnitude or more larger in radio-loud than in radio-quiet • Radio and IR are disconnected, implying different origins

  12. The IR and Blue bumps • LIR contains up to 1/3 of Lbol LBBB contains a significant fraction of Lbol • IR bump due to dust reradiation, BBB due to blackbody from an accretion disk • The 3000 A bump in 4000-1800 A: • Balmer Continuum • Blended Balmer lines • Forest of FeII lines

  13. The highest energies • Typically α=0.7-0.9 in 2-10 keV • Radio-loud AGN (BLRGs, QSRs) have flatter X-ray continua than radio-quiet • Soft X-ray excess is also observed, often smoothly connected to UV bump • The only AGN emitting at gamma-rays ( MeV) are blazars

  14. Blazars’ SEDs Blue blazars: PKS 2155-398 Red blazars: 3C279 Wehrle et al. 1999 Bertone et al. 2001

  15. Blazar SEDs main features • Two main components: • Radio to UV/X-rays • X-rays to gamma-rays • Component 1 is polarized and variable Synchrotron emission from jet • Component 2: possibly inverse Compton scattering

  16. A fundamental question How much of the AGN radiation is primary and how much is secondary? • Primary: due to particles powered directly by the central engine (e.g., synchrotron, accretion disk) • Secondary: due to gas illuminated by primary and re-radiating

  17. An important issue Isotropy of emitted radiation • Thermal radiation is usually isotropic • Non-thermal radiation can be highly directed (“beamed”). In this case: • We can not obtain the true luminosity of the AGN • We will not have a true picture of various AGN emission processes

  18. 1. UV-Optical Continuum Interpreting the BBB From accretion disk theory (last class), And the maximum emission frequency is at i.e., in the EUV/soft X-ray emission region. BBB=thermal disk emission?!

  19. Model Spectrum of an Accretion Disk

  20. Spectrum from an accretion disk • Optically thick, geometrically thin accretion disk radiates locally as a blackbody due to sheer viscosity • Total integrated spectrum goes like ~ν2 at low frequencies, decays exponentially at high frequencies • For intermediate frequencies spectrum goes as ~ ν1/3 • T=T(R) and T is max in the inner regions in correspondence of UV emission

  21. Observations of optical-to-UV continuum • After removing the small blue bump, the observed continuum goes as ν-0.3 • Removing the extrapolation of the IR power law gives ν-1/3 - but is the IR really described by a power law?? • More complex models predict Polarization and Lyman edge – neither convincingly observed Disk interpretation is controversial!

  22. Alternative interpretation • Optical-UV could be due to Free-free (bremsstrahlung) emission from many small clouds Barvainis 1993 • Slope consistent with observed (α~0.3), low polarization and weak Lyman edge predicted • Requires high T~106 K

  23. Is an accretion disk really there? Indirect evidence: • Fitting of SEDs • Double-peaked line profiles Direct evidence: • Water maser in NGC 4258

  24. Optical emission lines Eracleous and Halpern 1984

  25. Water Masers in NGC 4258 Within the innermost 0.7 ly, Doppler-shifted molecular clouds: • Obey Kepler’s Law • Massive object at center

  26. 2. The IR emission • In most radio-quiet AGN, there is evidence that the IR emission is thermal and due to heated dust • However, in some radio-loud AGN and blazars the IR emission is non-thermal and due to synchrotron emission from a jet

  27. Evidence for IR thermal emission • Obscuration : Many IR-bright AGN are obscured (UV and optical radiation is strongly attenuated) IR excess is due to re-radiation by dust

  28. Radial dependence of dust temperature From the balance between emission and absorption: With R in pc, Leff in erg/s, T in Kelvin Hotter dust lies closer to the AGN

  29. Evidence for IR thermal emission • IR continuum variability : IR continuum shows same variations as UV/optical but with significant delay variations arise as dust emissivity changes in response to changes of UV/optical that heats it

  30. Emerging picture • The 2μ-1mm region is dominated by thermal emission from dust (except in blazars and some other radio-loud AGN) • Different regions of the IR come from different distances because of the radial dependence of temperature

  31. The 1μ minimum • General feature of AGN • Consistent with the above picture: hottest dust has T~2000 K (sublimation temperature) and is at 0.1 pc • This temperature limit gives a natural explanation for constancy of the 1μ minimum flux

  32. 3. Radio properties of AGN I) Basic features of radio morphology II) Observed phenomena • Superluminal motion • Beaming

  33. Radio features Lobes Jet Hotspot Core

  34. Speed of Jets What is the speed of radio jets in AGN? Since this is non-thermal plasma where no spectral lines are seen, the Doppler-shift cannot be used to derive a jet velocity for the nucleus!

  35. Radio Telescopes: VLA, VLBI • The Very Large Array has angular resolution • At z=0.5 this is ~2 kpc • For the Very Long Baseline Interferometry, R~1m.a.s. • At z=0.5 this is ~2 pc

  36. The power of resolution Energy is transported by jets from the cores to the outer regions

  37. Superluminal Motion • VLBI observations of the inner jet of 3C273 shows ejected blobs moving at v~3-4c • This is called superluminal motion How is this possible??

  38. Historgram of observed v/c in 33 jets

  39. Explanation of apparent superluminal motion Explain apparent superluminal motion as an optical illusion caused by the finite speed of light. Consider a knot in the jet moving almost directly towards us at high speed: The blobs are moving towards us at an angle  measured from the line of sight. Photon emitted along the line of sight at time t=0, travels a distance d to us, taking a time t1 to arrive: t1 = d/c A second photon is emitted at a time te later, when The blob is a distance d – vte cos away from us. The second photon arrives at t2 = te + (d - vte cos)/c The observed difference in the time of arrival from photon 1 & 2 is: tobs = t2 - t1 = te (1 – vcos/c) < te

  40. The apparent transverse velocity is vT = vte sin / t = v sin / (1 – v cos /c) As v approaches c, vT can appear > than c! Superluminal motion, typically 5-10c! Let  = 1/(1- v2/c2)1/2, this is the Lorentz factor. Then: vT v (the maximum observed velocity) which occurs when cos  = v/c. We will only observe superluminal motion when the jets are pointed within an angle of 1/ towards the line of sight, but this light will be beamed and brightened.

  41. Relativistic motion of plasma • Relativistic bulk motion in radio sources has important consequences on the following observed quantities: • Frequency • Length and time • Intensity • Direction light is emitted

  42. Relativistic Doppler Effect Assume an emitting source moving at a speed v c at an angle q with respect to the observer. Time-dilation tells us that dt in the observers rest frame for a periodic signal with frequency n’ in the co-moving (primed) frame is: However, since the emitting source is moving almost as fast as the emitted photon, the source will be catching up on the photon, and travel a distance s = v Dtcos q . The time difference in the arrival time of the two photons will therefore be reduced by s/c, i.e

  43. and the observed frequency is This is the relativistic Doppler effect which defines the Doppler factor One can show (i.e. Rybicki & Lightman, chap. 4.9) that the ratio of the flux density Sn and the frequency cubed is invariant under Lorentz transormation: Since the observed frequency is n=Dn’, = we find that also the observed flux has to be (S’n= flux density in co-moving frame)

  44. Even for relatively modest relativistic velocities of v=0.97c, for example, the flux in the forward direction can be boosted by a factor 1000, while it is reduced by a factor 1000 in the backward direction! The transformation from a spherical to an elliptical polar diagram shows that angles are also transformed by relativistic effects. The so-called relativistic aberration (see Rybicki & Lightman, chap. 4.1) is given by: In the rest frame of the source, half of the radiation will be emitted from –p/2 to p/2, hence setting q’ = p/2 will give thus for g>> 1 half of the radiation will be emitted in a cone with half-opening angle

  45. Jet-sidedness Since we expect jets to be two-sided, we always have two angles under which the emission is seen by an observer: q and q+p . We can now calculate the flux ratio R between jet and counter-jet under the assumption of intrinsically symmetric jets: Even for mildly relativistic jets one side will always be significantly brighter than the other

  46. Most of the strong, compact radio cores seem to come from sources where the angle to the line of sight is small, these jets are always one-sided. • Even most of the large scale jets appear to be one- sided, even though 2 extended lobes are seen indicating that • really two jets are present. Nearby FRI radio galaxy and LINER galaxy M87 - no counter- Jet observed

  47. Summary: evidence for relativistic motion in AGN • Superluminal motion • One-sided jets (pc and kpc scales) Caveats • None of the above evidence proves that relativistic motion exists • Alternative explanation exist for each observed property (e.g., one-side jets) • But relativistic motion=beaming is the only and the simplest explanation forall of them at once

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