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Page 1: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion
Page 2: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Lecture 7

• Continuum Emission in AGN

• UV-Optical Continuum

• Infrared Continuum

• High Energy Continuum

• Radio Continuum - Jets and superluminal motion

Page 3: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 4: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Spectral Energy Distribution

AGN show emission lines in all astrophysically relevant wavelength regimes

Page 5: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Power Law Continuum• Emission observed from 108 Hz to 1027Hz:

α=energy index now know to differ in different

bands

ανAνL )(

Actual SED is a function of the AGN Class

Page 6: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

From last class:AGN Taxonomy

• Seyfert galaxies 1 and 2

• Quasars (QSOs and QSRs)

• Radio Galaxies

• LINERs

• Blazars

• Related phenomena

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

• 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

opticalradio LLR /

Page 8: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

The Continuum

A phenomenological approach:

• Power law continuum

• Thermal features

• Spectral Energy Distributions of

Radio-loud and Radio-quiet AGN

Page 9: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Observing the SEDs of AGN

Page 10: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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)

Page 11: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Spectral Energy Distributions

IR bump

Big Blue BumpEUV gap

Sanders et al. 1989

Page 12: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 13: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 14: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 15: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Blazars’ SEDs

Red blazars: 3C279 Blue blazars: PKS 2155-398

Wehrle et al. 1999 Bertone et al. 2001

Page 16: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 17: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 18: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 19: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Interpreting the BBBFrom accretion disk theory (last class),

And the maximum emission frequency is at

i.e., in the EUV/soft X-ray emission region.

Hz 106.3 16max ν

BBB=thermal disk emission?!

1. UV-Optical Continuum

Page 20: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Model Spectrum of an Accretion Disk

Page 21: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 22: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

• 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

Observations of optical-to-UV continuum

Disk interpretation is controversial!

Page 23: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 24: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Is an accretion disk really there?

Indirect evidence:

Fitting of SEDs Double-peaked line profiles

Direct evidence: Water maser in NGC 4258

Page 25: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Optical emission lines

Eracleous and Halpern 1984

Page 26: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Water Masers in NGC 4258

Within the innermost 0.7 ly, Doppler-shifted molecular clouds:

• Obey Kepler’s Law• Massive object at

center

Page 27: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 28: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 29: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Radial dependence of dust temperature

From the balance between emission and absorption:

With R in pc, Leff in erg/s, T in Kelvin

5/12

6 )(10R

LT eff

Hotter dust lies closer to the AGN

Page 30: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 31: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 32: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

Page 33: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

3. Radio properties of AGN

I) Basic features of radio morphologyII) Observed phenomena• Superluminal motion• Beaming

Page 34: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Radio features

Core

Lobes

JetHotspot

Page 35: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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!

Page 36: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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

"1106

kmcm

R

Page 37: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

The power of resolution

Energy is transported

by jets from the cores

to the outer regions

Page 38: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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??

Page 39: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Historgram of observed v/c in 33 jets

Page 40: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Explanation of apparent superluminal motionExplain 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 anangle measured from theline of sight.

Photon emitted along theline of sight at time t=0, travelsa distance d to us, taking a time t1 to arrive: t1 = d/c

A second photon is emitted at a time te later, whenThe 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

Page 41: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

The apparent transverse velocityis vT = vte sin / t = v sin / (1 – v cos /c)

As v approaches c, vT canappear > 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 occurswhen cos = v/c. We will only observe superluminal motion whenthe jets are pointed within an angle of 1/ towards the line of sight,but this light will be beamed and brightened.

Page 42: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion
Page 43: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion
Page 44: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Relativistic motion of plasma

• Relativistic bulk motion in radio sources has important consequences on the following observed quantities:

1. Frequency

2. Length and time

3. Intensity

4. Direction light is emitted

Page 45: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Relativistic Doppler Effect

Assume an emitting source moving at a speed v c at an angle with respect to the observer.

Time-dilation tells us that t in the observers rest frame for a periodic signal with frequency ’ 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 tcos . The time difference in the arrival

time of the two photons will therefore be reduced by s/c, i.e

Page 46: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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 S and the frequency cubed is invariant under Lorentz transormation:

Since the observed frequency is =D’, = we find that also the observed flux has to be (S’= flux density in co-moving frame)

Page 47: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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 –/2 to /2, hence setting ’ = /2 will give

thus for >> 1 half of the radiation will be emitted in a cone with half-opening angle

Page 48: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Jet-sidedness

Since we expect jets to be two-sided, we always have two angles

under which the emission is seen by an observer: and + . 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

Page 49: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

•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

Page 50: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

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 for all of them at once

Page 51: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Physics of AGNThe Emission-Line Regions (BLR, NRL)

An AGN produces a lot of ionizing radiation, most likely from theaccretion disk.

This emission is intercepted by gas and dust in the host galaxy.Correspondingly an AGN spectrum shows reprocessed radiation fromthis gas and dust. The respective features are:

• Broad-Line Region (BLR)• Narrow-Line Region (NLR)• IR-bump from a molecular (dusty) “torus” (which we talked about last class)

Reprocessed Radiation

Page 52: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

• FWHM several thousand km/sec up to 30000 km/sec FWZI (zero

intensity)• derived gas temperatures are several 104 K• Doppler broadening through bulk motion of gas in gravitational field

• with velocities as high as 0.1c, the distance from the Black Hole

can be as close as 100 Rs

• Comparison of continuum and BLR fluxes indicate that only 10% of

the continuum radiation is absorbed by BLR clouds• The volume filling factor is very low - a few millionth of the central

region is occupied by BLR 'clouds'• The necessary mass in the BLR to produce the observed luminosity

is only a few solar masses

• Broad-lines are very smooth - they are either made up of a huge

number of small clouds or represent a coherent structure

BLR:Properties

Broad, permitted emission lines (e.g., H) in the optical spectrum:

Page 53: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion
Page 54: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion
Page 55: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion
Page 56: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Lines from highly ionized gas (He II 1640, C IV 1549) respondfaster than lines from lower ionization levels (e.g. Balmer lines)

ionization structure in BLR

more highly excited lines are further in

Reverberation Mapping

Page 57: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

For Keplerian rotation, the FWHM of the lines should correspond to the typical velocity dispersion at the radius where the line is produced.

More highly ionized lines, which are closer in, should have largerFWHM and shorter time-lags.

Size of BLR

Reverberation Mapping

Page 58: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion
Page 59: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Luminous AGN are classified as:

• Seyfert galaxies (Type I and II)• Quasars, QSOs• BL Lacs• Radio galaxies (in `Broad line’ and `Narrow line’ variants)• LINERs

All powered by accretion onto supermassive black holes.But why so many classes - are these all physically distinctobjects?

AGN Unification Schemes

Page 60: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

An AGN consists of the following basic ingredients:• Black Hole (power source)• Accretion Disk (UV/x-rays)• Jet (radio)• -Core (compact, flat-spectrum, radio-to-gamma emission)• -Jet• -Lobes & Hotspots (extended, steep spectrum)

• Broad-Line Region (BLR)• Narrow-Line Region (NLR)• molecular (dusty) “torus" (feeding and obscuration)• host galaxy (feeding)

AGN: Basic Ingredients

Page 61: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Seyferts 1 and 2: Unification Scheme

In the broad-line region (BLR)

The Keplerian orbital speeds of the clouds around the central massive body will be large => lines are Doppler broadened.

Density is high => no forbidden lines are emitted

In the narrow-line region (NLR)

The Keplerian orbital speeds of the clouds will be much smaller => lines are narrow

Density is low => forbidden lines are emitted

Page 62: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

So, if the above Seyfert were viewed from direction (1), you would see:

Broad permitted lines

Narrow Forbidden Lines

Bright continuum from the central engine

i.e. a Seyfert 1

If, on the other hand, it were viewed from direction (2), you would see:

No broad permitted lines (obscured by dust torus)

Narrow Forbidden Lines

No bright continuum from the obscured central engine

except in the infrared and X-ray region, which gets through the dust

i.e. a Seyfert 2

Seyferts 1 and 2: Unification Scheme

Page 63: Lecture 7 Continuum Emission in AGN UV-Optical Continuum Infrared Continuum High Energy Continuum Radio Continuum - Jets and superluminal motion

Seyferts 1 and 2: Unification Scheme: Evidence for Torus

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