tema 6. continuum emission - agn - universidad de …papaqui/fisica_agn/tema_1.06.pdf · tema 6....

TEMA 6. Continuum EmissionAGN

Dr. Juan Pablo Torres-Papaqui

Departamento de AstronomıaUniversidad de Guanajuato

DA-UG (Mexico)

[email protected]

Division de Ciencias Naturales y Exactas,Campus Guanajuato, Sede Valenciana

Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 1 / 48

Continuum Emission

Give a general overview of the continuum emission from AGN.


Continuum Emission

Observing the SED of AGN


Continuum Emission

Types of Continuum Spectra

1) Blazars:Non-thermal emission from radio to gamma-rays(2 components)

2) Seyferts, QSOs, BLRGs:IR and UV bumps (thermal)radio, X-rays (non-thermal)

Spectral Energy Distributions (SEDs): plots of power versus frequency(log-log)


Continuum Emission

Continuum Emission in AGN

UV-Optical Continuum

Infrared Continuum

High Energy Continuum

Radio Continuum

Jetssuperluminal motion


Continuum Emission

Spectral Energy Distribution of Seyferts, QSOs, BLRGs

Radio Quiet Quasars Radio Loud Quasars


Continuum Emission



Continuum Emission

Spectral Energy Distribution of Blazars

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


Continuum Emission

AGN: Spectral Energy Distribution


Continuum Emission

AGN: Spectral Energy DistributionMany different types of AGN SEDs


Continuum Emission

AGN: Spectral Energy Distribution


Continuum Emission

The Blue and IR bumps

LIR contains up to 1/3 of Lbol

LBBB contains a significant fraction of Lbol (Big Blue Bump)

IR bump due to dust reradiation, BBB due to blackbody from anaccretion disk

The 3000A bump in 1800-4000A:

Balmer ContinuumBlended Balmer linesForest of FeII lines


Continuum Emission

The 3000A Bump


Continuum Emission

AGN: (Non-)Thermal Emission

Fundamental Questions:

(1) How much and which part of AGN SED is thermal and non-thermal?

a) Thermal: Particles have Maxwellian velocity distribution due tocollisions

b) Non-Thermal: e.g. Synchrotron radiation with power-law energydistribution of particles

(2) How much emission is primary and secondary?

From Central Engine (primary)Re-radiation (secondary)


Continuum Emission


“Big Blue Bump” is assumed to be thermal emission from the accretiondisk with T = 105±1 K (100A)

What emission is expected from an accretion disk?

Assumptions:

Locally the disk emits like a Black Body

Geometrically/optically thin/thick disk


Continuum Emission


Gravitational potential energy is released half (virial theorem) into kineticenergy and half in to radiation:

L =G M M

2r= 2πr2σT 4

From which is follows:

T =

(G M M

4πσr3

)1/4

(this is an approximation, averaged over the disk)


Continuum Emission

UV-Optical ContinuumIn reality, energy is dissipated locally in the disk through viscosity. Thisyields:

T (r) =

[3GMM

8πσr3{1− (Ri/r)1/2}

]1/4

or

T (r) =

[3GMM

8πσR3s

]1/4(r

Rs

)−3/4

when r >> Ri and Ri ≈ Rs


Continuum Emission


Inserting Rs = 2GM/c2, we find:

T (r) = 6× 105

(M

ME

)1/4

M−1/48

(r

Rs

)−3/4

K

This peaks at ∼100A for this typical temperature of a million K .

Hence the accretion disk continuum spectrum is a superposition of manyBB’s with different temperatures.


Continuum Emission


When we assume that the disk is optically thick (hence the luminositydoes not depend on the surface mass density of the disk), then:

dLν(r) = 2π r cos(i)(πBν)dr

is the luminosity from a bin of width dr .

Lν =4π2hν3cos(i)

c2

∫ Ro

Ri

r dr

exp(hν/kT (r))− 1

This does not have a simple solution, but ...


Continuum Emission


At low frequency all BBs emit like a Wien spectrum, hence:

Lν ∝ ν2

(long wavelengths)

At high frequencies, the spectrum has an exponential cutoff, determinedby the highest temperature (at Ri )

Lν ∝ ν3exp(−hν/kT (Ri ))

(short wavelengths)


Continuum Emission


In the intermediate regime, if we define:

x =hν

kT (r)=

hν

kTs(r/Rs)3/4

Substituting this back into the previous equation, we find:

Lν ∝ ν1/3

(intermediate wavelengths)


Continuum Emission

UV-Optical ContinuumThe superposition of these BB spectra will thus look like:


Continuum Emission


Homework #5: Compute the four velocity of the Black Bodiesshowed in the last figure.


Continuum Emission

Observations of Optical-to-UV Continuum

After removing the small blue bump, the observed continuum goes asν−0.3

Removing the extrapolation of the IR power law gives ν−1/3 - but isthe IR really described by a power law?

More complex models predict Polarization and Lyman edge - neitherconvincingly observed

Disk interpretation is controversial!


Continuum Emission

Variability

More observational clues come from variability:

UV & Optical vary in phase

Variation are larger at higher frequencies

Small variations in UV are smoothed out at lower freq.

Most variability at longer time-scales [P(f ) ∝ 1/f 2−3]

Simultaneous UV & Optical variability is a major problem for models withthe T-gradient outward!


Continuum Emission

Alternative interpretation?

Optical-UV could be due to free-free (bremsstrahlung) emission frommany small clouds in a optically thin disk (Barvainis 1993)

Slope consistent with observed (α ∼ 0.3), low polarization and weakLyman edge predicted

Requires high T ∼ 106 K

Disadvantage: low efficiency!


Continuum Emission

Infrared Continuum

In most radio-quietAGN, there is evidencethat the IR emission isthermal and due toheated dust

However, in some radio-loud AGN and blazars the IR emission isnon-thermal and due to synchrotron emission from a jet.


Continuum Emission

Infrared Continuum: Evidence

Obscuration:

Many IR-bright AGN are obscured (UV and optical radiation isstrongly attenuated)

IR excess is due tore-radiation by dust


Continuum Emission

Infrared Continuum: Evidence

IR continuum variability:

IR continuum shows same variations as UV/optical but withsignificant delay

Variations arise as dust emissivity changes in response to changes ofUV/optical that heats it


Continuum Emission

Dust Reverberation


Continuum Emission

Dust Reverberation

Optical varied by factor∼20

IR variations follow by ∼1year

IR time delays increasedwith increasingwavelength

Evidence for dust(torus) alight year from the AGNnucleus, with decreasing T asfunction of radius


Continuum Emission

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 Sub-mm break from differentdistances because of the radial dependence of temperature

1µ minimum: hottest dust has T ∼2000 K (sublimation T ) and is at∼0.1 pc (generic feature of AGN)


Continuum Emission

Emerging picture


Continuum Emission

X-ray emission

AGN are easy to find in X-rays. Away from the Galactic plane mostX-ray sources are AGN. Many X-ray selected AGN show weak or nooptical signatures.

X-rays come from very close to the SMBH. The most rapid variabilityis seen in X-rays.

The only spectral lines observed that come from close to the MBHare in the X-ray band. The strongest line is from Fe at ∼6.4 keV butother lines have been observed.

All types of AGN are strong X-ray sources.

We can “X-ray” the material around AGN using the emission fromclose to the MBH as a background source.


Continuum Emission

X-ray emission: Origin

Accretion flow surrounded by dusty torus

BBB radiation from disk → ‘big blue bump’

B-field loops → optically thin corona

Isotropic X-rays from Comptonization of disk photons in hot corona

Power law spectrum


Continuum Emission

X-ray emission: Origin


Continuum Emission

Reflection and Fluorescence

The MBH is surrounded by anaccretion disk. Suppose thatX-rays are generated above thedisk:

We observe some photonsdirectly.

Others hit the accretiondisk. Some are reflected.Some eject an inner shellelectron from an atom togive fluorescent lineemission.


Continuum Emission

NGC 4945


Continuum Emission

Reflected X-ray Spectra


Continuum Emission

Astrophysical Jets in Radio-Loud AGNChandra commonly resolves kpc-scale X-ray jet emission in nearby RLAGN:

FRIs → kpc X-ray emission synchrotron in nature (e.g. Hardcastle et al. 2001, 2003, 2005)

FRIIs → X-ray emission tends to be inverse-Compton

What about (unresolved) parsec-scale X-ray jets?


Continuum Emission

Radio-Galaxy Nuclei - Two Competing Models

Is nuclear X-ray emissiondominated by:

The parsec-scale jet?-or-

The accretion flow?


Continuum Emission

Evidence for jet-dominated nuclear X-ray emission

Correlations between the ROSATsoft X-ray and VLA radio corefluxes

Parsec-scale radio emission isjet-generated and stronglyaffected by beaming

Tight correlations suggest X-rayemission affected by beaming insame manner as radio NGC 6251- 5 GHz VLBI

Soft X-ray emission originates ina jet

Double-peaked SED (modeled

with syn+SSC)


Continuum Emission

Evidence for accretion-dominated nuclear X-ray emission

Short (∼ks) timescale variability in broad-line FRII 3C 390.3 (Gliozziet al. 2005)

Broadened Fe K → line emission in narrow-line FRI NGC 6251(Gliozzi et al. 2004)

Implies Fe K → origin in inner regions of accretion flow


Continuum Emission

Evidence for accretion-dominated nuclear X-ray emission


Continuum Emission


X-ray continuum emission in the nuclei of RL AGN consists of:

“Radio-quiet” accretion-related component“Radio-loud” jet-related component


Continuum Emission



Continuum Emission

X-ray emission: some conclusions

X-ray emission of FRI radio-galaxy nuclei is unabsorbed anddominated by a parsec-scale jet

X-ray emission of FRII radio-galaxy nuclei is heavily absorbed andaccretion-related

Each FRII also has an unabsorbed component of X-ray emission →jet origin

Data do not exclude the presence of a heavily obscured,accretion-related emission in FRI-type source


tema 6. continuum emission - agn - universidad de …papaqui/fisica_agn/tema_1.06.pdf · tema 6....

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