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Transcript of 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)
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: AGN J.P. Torres-Papaqui Physics of AGN 2 / 48
Continuum Emission
Observing the SED of AGN
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 3 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 4 / 48
Continuum Emission
Continuum Emission in AGN
UV-Optical Continuum
Infrared Continuum
High Energy Continuum
Radio Continuum
Jetssuperluminal motion
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 5 / 48
Continuum Emission
Spectral Energy Distribution of Seyferts, QSOs, BLRGs
Radio Quiet Quasars Radio Loud Quasars
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 6 / 48
Continuum Emission
Spectral Energy Distribution of Seyferts, QSOs, BLRGs
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 7 / 48
Continuum Emission
Spectral Energy Distribution of Seyferts, QSOs, BLRGs
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 8 / 48
Continuum Emission
Spectral Energy Distribution of Blazars
Red blazars: 3C279 Blue blazars: PKS 2155-398
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 9 / 48
Continuum Emission
AGN: Spectral Energy Distribution
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 10 / 48
Continuum Emission
AGN: Spectral Energy DistributionMany different types of AGN SEDs
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 11 / 48
Continuum Emission
AGN: Spectral Energy Distribution
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 12 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 13 / 48
Continuum Emission
The 3000A Bump
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 14 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 15 / 48
Continuum Emission
UV-Optical Continuum
“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: AGN J.P. Torres-Papaqui Physics of AGN 16 / 48
Continuum Emission
UV-Optical Continuum
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: AGN J.P. Torres-Papaqui Physics of AGN 17 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 18 / 48
Continuum Emission
UV-Optical Continuum
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: AGN J.P. Torres-Papaqui Physics of AGN 19 / 48
Continuum Emission
UV-Optical Continuum
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: AGN J.P. Torres-Papaqui Physics of AGN 20 / 48
Continuum Emission
UV-Optical Continuum
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: AGN J.P. Torres-Papaqui Physics of AGN 21 / 48
Continuum Emission
UV-Optical Continuum
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: AGN J.P. Torres-Papaqui Physics of AGN 22 / 48
Continuum Emission
UV-Optical ContinuumThe superposition of these BB spectra will thus look like:
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 23 / 48
Continuum Emission
UV-Optical Continuum
Homework #5: Compute the four velocity of the Black Bodiesshowed in the last figure.
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 24 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 25 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 26 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 27 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 28 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 29 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 30 / 48
Continuum Emission
Dust Reverberation
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 31 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 32 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 33 / 48
Continuum Emission
Emerging picture
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 34 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 35 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 36 / 48
Continuum Emission
X-ray emission: Origin
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 37 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 38 / 48
Continuum Emission
NGC 4945
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 39 / 48
Continuum Emission
Reflected X-ray Spectra
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 40 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 41 / 48
Continuum Emission
Radio-Galaxy Nuclei - Two Competing Models
Is nuclear X-ray emissiondominated by:
The parsec-scale jet?-or-
The accretion flow?
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 42 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 43 / 48
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: AGN J.P. Torres-Papaqui Physics of AGN 44 / 48
Continuum Emission
Evidence for accretion-dominated nuclear X-ray emission
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 45 / 48
Continuum Emission
Radio-Galaxy Nuclei - Two Competing Models
X-ray continuum emission in the nuclei of RL AGN consists of:
“Radio-quiet” accretion-related component“Radio-loud” jet-related component
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 46 / 48
Continuum Emission
Radio-Galaxy Nuclei - Two Competing Models
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 47 / 48
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
Continuum Emission: AGN J.P. Torres-Papaqui Physics of AGN 48 / 48