Magnetic fields in AGN
JET POLARIMETRY TEAM: Eric Perlman, Mihai Cara, Markos
Georganopoulos, Diana Worrall, Mark Birkinshaw, Bill Sparks,
Chris O’Dea, Stefi Baum, Dave Axon, Teddy Cheung,
Yasunobu Uchiyama, Paolo Coppi, Mitch Begelman, Herman
Marshall, Lukasz Stawarz
IR POLARIMETRY TEAM: Enrique Lopez-Rodriguez, Chris
Packham, Eric Perlman, Nancy Levenson, Rachel Mason,
Stuart Young, Cristina Ramos-Almeida, Moshe Elitzur, Thomas
Jones
The Unified AGN Model
Supermassive (107-1010 M) black
hole. M = 108M RG~2 AU
Accretion disk – thermal UV/Xray lines from highly ionized
atoms (R~3-100 RG)
High velocity (>1000 km/s)
broad-line clouds (R~103-4 RG)
Dusty torus, which orbits in/near
plane of accretion disk (R~104-5
RG)
Lower velocity (few hundred
km/s) narrow-line clouds (R~105-7
RG)
Relativistic jet ( ~ 5-30) – may
be collimated on ~50 RG scales,
can extend for many
kiloparsecs
Observed properties vary with
viewing angle
Urry & Padovani 1995
Radiation Processes in Jets
Synchrotron radiation
emitted by relativistic
particles in magnetic field
Inverse-Compton – scattering
interaction between photon and a
relativistic particle that results in a
higher-energy photon.
Radiation from jets emitted by two processes: synchrotron and inverseCompton.
For inverse-Compton, the ‘scattered’ photon can be either from
within the jet (often called synchrotron self-Compton) or some
external source (e.g, the cosmic microwave background or emission
line regions).
Dust – multiple
polarizing
mechanisms
Dust can polarize radiation via
a number of mechanisms:
Optical: Electron scattering
NIR: Dichroic absorption &
transmission
MIR: Dichroic emission
To understand the dust
properties, you need multiband observations!
IC5063
NIR polarimetry observations of IC5063
Observations on the 4-m AAT
J
H
Kn
IC5063
Mechanisms of polarization
?
?
J
scattering
H
radio axis
Vector at top right of panels – 5%.
?
Kn
Polarization Model
Observer
Central engine
unpolarized power-law
Fν ∝ ν-0.5
Torus
Dichroic absorption
Pλ ∝ Aλ x λ-γ
Extinction: Av(tor)
Ionization cones
Electron scattering
Pλ = λ 0
Host galaxy
Dichroic absorption
P/Pmax = exp(-ln(λ/λmax)2)
Extinction: Av(gal)
Polarization Model
Degree of polarization
Total intensity
AGN extinguished
Host galaxy
Ionization cones
Model
Polarized intensity
μm
μm
Extinction Host galaxy
Torus
AGN extinguished
Host galaxy
Ionization cones
Model
mJy
%
mJy
AGN extinguished
Host galaxy
Ionization cones
Model
μm
Av(gal) = 6 ± 2 mag.
Av(tor) = 48 ± 2 mag.
Intrinsic polarization at Kn
dichroic absorption Pint = 12.5 ± 2.7
by
%
Nuclear Extinction: Interpretation
HOMOGENEOUS TORUS
MIR
FIR
CLUMPY TORUS
NIR
~100 pc
few pc
NIR total flux is emitted from inner facing clumps and/or central
1.
engine
2. MIR total flux is emitted from the warm dust of clumps
NIR Polarization in the Torus: Interpretation
NIR polarization produced by the passage of light through the
aligned dust grains in the clumps of the torus
2
1
central engine
NIR
3
single cloud
1. Radiation from the central engine
2. Passage of light through the aligned dust grains to the observer
3. Radiation is extinguished
Observer
Magnetic Fields in the Torus of IC5063
Best model in literature by Vrba, Coyne & Tapia (1981) for
molecular clouds, tested in optical and IR wavelengths
Main parameters: Tgas, Tgr, a, n
- Other parameters are dependent of these four ones.
Estimated an intrinsic polarization of 12.5 ± 2.7 %
Estimated a torus obscuration of Av(tor) = 48 ± 2 mag.
Magnetic field for the torus of IC5063 is calculated to
be:
B = 12 - 128 mG
Extended NIR Polarization
From warm dust in narrow-line region – work still ongoing.
Magnetic fields in the Torus of IC5063
Magnetic field for the torus of IC5063 is calculated to
be:
B = 12 - 128 mG
Physical environment in AGN torus is extreme, in comparison with
the typical molecular clouds in the ISM
Note the model is for quiescent molecular clumps
Lower-limit of the magnetic field is estimated
Literature:
Water vapor maser clouds in NGC 3079 at
1.
B > 11 mG
0.64 pc
Vlemmings et al. (2007)
Water vapor maser clouds in NGC 4258 at 0.2
2.
B ~ 130 mG
pc
Modjaz et al. (2005)
3 VLA circular polarization in NGC4258
B < 200 mG
4 Clumpy magnetic disk-driven wind model
Herrstein et al. (1998)
B > 20 mG
Kartje et al. (1999)
At high energies:
Synchrotron or IC/CMB?
• Without polarimetry, we can’t tell the difference
– either model works fine.
Polarimetry can make a critical difference:
o IC/CMB should not be polarized except for electrons with γ~1
o Synchrotron emission highly polarized at all wavelengths
• IC/CMB requires a very unlikely set of parameters:
Γ>30, θ<4°.
• Syncrotron has no such requirement
At high energies:
Synchrotron or IC/CMB?
• Synchrotron is favored, but IC/CMB cannot be
formally ruled out.
o Requires a separate, high-energy population of electrons reaching
ϒ=108.
o IC/CMB requirements would correspond to a blazar – does not
match observational properties of PKS1136
At high energies:
Synchrotron or IC/CMB?
• Synchrotron is favored, but IC/CMB cannot be
formally ruled out.
o Requires a separate, high-energy population of electrons reaching ϒ=108.
o IC/CMB requirements would correspond to a blazar – does not match
observational properties of PKS1136
o Further observations needed.
• IC/CMB can only be sub-Eddington if the jet is
leptonic.
o Lower solid trace – Electron power required by X-ray and radio
emisison.
o Upper solid trace – Jet power required for one proton per electron.
Conclusions
o Imaging Polarimetry is a powerful tool for many regions of AGN
In Torus, allows you to penetrate the mechanisms by which dust
scatters and re-emits light
Multiple Emission mechanisms must be at work in different
bands – it is NOT all scattering.
Multi-waveband view is necessary.
In process: Mid-IR polarimetry of bright AGN with
CanariCAM
In jet, probes directly the magnetic field orientation in the emission
region
Radio: low-energy electrons
Optical/UV – dynamic processes, linked to X-ray emission?
Test emission mechanism
In at least one object – IC/CMB model very difficult to
support
More objects need to be done.