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Multi-Wavelength Polarizations of
Western Hotspot of Pictor A
Mahito Sasada (Kyoto University)
S. Mineshige (Kyoto Univ.), H. Nagai (NAOJ), M. Kino (JAXA),
K. Kawabata (Hiroshima Univ.), H. Nagayama (Nagoya Univ.)
Outline of My Talk
• Introduction
• AGN jet and hot spot
• Pictor A
• Observational results
• Near-infrared polarizations obtained by IRSF/SIRPOL
• An optical imaging polarimetry obtained by VLT/FORS1
• Comparison between the polarizations from radio to optical
bands
• Inclination angle and compression in the jet estimating
from the polarization
Structures of AGN Jet
Radio image of Cygnus A
• AGN jets often have several
structures.
• Knot
• Hotspot
• Radio lobe
• In a hot spot, there are relativistic
electrons accelerated in a shock
region where the jet interacts with
ICM.
• In the hot spot, there is not a
simple point source but complex
structures.
Hot Spot
Core
Radiation in Hot Spot
• A hot spot has a broad-band
radiation from radio to X-ray
bands.
• Two radiations in the hot spot.
Multi-wavelength Spectrum
• Low energy; Synchrotron radiation
• High energy; Inverse-Compton
radiation
• There are varieties of spectra in
individual hot spots.
Stawarz+ 2007
Radio Polarimetry in AGN Jet
• Spatial distributions of radio
polarization in AGN jets, hot spots
and radio lobes can be obtained by
VLBI observations.
• The synchrotron radiation is
dominated in the radio band.
Polarimetric observations are mainly
performed in the radio band.
Derher+ 1987
Optical and Radio Polarizations in M87 jet
• M87 is one of the nearest radio
galaxies.
• M87 jet is highly polarized in optical
and radio bands; 40%-50%.
• In bright knots, the magnetic field is
distributed perpendicular to jet axis.
• In other regions, the magnetic field
are parallel to jet direction.
• Radio and optical polarizations are
different in bright knots.
There are few optical polarization
observations in the jet hot spots.
Optical and Radio Polarimetries; M87
Perlman+ 1999
Advantage in Optical Polarization
• Magnetic fields in a hot spot and a knot are evolved by the shock interacted with
ICM.
• Energies emitting radio and optical synchrotron photons are different.
→ Synchrotron cooling timescales are different.
(𝑡1/2 ∝ 𝐵−2 𝛾 −1 ； 𝑡1 2;radio ~260 × 𝑡1 2;opt )
• We can trace the particle-accelerated regions from the optical synchrotron
radiation because of the rapid cooling timescale.
We study a relation between the particle-accelerated region and its
magnetic field by observing the optical polarization.
We investigate the emitting regions and its magnetic field to compare
the radio and optical emissions and polarizations.
Polarization in Pictor A
• One of the most famous FRII type radio
galaxies.
• There are two-side radio jets, hot spots
in terminals of the jet, and radio lobes.
• There is a highly polarized emission in
the western hot spot (WHS); 30%-60%.
• The emission is polarized parallel to the
jet direction.
• Detected polarizations are distributed
perpendicular to the radio lobe.
Perley+ 1997
IRSF/SIRPOL
• We performed simultaneous J-, H- and Ks-band
near-infrared (NIR) polarimetries to WHS of
Pictor A using IRSF/SIRPOL.
• Location; Sutherland in South Africa
• Seeing size; typically ~1”
SIRPOL
• Observing bands; NIR J, H and Ks bands,
simultaneously
• A single-beam polarimeter; a half-wave plate
rotator unit and a fixed wire-grid polarizer
Three-band NIR Imaging Polarimetries
• We obtained three-band NIR polarizations
to the WHS of Pictor A.
• The emission from WHS is highly polarized.
• Degrees of Polarization
Obtained images of WHS field
HWP=0°
HWP=22.5°
(PJ, PH, PK ) = (47% ± 6%, 46% ± 2%, 45% ± 3%)
• Angle of Polarization
(PAJ, PAH, PAK) = (107±4deg, 113±1deg, 111±2deg)
• There is no difference between the
polarization in each band.
HWP=45°
HWP=67.5°
Multi-Wavelength Images
• Radio, optical and X-ray images
of Pictor A.
• An optical image was obtained by
VLT/FORS1.
• The WHS of Pictor A is bright in
the radio, optical and X-ray bands.
• There is a distinct jet knot in the
X-ray band.
Radio
Core
Hot spot
Perley+ 1997
Optical
X-ray
Hardcastle+ 2005
Optical and Radio Images ①
• There are extended structure in
the WHS of Pictor A both in the
radio and optical bands.
VLA radio image
• Hot spot
• Filament
VLT optical image
Hot spot
• The hot spot is 10 times brighter
than the filament.
• There is more effective particleacceleration and cooling in the hot
spot.
Filament
Perley+ 1997
Optical and Radio Images ②
• Sizes of the emitting region
• Radio；16.8 kpc
• Optical；10.5 kpc (Filament)
4.8 kpc (Hot spot)
• Light travel time distances calculated
from the synchrotron cooling
timescales.
VLA radio image
VLT optical image
24’’
6.8’’
• Radio；110 kpc
• Optical and near-infrared；0.5-1 kpc
• In the hot spot, a particle-accelerated
region corresponding to the shock is
extended to 4.8 kpc.
• Particles should be accelerated in the
filament.
15’’
Perley+ 1997
Opt Pol.
Optical Polarization
• Polarizations in the radio and optical bands are
approximately the same distributions.
• Polarization vectors are almost parallel to the jet
direction.
• The degree of polarization in each region of the
hot spot is different; P=32%-53%.
• A terminal region of the hot spot is the most
polarized among four regions.
• Degrees of polarization in the hot spot are more
polarized than those of filaments; P=16%-21%.
Jet flow
50%
Radio Pol.
Perley+ 1997
1
3
Optical and NIR Polarizations
1
2
2
• Polarization vectors in the hot spot
are distributed almost the same
regions in the Q-U plane.
• Angles of polarization are the same.
• Degrees of polarization are different.
• Polarization vectors in the filaments
are distributed different regions in
the Q-U plane.
Magnetic fields are different between
in the hot spot and the filaments.
4
3
Optical and NIR polarizations in Q-U plane
0.4
0.2
0
-0.2
Filament
-0.4
Hot spot
-0.4
-0.2
0
Q/I
0.2
0.4
Multi-wavelength Polarization
• Wavelength dependence of
polarization in the hot spot from
radio to optical bands.
• Polarization vectors are
approximately the same.
There are the same
distributions of the magnetic
field in the regions
distributing from the radioto optical-emitting electrons.
Multi-wavelength polarization; radio-optical
Radio
Perley+ 1997
NIR
Opt.
Our work
Magnetic Field in Hot Spot
Meisenheimer+ 1989
• A magnetic field in the shock region is
compressed.
A magnetic field distributes perpendicular to
the direction of jet flow.
The polarization vector should be parallel to
the jet direction.
Compress
• Accelerated particles move outward
through a back flow.
• There is no wavelength dependence of
polarization.
High-energy particles radiated radio and
optical emissions exist in a same distribution
of magnetic field.
Radio and optical lights are emitted from the same magnetic field aligned
by the shock.
Polarization by Compression
• The observed degree of polarization is
proportional to the degree of alignment
of the magnetic field at the emission
region.
Random magnetic field
Laing 1980
Compress
Alignment of magnetic field
• There is a compression generated by the
interaction between the jet flow and ICM.
• The side surface of magnetic field is
aligned by its compression
• The degree of alignment of the magnetic
field becomes large, when the
compression is strong.
• The degree of alignment is different by
the inclination angle 𝜃.
𝜃
Polarization and Inclination Angle
• An observed degree of polarization is related to the compression
parameter 𝜂 and inclination angle 𝜃.
P = Π𝑠
1−𝜂 −2 sin2 𝜃
2− 1−𝜂 −2 sin2 𝜃
; Π𝑠 =
𝛼+1
~73%
𝛼+5/3
(𝛼 = 0.8)
Hughes+ 1985
• 𝜂 is the ratio between densities in compress and uncompress; 𝜂 = 𝑛c 𝑛unc > 1.
• 𝜂 is constrained by the Rankine-Hugoniot relation. We assume the non-relativistic
adiabatic case for simplicity; 1 < 𝜂 < 4.
• We can constrain the lower limit of 𝜃 using the observed degree of
polarization (53%) at the terminal of WHS of Pictor A, assuming 𝜂 = 4;
71° < 𝜃 < 90° .
Constraint from Hydrodynamics Simulation
• A hot spot and filament of WHS of Pictor A are simulated in the
hydrodynamics. (Saxton+ 2002)
• The inclination angle 𝜃 of its jet is constrained from the
geometry of simulated filament.
°
• A filament becomes bar-like geometry when 𝜃 is larger than 60 .
𝜃 = 45°
Filament
Hot spot
• The 𝜃 is determined by the constraints of simulation and a
limitation of the brightness ratio between the X-ray fluxes of
the jet and counter jet.
 𝜃 = 70°
𝜃 = 60°
We calculate the compression° parameter 𝜂 from the observed
polarization assuming 𝜃 = 70 .
• 𝜂~4.6 (𝑃 = 53%；terminal region of WHS)
It can not be considered in a simple adiabatic case in
the terminal shock of Pictor A.
→ We need the relativistic case in the hot spot.
𝜃 = 90°
Saxton+ 2002
Summary
• The emission in the WHS of Pictor A is highly polarized; 40% 50%.
• The angle of polarization is parallel to the jet direction.
• The emission from the terminal of the hot spot is the most
polarized in the optical band.
• The polarizations in the radio and optical bands are the same
distributions.
The radio- and optical-emitting regions have the same
distributions of the magnetic field.
Wilson+ 2001