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• 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.

Radio image of Cygnus A

Core

Hot Spot

Radiation in Hot Spot• A hot spot has a broad-band

radiation from radio to X-ray bands.

• Two radiations in the hot spot.• Low energy; Synchrotron radiation• High energy; Inverse-Compton

radiation• There are varieties of spectra in

individual hot spots.Stawarz+ 2007

Multi-wavelength Spectrum

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.

Perlman+ 1999

Optical and Radio Polarimetries; M87

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. ( ； )

• 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.

• 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.

Polarization in Pictor A

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 (PJ, PH, PK ) = (, , )• Angle of Polarization(PAJ, PAH, PAK) = (107±4deg, 113±1deg, 111±2deg)

• There is no difference between the polarization in each band.

HWP= HWP=

HWP= HWP=

Obtained images of WHS field

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

Optical

X-ray

Core Hot spot

Perley+ 1997

Hardcastle+ 2005

Optical and Radio Images ①• There are extended structure in

the WHS of Pictor A both in the radio and optical bands.

• Hot spot• Filament

• The hot spot is 10 times brighter than the filament.

• There is more effective particle-acceleration and cooling in the hot spot.

VLA radio image VLT optical image

Perley+ 1997

Hot spot

Filament

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.

• 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.

VLA radio image VLT optical image

24’’

15’’

6.8’’

Perley+ 1997

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%.

Opt Pol.

Jet flow

50%

Perley+ 1997

Radio Pol.

Optical and NIR Polarizations

• 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.

Filament

Optical and NIR polarizations in Q-U plane

U/I

Q/I

1

23

4

1

2

3

0.4

0.2

0

-0.2

-0.4

-0.4 -0.2 0 0.40.2

Hot spot

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 radio- to optical-emitting electrons.

Multi-wavelength polarization; radio-optical

Perley+ 1997 Our work

Radio NIR Opt.

Magnetic Field in Hot Spot• 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.

• 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.

Meisenheimer+ 1989

Compress

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.

• 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 .

Laing 1980

Compress

𝜃

Random magnetic field

Alignment of magnetic field

Polarization and Inclination Angle• An observed degree of polarization is related to the compression

parameter and inclination angle .;

• is the ratio between densities in compress and uncompress; . • is constrained by the Rankine-Hugoniot relation. We assume the non-

relativistic adiabatic case for simplicity; .

• We can constrain the lower limit of using the observed degree of polarization (53%) at the terminal of WHS of Pictor A, assuming ;　 　 .

Hughes+ 1985

Constraint from Hydrodynamics Simulation

• A hot spot and filament of WHS of Pictor A are simulated in the hydrodynamics. (Saxton+ 2002)

• The inclination angle of its jet is constrained from the geometry of simulated filament.

• A filament becomes bar-like geometry when is larger than .

• The 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.

We calculate the compression parameter from the observed polarization assuming .

• ( ； 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.

Saxton+ 2002

Filament Hot spot

𝜃=45°

𝜃=60°

𝜃=90°

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