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 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 Optical and Radio Polarimetries; M87

Advantage in Optical Polarization

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 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 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 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 ； 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 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 Q/I 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 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 Compress Radio and optical lights are emitted from the same magnetic field aligned by the shock.

Polarization by Compression Laing 1980 Compress Random magnetic field Alignment of magnetic field

Polarization and Inclination Angle Hughes+ 1985

Constraint from Hydrodynamics Simulation Saxton Filament Hot spot

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