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
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