Gamma-Ray Burst Polarization Kenji TOMA (Kyoto U/NAOJ) Collaborators are: Bing Zhang (Nevada U), Taka Sakamoto (NASA), POET team Ryo Yamazaki, Kunihito Ioka, Takashi Nakamura

GRB polarization One of the GRB frontiers is polarization observations! The GRB mechanism has been studied mainly through light curves and spectra so far. Measuring multi-wavelength polarization can give us much new information. k B e

GRB polarization: current status Central engine Relativistic jet BurstAfterglow (Synchrotron emission) GRB polarization has been detected only in the late, optical afterglow.  L,opt ~ 1-3 % ~ 10 sec   ~ % (Coburn & Buggs 03) This result is quite controversial.

Near future prospects We will obtain the multi-wavelength polarizations in the near future. => X-ray and gamma-ray pol Many satellites are proposed to operate from about POET (USA), PoGO (USA, Japan), XPOL (Europe), POLAR (Europe), Polaris (Japan) POlarimeters for Energetic Transients => (early) optical pol Kanata (Japan) and Liverpool (UK) can detect early (>~a few minutes) optical polarization. => Radio pol ALMA (USA, Japan, Europe) is planned to operate from about 2010.

What can be explored (1) Emission mechanism Synchrotron emission? Compton scattering? Thermal (photospheric) emission? (2) Geometry of source Magnetic field configuration Jet? Spherical outflow? (3) Composition of source Electron energy distribution Electron-proton? Electron-positron? Polarization is changed through the source. => Particle acceleration, Jet acceleration

Afterglow Polarization

Afterglow polarization The (late) afterglow is widely explained as due to synchrotron emission of electrons accelerated in the external shock. Shocked fluid Accelerated electrons Strong magnetic field k B e We have obtained some implications for the magnetic field configuration in the shocked fluid from the optical polarimetry. The field configuration is important for particle acceleration. (Meszaros & Rees 97; Sari et al. 98)

Electron energy distribution  L,opt ~ 1-3 %  L ~ 70% The magnetic field is not perfectly ordered. Optical afterglow Large-scale random field, or small-scale random field? Shocked fluid Accelerated electrons Strong magnetic field Ordered field

Small-scale random field case It is possible that the field is generated by some plasma instabilities in the collisionless shocks. In this case, the field may be coherent on tiny scales (~10 6 cm). local polarization survives. GRB jet The visible angular size is ~  -1 because of the beaming effect.  can be observed around when  -1 ~  j. Polarization angle will change by 90 degrees. (Medvedev & Loeb 99; Sari 99; Ghisellini & Lazzati 99)

Large-scale random field GRB jet Visible region To reproduce the optical detection, N ~ (coherence length ~ cm) If the strong field is generated by macroscopic inhomogeneity (e.g., vorticity), it is coherent on large scales. In this case, polarization should be subject to erratic variations of polarization angle on dynamical time scales. (Sironi & Goodman 07; Gruzinov & Waxman 99)

Observational results GRB : smoothest light curve GRB : least smooth light curve Change of polarization angle by 90 degrees is not seen. Erratic variation of polarization angle on dynamical time scales. Large-scale random field is suggested. Early observations are crucial!  L,opt < 8% (t ~ 203 sec) (Mundell et al. 07)

Radio afterglow 0.5  2 1/3 optical -(p-1)/2 radio VLA ALMA  has not be detected in the radio band, although the synchrotron  is little dependent on frequency. This seems because the self- absorption frequency is typically in the VLA band. a In the frequencies lower than the self-absorption frequency, the radiation is strongly coupled to the particles, and is similar to blackbody radiation. ~1 day ALMA!

Radio afterglow: plasma effects ＋ ＝ The radio polarimetry can be used to diagnose plasma composition in the shocked region, because plasma effects are stronger in lower frequencies. Faraday rotation effect Faraday depolarization Two natural modes with different phase velocities The polarization plane rotates.   ’  ” Linear polarization cancels out. (Sazonov 69; Matsumiya & Ioka 03)

Efficiency of acceleration (KT, Ioka, Nakamura 08) 1-ff  It is possible that only a small fraction f of electrons are accelerated. n’ = n/f, E’ = E/f (Eichler & Waxman 05) The true total energy is larger than previously estimated! Observed afterglow Depolarization existence of non-accelerated electrons, large-scale coherent field

Burst Polarization

Burst polarization The emission mechanism of the burst is highly debated. Measuring the burst polarization is a powerful tool. Synchrotron emission? Synchrotron Self-Compton scattering? Bulk Compton scattering? Photospheric emission? Jitter radiation?

Burst polarization Synchrotron with ordered field (Granot 03; Lyutikov et al. 03) Synchrotron with small-scale random field (Granot 03; Nakar et al. 03) Bulk Compton scattering (Lazzati et al. 04) BB Seed optical photon It has been shown that high degree of polarization can be obtained in the following 3 models. Visible region

Statistical approach POET satellite is designed to detect ~ 100 bursts in keV in 2 yr operation. The 3 models can be distinguished. Monte Carlo simulation (KT et al. in prep.) All the bursts have high  in the ordered field synchrotron model.  distributions in the random field synchrotron model and in the bulk Compton model, but  near 100% is possible in the bulk Compton model.

Polarization spectrum  m a >10%? V If the magnetic field is ordered on large scales, cooled electrons will affect . (KT in prep.) Faraday depolarization Synchrotron model

Photospheric emission model  ~ 1 Progenitor star Seed blackbody emission Compton up- scattered Compton down- scattered polarized unpolarized The spectral peak might be produced by the photospheric blackbody radiation. (e.g., Ryde et al., Thompson et al., Ioka et al.) This model shows a unique  spectrum.

Summary We will obtain the multi-wavelength polarizations in the near future. Measuring time- and energy-dependent polarization can reveal many aspects which are not available with the more traditional light curves and spectra. Measuring early optical polarization is crucial for determining the magnetic field configuration in the external shock. The electron energy distribution (and even the total explosion energy) can be probed by the radio polarimetry. The X-ray and  -ray polarimetry is powerful tool to understand the burst emission mechanism, magnetic field configuration in the jet, and the composition of the jet.