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Gamma-Ray Burst Polarization

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!.

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Gamma-Ray Burst Polarization

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  1. 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

  2. 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. B e k

  3. GRB polarization: current status GRB polarization has been detected only in the late, optical afterglow. Relativistic jet Burst Afterglow (Synchrotron emission) Central engine ~ 10 sec Pg ~ 80 +- 20 % (Coburn & Buggs 03)This result is quite controversial. PL,opt ~ 1-3 %

  4. 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 2010. 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.

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

  6. Afterglow Polarization

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

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

  9. 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 (~106cm). (Medvedev & Loeb 99; Sari 99; Ghisellini & Lazzati 99) local polarization survives. GRB jet The visible angular size is ~G-1 because of the beaming effect. P can be observed around when G-1 ~ qj. Polarization angle will change by 90 degrees.

  10. Large-scale random field If the strong field is generated by macroscopic inhomogeneity (e.g., vorticity), it is coherent on large scales. (Sironi & Goodman 07; Gruzinov & Waxman 99) Visible region To reproduce the optical detection, N ~ 103. (coherence length ~ 1013cm) GRB jet In this case, polarization should be subject to erratic variations of polarization angle on dynamical time scales.

  11. Observational results GRB 030329: least smooth light curve GRB 020813: smoothest light curve Change of polarization angle by 90 degrees is not seen. Early observations are crucial! Erratic variation of polarization angle on dynamical time scales. Large-scale random field is suggested. PL,opt < 8% (t ~ 203 sec) (Mundell et al. 07)

  12. Radio afterglow P has not be detected in the radio band, although the synchrotron P is little dependent on frequency. n-(p-1)/2 n1/3 This seems because the self-absorption frequency is typically in the VLA band. n2 VLA ALMA optical ~1 day radio In the frequencies lower than the self-absorption frequency, the radiation is strongly coupled to the particles, and is similar to blackbody radiation. Pn 0.5 ALMA! na

  13. 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 ＋ The polarization plane rotates. ＝ Two natural modes with different phase velocities (Sazonov 69; Matsumiya & Ioka 03) Faraday depolarization Dc Linear polarization cancels out. Dc’ Dc”

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

  15. Burst Polarization

  16. 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?

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

  18. Statistical approach POET satellite is designed to detect ~ 100 bursts in 50-500 keV in 2 yr operation. The 3 models can be distinguished. Monte Carlo simulation (KT et al. in prep.) All the bursts have high P in the ordered field synchrotron model. P distributions in the random field synchrotron model and in the bulk Compton model, but P near 100% is possible in the bulk Compton model.

  19. Polarization spectrum If the magnetic field is ordered on large scales, cooled electrons will affect P. (KT in prep.) Synchrotron model >10%? P na nV nm n Faraday depolarization

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

  21. 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 g-ray polarimetry is powerful tool to understand the burst emission mechanism, magnetic field configuration in the jet, and the composition of the jet.

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