GammaRay Burst Polarization

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
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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.
gt 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
gt (early) optical pol
Kanata (Japan) and Liverpool (UK) can detect
early (gta few minutes) optical polarization.
gt Radio pol
ALMA (USA, Japan, Europe) is planned to operate
from about 2010.
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What can be explored
(1) Emission mechanism
Synchrotron emission?
Compton scattering?
Thermal (photospheric) emission?
(2) Geometry of source
Magnetic field configuration
gt Particle acceleration, Jet acceleration
Jet? Spherical outflow?
(3) Composition of source
Electron energy distribution
Polarization is changed through the source.
Electron-proton? Electron-positron?
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Afterglow Polarization
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Afterglow polarization
The (late) afterglow is widely explained as due
to synchrotron 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 for particle
acceleration.
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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?
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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.
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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.
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Observational results
GRB 020813 smoothest light curve
GRB 030329 least smooth 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 lt 8 (t 203 sec)
(Mundell et al. 07)
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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
optical
VLA ALMA
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
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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
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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 ltgt existence of non-accelerated
electrons, large-scale coherent field
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Burst Polarization
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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?
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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
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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.
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Polarization spectrum
If the magnetic field is ordered on large scales,
cooled electrons will affect P. (KT in prep.)
Synchrotron model
gt10?
P
nm
na
nV
n
Faraday depolarization
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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
polarized
unpolarized
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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.