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.
B
e
k
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 %
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)
=> (early) optical pol
POlarimeters for
Energetic Transients
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
=> Particle acceleration, Jet acceleration
Jet? Spherical outflow?
(3) Composition of source
Electron energy distribution
Electron-proton? Electron-positron?
Polarization is changed through the
source.
Afterglow Polarization
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.
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?
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.
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
GRB jet
To reproduce the optical
detection, N ~ 103.
(coherence length ~ 1013cm)
In this case, polarization should be subject to erratic
variations of polarization angle on dynamical time scales.
Observational results
GRB 030329: least smooth light curve
GRB 020813:
smoothest 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!
PL,opt < 8% (t ~ 203 sec)
(Mundell et al. 07)
Radio afterglow
n1/3
n-(p-1)/2
This seems because the selfabsorption frequency is
typically in the VLA band.
n2
VLA ALMA
radio
Pn
0.5
na
P has not be detected in
the radio band, although
the synchrotron P is
little dependent on
frequency.
optical
~1 day
In the frequencies lower
than the self-absorption
frequency, the radiation is
strongly coupled to the
particles, and is similar to
blackbody radiation.
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
＋
The polarization
plane rotates.
＝
Two natural modes with
different phase velocities
(Sazonov 69; Matsumiya & Ioka 03)
Faraday depolarization
Dc
Dc’
Dc”
Linear polarization
cancels out.
Efficiency of acceleration
Observed afterglow
1-f
f
G
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
It is possible that only a
small fraction f of
electrons are accelerated.
(Eichler & Waxman 05)
(KT, Ioka, Nakamura 08)
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
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
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.
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
Faraday
depolarization
n
Photospheric emission model
t~1
Progenitor
star
The spectral peak might be
produced by the
photospheric blackbody
radiation. (e.g., Ryde et al.,
Thompson et al., Ioka et al.)
Compton downscattered
Compton upscattered
This model shows a
unique P spectrum.
Seed blackbody emission
polarized unpolarized
polarized
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.