Constraining the magnetic field structure in collisionless relativistic shocks with a radio afterglow polarization upper limit in GW170817. (arXiv:1910.05687v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gill_R/0/1/0/all/0/1">Ramandeep Gill</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Granot_J/0/1/0/all/0/1">Jonathan Granot</a>
Gamma-ray burst afterglows arise from relativistic collisionless shocks in
which the postshock tangled magnetic field $vec{B}$ is produced by the
two-stream and/or Weibel instabilities on plasma skin-depth scales
$(c/omega_p)$. The field is expected to be oriented predominantly within the
shock plane ($B_{perp}$; transverse to the shock normal, $hat{n}_{rm{sh}}$),
and is often approximated to be completely within it
($B_parallelequivhat{n}_{rm{sh}},cdot,vec{B}=0$). Current 2D/3D
particle-in-cell simulations are limited to short timescales and box sizes
$lesssim10^4(c/omega_p)ll R/Gamma_{rm{sh}}$ much smaller than the shocked
region’s comoving width, and cannot probe the asymptotic downstream $vec{B}$
structure. We constrain the latter using the linear polarization upper limit,
$vertPivert<12%$, on the radio afterglow of GW170817/GRB170817A. Afterglow
polarization depends on the jet's angular structure, our viewing angle, and the
$vec{B}$ structure. In GW170817/GRB170817A the latter can be tightly
constrained since the former two are constrained from observations. We model
$vec{B}$ as an isotropic field in 3D that is stretched along
$hat{n}_{rm{sh}}$ by a factor $xiequiv{}B_{parallel}/B_{perp}$, whose
initial value $xi_fequiv{}B_{parallel,f}/B_{perp,f}$ describes the field
that survives downstream on plasma scales $ll{}R/Gamma_{rm{sh}}$. We
calculate $Pi(xi_f)$ by integrating over the shocked volume for
core-dominated structured jets, with a local Blandford-McKee self-similar
radial profile. We find that independent of the exact jet structure, $vec{B}$
has a finite, but initially sub-dominant, parallel component:
$0.57lesssimxi_flesssim0.89$, making it less anisotropic. While this
motivates numerical studies of the asymptotic $vec{B}$ structure in
relativistic collisionless shocks, it may be consistent with turbulence
amplified magnetic field.
Gamma-ray burst afterglows arise from relativistic collisionless shocks in
which the postshock tangled magnetic field $vec{B}$ is produced by the
two-stream and/or Weibel instabilities on plasma skin-depth scales
$(c/omega_p)$. The field is expected to be oriented predominantly within the
shock plane ($B_{perp}$; transverse to the shock normal, $hat{n}_{rm{sh}}$),
and is often approximated to be completely within it
($B_parallelequivhat{n}_{rm{sh}},cdot,vec{B}=0$). Current 2D/3D
particle-in-cell simulations are limited to short timescales and box sizes
$lesssim10^4(c/omega_p)ll R/Gamma_{rm{sh}}$ much smaller than the shocked
region’s comoving width, and cannot probe the asymptotic downstream $vec{B}$
structure. We constrain the latter using the linear polarization upper limit,
$vertPivert<12%$, on the radio afterglow of GW170817/GRB170817A. Afterglow
polarization depends on the jet’s angular structure, our viewing angle, and the
$vec{B}$ structure. In GW170817/GRB170817A the latter can be tightly
constrained since the former two are constrained from observations. We model
$vec{B}$ as an isotropic field in 3D that is stretched along
$hat{n}_{rm{sh}}$ by a factor $xiequiv{}B_{parallel}/B_{perp}$, whose
initial value $xi_fequiv{}B_{parallel,f}/B_{perp,f}$ describes the field
that survives downstream on plasma scales $ll{}R/Gamma_{rm{sh}}$. We
calculate $Pi(xi_f)$ by integrating over the shocked volume for
core-dominated structured jets, with a local Blandford-McKee self-similar
radial profile. We find that independent of the exact jet structure, $vec{B}$
has a finite, but initially sub-dominant, parallel component:
$0.57lesssimxi_flesssim0.89$, making it less anisotropic. While this
motivates numerical studies of the asymptotic $vec{B}$ structure in
relativistic collisionless shocks, it may be consistent with turbulence
amplified magnetic field.
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