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Register a new userCoherence scale of magnetic fields generated in early-time forward shocks of GRBs
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Added by
Nuria Jordana-Mitjans
Publication date
2021
fields
Physics
and research's language is
English
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We report the earliest-ever detection of optical polarization from a GRB forward shock (GRB 141220A), measured $129.5-204.3,$s after the burst using the multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous robotic Liverpool Telescope. The temporal decay gradient of the optical light curves from $86,$s to $sim 2200,$s post-burst is typical of classical forward shocks with $alpha = 1.091 pm 0.008$. The low optical polarization $P_{BV} = 2.8_{- 1.6} ^{+ 2.0} , %$ (2$sigma$) at mean time $sim 168,$s post-burst is compatible with being induced by the host galaxy dust ($A_{V, {rm HG}}= 0.71 pm 0.15 ,$mag), leaving low polarization intrinsic to the GRB emission itself -- as theoretically predicted for forward shocks and consistent with previous detections of low degrees of optical polarization in GRB afterglows observed hours to days after the burst. The current sample of early-time polarization data from forward shocks suggests polarization from (a) the Galactic and host galaxy dust properties (i.e. $P sim 1%-3%$), (b) contribution from a polarized reverse shock (GRB deceleration time, jet magnetization) or (c) forward shock intrinsic polarization (i.e. $P leq 2%$), which depends on the magnetic field coherence length scale and the size of the observable emitting region (burst energetics, circumburst density).
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We present a semi-analytical kinetic calculation of the process of non-linear diffusive shock acceleration (NLDSA) which includes the magnetic field amplification due to cosmic ray induced streaming instability, the dynamical reaction of the amplified magnetic field and the possible effects of turbulent heating. The approach is specialized to parallel shock waves and the parameters we chose are the ones appropriate to forward shocks in Supernova Remnants. Our calculation allows us to show that the net effect of the amplified magnetic field is to enhance the maximum momentum of accelerated particles while reducing the concavity of the spectra, with respect to the standard predictions of NLDSA. This is mainly due to the dynamical reaction of the amplified field on the shock, which noticeably reduces the modification of the shock precursor. The total compression factors which are obtained for parameters typical of supernova remnants are $R_{tot}sim 7-10$, in good agreement with the values inferred from observations. The strength of the magnetic field produced through excitation of streaming instability is found in good agreement with the values inferred for several remnants if the thickness of the X-ray rims are interpreted as due to severe synchrotron losses of high energy electrons. We also discuss the relative role of turbulent heating and magnetic dynamical reaction in driving the reduction of the precursor modification.
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