ALMA Detection of a Linearly Polarized Reverse Shock in GRB 190114C. (arXiv:1904.07261v1 [astro-ph.HE])

ALMA Detection of a Linearly Polarized Reverse Shock in GRB 190114C. (arXiv:1904.07261v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Laskar_T/0/1/0/all/0/1">Tanmoy Laskar</a>

We present ALMA 97.5 GHz total intensity and linear polarization observations
of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the
burst. We detect linear polarization at the $approx 5,sigma$ level,
decreasing from $Pi=(0.87pm0.13)%$ to $(0.60pm0.19)%$, and evolving in
polarization position angle from $(10pm5)^circ$ to $(-44pm12)^circ$ during
the course of the observations. This represents the first detection of
polarized millimeter emission in a $gamma$-ray burst. We show that the optical
and X-ray observations between $0.03$ days and $sim0.3$ days are consistent
with a fast cooling forward shock expanding into a wind environment. However,
the optical observations at $lesssim0.03$ days, as well as the radio and
millimeter observations arise from a separate component, which we interpret as
emission from the reverse-shocked ejecta. Using the measured linear
polarization, we constrain the coherence scale of tangled magnetic fields in
the ejecta to an angular size of $theta_{rm B} approx10^{-3}$ radian, while
the rotation of the polarization angle rules out the presence of large scale,
ordered axisymmetric magnetic fields, and in particular a large scale toroidal
field, in the jet.

We present ALMA 97.5 GHz total intensity and linear polarization observations
of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the
burst. We detect linear polarization at the $approx 5,sigma$ level,
decreasing from $Pi=(0.87pm0.13)%$ to $(0.60pm0.19)%$, and evolving in
polarization position angle from $(10pm5)^circ$ to $(-44pm12)^circ$ during
the course of the observations. This represents the first detection of
polarized millimeter emission in a $gamma$-ray burst. We show that the optical
and X-ray observations between $0.03$ days and $sim0.3$ days are consistent
with a fast cooling forward shock expanding into a wind environment. However,
the optical observations at $lesssim0.03$ days, as well as the radio and
millimeter observations arise from a separate component, which we interpret as
emission from the reverse-shocked ejecta. Using the measured linear
polarization, we constrain the coherence scale of tangled magnetic fields in
the ejecta to an angular size of $theta_{rm B} approx10^{-3}$ radian, while
the rotation of the polarization angle rules out the presence of large scale,
ordered axisymmetric magnetic fields, and in particular a large scale toroidal
field, in the jet.

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