Polarization constraints on the geometry of the magnetic field in the external shock of gamma-ray bursts. (arXiv:2002.04614v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Stringer_E/0/1/0/all/0/1">Eric Stringer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lazzati_D/0/1/0/all/0/1">Davide Lazzati</a> (Oregon State University)
We study the ensemble of linear polarization measurement in the optical
afterglows of long-duration gamma-ray bursts. We assume a non
sideways-expanding top-hat jet geometry and use the relatively large number of
measurements under the assumption that they represent a statistically unbiased
sample. This allows us to constrain the ratio between the maximum predicted
polarization and the measured one, which is an indicator of the geometry of the
magnetic field in the downstream region of the external shock. We find that the
measured polarization is substantially suppressed with respect to the maximum
possible for either a completely ordered magnetic field parallel to the shock
normal or to a field that is entirely contained in the shock plane. The
measured polarization is limited, on average, to between 25 and 30% of the
maximum theoretically possible value. This reduction requires the perpendicular
component of the magnetic field to be dominant in energy with respect to the
component parallel to the shock front, as expected for a shock generated and/or
shock compressed field. We find, however, that the data only marginally support
the assumption of a simple top-hat jet, pointing towards a more complex
geometry for the outflow.
We study the ensemble of linear polarization measurement in the optical
afterglows of long-duration gamma-ray bursts. We assume a non
sideways-expanding top-hat jet geometry and use the relatively large number of
measurements under the assumption that they represent a statistically unbiased
sample. This allows us to constrain the ratio between the maximum predicted
polarization and the measured one, which is an indicator of the geometry of the
magnetic field in the downstream region of the external shock. We find that the
measured polarization is substantially suppressed with respect to the maximum
possible for either a completely ordered magnetic field parallel to the shock
normal or to a field that is entirely contained in the shock plane. The
measured polarization is limited, on average, to between 25 and 30% of the
maximum theoretically possible value. This reduction requires the perpendicular
component of the magnetic field to be dominant in energy with respect to the
component parallel to the shock front, as expected for a shock generated and/or
shock compressed field. We find, however, that the data only marginally support
the assumption of a simple top-hat jet, pointing towards a more complex
geometry for the outflow.
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