GRB 210619B optical afterglow polarization

TL;DR

This paper presents optical polarimetry observations of the bright GRB 210619B afterglow, detecting intrinsic polarization and analyzing its constancy over time, providing insights into the magnetic field structure of the burst.

Contribution

First optical polarization measurement of GRB 210619B's afterglow, demonstrating intrinsic polarization and its stability, advancing understanding of magnetic fields in gamma-ray burst afterglows.

Findings

Detected 1.5% polarization at 5 sigma significance

Polarization remained constant during observations

Polarization is intrinsic to the afterglow, not due to interstellar medium

Abstract

We report on the follow-up of the extremely bright long gamma-ray burst GRB~210619B with optical polarimetry. We conducted optopolarimetric observations of the optical afterglow of GRB~210619B in the SDSS-r band in the time window ~ 5967 - 8245 seconds after the burst, using the RoboPol instrument at the Skinakas observatory. We report a $5\,\sigma$ detection of polarization $P=1.5\pm0.3$ at polarization angle $EVPA=8\pm6^\circ$. We find that during our observations the polarization is likely constant. These values are corrected for polarization induced by the interstellar medium of the Milky Way and host-induced polarization is likely negligible. Thus the polarization we quote is intrinsic to the GRB afterglow.