GRB 210619B optical afterglow polarization

Aims. We report on the follow-up of the extremely bright long gamma-ray burst (GRB) GRB 210619B with optical polarimetry. Methods. We conducted optopolarimetric observations of the optical afterglow of GRB 210619B in the SDSS- r band in the time window ∼5967 − 8245 s after the burst, using the RoboPol instrument at the Skinakas observatory. Results. We report a 5 σ detection of polarization P = 1.5 ± 0.3 at polarization angle EVPA = 8 ± 6°. We find that during our observations, the polarization is likely constant. These values have been 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.