GRB 191016A: A highly collimated gamma-ray burst jet with magnetised energy injection

TL;DR

This paper studies GRB 191016A, revealing a highly collimated jet with magnetized energy injection, polarization evidence, and a jet break, advancing understanding of gamma-ray burst jet structures and energy dynamics.

Contribution

It presents the first detailed polarization and light curve analysis of GRB 191016A, indicating magnetized energy injection and a narrow jet opening angle.

Findings

Polarization detected at all phases, including during flattening.

Evidence of magnetized energy injection with ΔE/E > 0.36.

Jet break observed with a 2° opening angle.

Abstract

Long gamma-ray burst GRB 191016A was a bright and slow rising burst that was detected by the \textit{Swift} satellite and followed up by ground based Liverpool Telescope (LT). LT follow-up started $2411$-s after the \textit{Swift} Burst Alert Telescope (BAT) trigger using imager IO:O around the time of the late optical peak. From $3987-7687$-s, we used the LT polarimeter RINGO3 to make polarimetric and photometric observations of the GRB simultaneously in the $V,R$ and $I$ bands. The combined optical light curve shows an initial late peak followed by a decline until 6147-s, 6087-s, and 5247-s for $I,R$ and $V$ filters respectively followed by a flattening phase. There is evidence of polarization at all phases including polarization ($P = 14.6 \pm 7.2 \%$) which is coincident with the start of the flattening phase. The combination of the light curve morphology and polarization measurement favours an energy injection scenario where slower magnetised ejecta from the central engine catches up with the decelerating blast wave. We calculate the minimum energy injection to be $\Delta E / E>0.36$. At a later time combining the optical light curve from BOOTES (reported via GCN) and IO:O we see evidence of a jet break with jet opening angle 2 \degree.