2022MNRAS.509.5964S

Long gamma-ray burst GRB 191016A was a bright and slow rising burst that was detected by the Swift satellite and followed up by ground based Liverpool Telescope (LT). LT follow up started 2411 s after the 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 ±7.2 per cent) 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 magnetized ejecta from the central engine catches up with the decelerating blast wave. We calculate the minimum energy injection to be ΔE/E > 0.36. At a later time, combining the optical light curve from Burst Observer and Optical Transient Exploring System (BOOTES) (reported via GCN) and IO:O we see evidence of a jet break with jet opening angle 2°.