Limits on optical polarization during the prompt phase of GRB 140430A

TL;DR

This study presents early optical observations and polarization limits of GRB 140430A, revealing complex emission mechanisms and low polarization levels during the prompt phase, challenging standard models of GRB emission.

Contribution

First densely sampled optical light curves and polarization limits during the prompt phase of GRB 140430A, highlighting complex emission processes beyond standard external shock models.

Findings

Optical polarization during the prompt phase is constrained to P < 12%.

Optical light curves show complex behavior not explained by simple shock models.

Early optical emission likely involves internal shock contributions.

Abstract

Gamma-ray burst GRB 140430A was detected by the Swift satellite and observed promptly with the imaging polarimeter RINGO3 mounted on the Liverpool Telescope, with observations beginning while the prompt $\gamma$-ray emission was still ongoing. In this paper, we present densely sampled (10-second temporal resolution) early optical light curves in 3 optical bands and limits to the degree of optical polarization. We compare optical, X-ray and gamma-ray properties and present an analysis of the optical emission during a period of high-energy flaring. The complex optical light curve cannot be explained merely with a combination of forward and reverse shock emission from a standard external shock, implying additional contribution of emission from internal shock dissipation. We estimate an upper limit for time averaged optical polarization during the prompt phase to be as low as P < 12% (1$\sigma$). This suggests that the optical flares and early afterglow emission in this GRB are not highly polarized. Alternatively, time averaging could mask the presence of otherwise polarized components of distinct origin at different polarization position angles.