Study of emission mechanism of Gamma-Ray Bursts by the gamma-ray polarization with IKAROS-GAP

Abstract

We report the polarization measurement in prompt γ-ray emission of GRB 100826A, GRB 110301A and GRB 110721A with the Gamma-Ray Burst Polarimeter (GAP) aboard the small solar power sail demonstrator IKAROS. We detected the firm change of polarization angle (PA) during the prompt emission of GRB 100826A with 99.9 % (3.5 σ ) confidence level, and the average polarization degree (Π) of 27 ± 11 % with 99.4 % (2.9 σ ) confidence level. For the later two events, we detected the strong polarization of Π = 70 ± 22 % and Π = 84 +16 −28 % with the statistical significance of 3.7 σ and 3.3 σ , respectively. Here the quoted errors are given at 1 σ confidence level for two parameters of interest. We conclude the linear gamma-ray polarization exists in the prompt GRBs from GAP observations. The non-axisymmetric (e.g., patchy) structures of the magnetic fields and/or brightness inside the relativistic jet are therefore required within the observable angular scale of ∼ Γ −1 . Our observation strongly indicates that the polarization measurement is a powerful tool to constrain the GRB production mechanism, and more theoretical works are needed to discuss the data in more details.

Preface

“…• the significant detection of gamma-ray polarization in a few GRBs by the GAP experiment on IKAROS [9]. Such measurements have been hard to come by with previous instrumentation, and the new dedicated GAP polarimeter has finally achieved the break-through.…”