Detection of Gamma-Ray Polarization in Prompt Emission of GRB 100826a

Abstract

We report the polarization measurement in prompt γ-ray emission of GRB 100826A 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 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. Here the quoted errors are given at 1 σ confidence level for two parameters of interest. The systematic errors have been carefully included in this analysis, unlike any previous reports. Such a high Π can be obtained in several emission models of gamma-ray bursts (GRBs), including synchrotron and photospheric models. However, it is difficult to explain the observed significant change of PA within the framework of axisymmetric jet as considered in many theoretical works. 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.

Modeling the Early Afterglow in the Short and Hard GRB 090510

“…For lGRBs, the strong gamma-ray polarization (Coburn & Boggs (2003); Willis et al (2005) but see Rutledge & Fox (2004) and more recently Yonetoku et al (2011)) and a stronger magnetic field in reverse shock (RS) than in forward shock (FS) as a result of a full description of a dozen of lGRBs (Zhang & Kobayashi 2005;Zhang et al 2003;Fan et al 2004;Fraija 2015), have supported the idea that the central engines of lGRBs could be endowed with primordial magnetic fields (Usov 1992;Wheeler et al 2000). In the framework of sGRBs, the question of what field strengths could be reached in a merger remnant before it collapses to a BH has so far remained unanswered, neither evidence of magnetization in the jet has been found.…”

e-ASTROGAM mission: a major step forward for gamma-ray polarimetry

“…42 GAP notably detected GRB 100826A, with an average polarization degree of 27% ± 11%, 2.9 σ confidence level. 43 The polarisation angle was observed to vary by ∼ 90 • during the prompt emission, which is difficult to explain for an axi-symmetric jet in the above scenarios. The polarisation swing during the burst can be interpreted as evidence for patchy emission, patchy magnetic field, or as photospheric emission from a variable jet.…”

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

“…We discussed several emission model in [16]. When we assume the helical magnetic fields to drive the relativistic jet, we expect the toroidal magnetic fields on the jet surface.…”

“…The significance of detection of polarization is 99.4 % (2.9 σ ) confidence level. These results are summarized in [16].…”