Yoshiyuki Morihara

DETECTION OF GAMMA-RAY POLARIZATION IN PROMPT EMISSION OF GRB 100826A

We report the polarization measurement in prompt γ-ray emission of GRB 100826A with the Gamma-Ray Burst Polarimeter on board the small solar-power-sail demonstrator IKAROS. We detected the firm change of polarization angle (PA) during the prompt emission with 99.9% (3.5σ) confidence level, and the average polarization degree (Π) of 27% ± 11% with 99.4% (2.9σ) confidence level. Here the quoted errors are given at 1σ confidence level for the two parameters of interest. The systematic errors have been carefully included in this analysis, unlike other 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 detail.

Magnetic Structures in Gamma-Ray Burst Jets Probed by Gamma-Ray Polarization

We report polarization measurements in two prompt emissions of gamma-ray bursts, GRB 110301A and GRB 110721A, observed with the gamma-ray burst polarimeter (GAP) on borad the IKAROS solar sail mission. We detected linear polarization signals from each burst with polarization degree of {Pi} = 70 {+-} 22% with statistical significance of 3.7{sigma} for GRB 110301A, and {Pi} = 84{sup +16}{sub -28}% with 3.3{sigma} confidence level for GRB 110721A. We did not detect any significant change of polarization angle. These two events had shorter durations and dimmer brightness compared with GRB 100826A, which showed a significant change of polarization angle, as reported in Yonetoku et al. Synchrotron emission model can be consistent with the data of the three GRBs, while the photospheric quasi-thermal emission model is not favored. We suggest that magnetic field structures in the emission region are globally ordered fields advected from the central engine.

Possible existence of the Ep-Lp and Ep-Eiso correlations for short gamma-ray bursts with a factor 5–100 dimmer than those for long gamma-ray bursts

We analyzed correlations among the rest frame spectral peak energy

Possible Origins of Dispersion of the Peak Energy–Brightness Correlations of Gamma-Ray Bursts

E_{\rm p}

Strict Limit on CPT Violation from Polarization of Gamma-Ray Burst

, the observed frame 64ms peak isotropic luminosity

Identifying Subclasses of Long Gamma-Ray Bursts with Cumulative Light-Curve Morphology of Prompt Emissions

L_{\rm p}

Gamma-Ray Burst Polarimeter aboard IKAROS

and the isotropic energy

Improved Ep-TL-Lp Diagram and a Robust Regression Method

E_{\rm iso}

Study of emission mechanism of GRBs probed by the gamma-ray polarization with IKAROS-GAP

for 13 Short Gamma Ray Burst (SGRB) candidates having the measured redshift

Gamma‐Ray Polarimetry of the Prompt Emission by IKAROS‐GAP

z