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

Abstract

The small solar power sail demonstrator “IKAROS” is a Japanese engineering verification spacecraft launched by H‐IIA rocket on May 21, 2010 at JAXA Tanegashima Space Center. IKAROS has a 20 m diameter sail which is made of thin polyimide membrane. This sail converts the solar radiation‐pressure into the propulsion force of IKAROS and accelerates the spacecraft. The Gamma‐Ray Burst Polarimeter (GAP) aboard IKAROS is the first polarimeter specifically designed to measure the polarization of Gamma‐Ray Bursts (GRBs) from space, and will do so in the cruising phase of the IKAROS mission. GAP is a modest detector of 3.8 kg in weight and 17 cm in size with an energy range between 50–300 keV. The GAP detector can be a member of the interplanetary network (IPN) for the determination of the GRB direction. The detection principle of gamma‐ray polarization is the anisotropy of the Compton scattering. Coincidence between the central plastic Compton scattering medium and discrete CsI detectors distributed around the sides of the plastic defines the Compton scattering angle, which is expected to show an angular dependence if polarization is present in a given GRB. We presented the GAP detector and its ground and onboard calibrations.