Preliminary observation of polarization for Crab Nebula by PHENEX polarimeter

We have been developing a hard X-ray polarimeter called "PHENEX" (Polarimetry for High ENErgy X rays). The PHENEX is Compton-scattering-type polarimeter. It is constructed from an array of identical small polarimeter called "unit counter". In Jun. 13 2006, we had carried out balloon-borne experiment to study the polarization of hard X rays from the Crab Nebula, loading the PHENEX polarimeter with four unit counters surrounded by active shields of CsI (Tl). The PHENEX accomplished the level flight at the altitude of 37.5 km over 6 hours and the detector system operated well over the duration. However, attitude control system did not function correctly and the line of the sight wandered around the Crab Nebula. Even so the PHENEX polarimeter can observe the Crab Nebula for about one hour and also a blank region of the sky for about one hour. Calculating the effect of the wandering by computer simulation, we carried out the analysis about the polarization of the Crab Nebula. Though the statistical error is large and the significance is low, the modulation to indicate the polarization can be found for the data of Crab Nebula. In near future, we will carry out balloon borne experiment again, improving the PHENEX polarimeter and fixing the trouble of the attitude control system.