Observation of Crab Nebula with hard x-ray polarimeter: PHENEX

We have been developing a hard X-ray polarimeter to open a new window for hard X-ray astronomy. The project is called as PHENEX (Polarimetry for High ENErgy X rays). The PHENEX detector is Compton scattering type polarimeter and it is constructed by several unit counters. The unit counter can achieve the modulation factor and the detection efficiency of 53% and 20% at 80 keV, respectively. Installing four unit counters, we have carried out balloon-borne experiment in Jun.13 2006 to preliminarily observe the polarization of the Crab Nebula in hard X-ray band. The PHENEX polarimeter successfully operated on the level flight and observed the Crab Nebula for about one hour. From the analysis of the obtained data, it was recognized that the PHENEX polarimeter does not make much spurious modulation and that the ratio of the signal from the Crab Nebula to the background from the blank sky is 1:3. Though we can not precisely determine the degree and the direction of the polarization for the Crab Nebula because of the trouble of the attitude control system, the obtained results were not inconsistent with those in the X-ray band. We will carry out balloon-borne experiment again, fixing the trouble of the attitude control system.