Balloon-borne experiment for deep sky survey of MeV gamma rays using an electron-tracking Compton camera

Despite the scientific importance of MeV gamma-ray studies, sufficient observations have not been performed due to the large radiation backgrounds and the unclearness of MeV gamma-ray imaging. To advance the MeV gamma-ray astronomy, we have developed an Electron-Tracking Compton Camera (ETCC) with a gaseous electron tracker. By measuring three dimensional tracks of Compton-recoil electrons, our ETCC has attained the high-quality imaging and powerful background rejection. In order to verify such performance of an ETCC, we have carried out the balloon-borne experiments, Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment (SMILE) since 2006. The performance of current ETCC has already been surpassed the requirements to detect the Crab Nebula for 5 sigma level with several hours balloon observations. In addition, the ability of the polarization measurements has been revealed. The modulation factor is estimated to be 0.6 for the energy region below 200 keV by the Monte Carlo simulation. We have measured the polarization ability using the polarized X-ray beam-line at SPring-8, and then modulation factor of 0.6 is obtained at 130 keV, which is consistent with the results of the simulation and shows that the ETCC has an excellent performance as a sub-MeV gamma-ray polarimeter. By using the pressured CF4 based gas at 3 atm, the detection efficiency of the ETCC will be increased one order. Therefore we have a plan of the long duration observation for deep sky survey with polarization measurements of bright sources including Gamma-Ray Bursts. Here we present the concept of ETCC and the future prospects based on the performance of the current ETCC.