Fermi Gamma-ray Space Telescope

GRAINE project, high resolution imaging and polarization sensitive gamma-ray observation with balloon-borne large aperture emulsion telescope

Satoru Takahashi
GRAINE collaboration


The observation of high energy cosmic gamma-rays provides us with direct knowledge of high energy phenomena in the universe. Currently, AGILE and Fermi-LAT are observing gamma-ray sky. Fermi-LAT detected 1873 gamma-ray sources and is strongly pushing forward gamma-ray observation. As a result, many understandings of high energy phenomena in the universe are being brought to us. On the other hand, past and current gamma-ray observations have several problems. Angular resolution is orders of magnitude poorer than another wave length observations in spite of critical performance for astronomical observation. Polarization observation have never been performed in spite of the one of independent informations of photon. Improvement of angular resolution and polarization observation lead gamma-ray observation to the next stage. The interaction of high energy gamma-ray with matter is dominated by electron-pair creation process. An electron pair has gamma-ray informations of arrival direction, energy, arrival timing and polarization. Suppressing multiple coulomb scattering and precise tracking provide us with precise determination of gamma-ray direction and a sensitivity to gamma-ray polarization. Nuclear emulsion is a powerful tracking device which can record 3-dimensional trajectory of charged particle within 1$mu$m accuracy. By detecting a beginning of electron pair converted from a gamma-ray with the emulsion film which is thin ($sim2.0times10^{-3}$ radiation lengths) and precise ($sim$submilliradian), gamma-ray direction can be determined precisely and it is sensitive to gamma-ray linear polarization. We are furthering the project of 10MeV-100GeV cosmic gamma-ray observation with precise (0.08deg@1-2GeV) and polarization sensitive large aperture area ($sim$10m$^2$) emulsion telescope by repeating long duration balloon flights. We call this project GRAINE (Gamma-Ray Astro-Imager with Nuclear Emulsion). An overview and a status of GRAINE project are presented.