GRAINE project: precise gamma-ray observations with balloon-borne emulsion telescope

Observation of cosmic gamma rays is important in the understanding of high-energy objects or phenomena in the universe. Since 2008, the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope (Fermi-LAT) has surveyed the sub-GeV/GeV gamma-ray sky and achieved high statistics measurements. However, observation at low galactic latitudes remains difficult owing to the lack of angular resolution, and new issues following the operation of Fermi-LAT have arisen. We devised a precise gamma-ray observation project, Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE), using balloon-borne emulsion gamma-ray telescopes to realize high angular resolution, polarization-sensitive, and large-aperture observations in the 10 MeV-100 GeV energy region. Following basic developments on the ground, we performed three balloon-borne experiments with upgraded instruments. In this paper, we present results from the second balloon experiment in 2015, a report on the latest balloon experiment conducted on April 26, 2018, and a recent study on hadronic interactions using proton beams.