GRAINE Project: balloon-borne emulsion gamma-ray telescope

Abstract

Observations of cosmic gamma rays emitted from black holes, pulsars, super nova remnants, etc. are important to understand such high-energy objects or phenomena in the universe. Some gamma-ray telescopes loaded on satellites have surveyed the sub-GeV/GeV gamma-ray sky, and contributed to the development of gamma-ray astronomy. On the other hand, new issues have come to light. Especially, observations at the low galactic latitude remain difficult because of a lack of angular resolution. The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) project aims at precise observations of gamma-ray sources by a balloon-borne gamma-ray telescope. The high-angular resolution gamma-ray telescope, called emulsion telescope, consists of nuclear emulsion films. Its angular resolution is 0.9 deg at 100 MeV or 0.1 deg at 1 GeV. Our second balloon experiment, GRAINE 2015, was performed in 2015 to detect gamma-ray sources, and demonstrate the imaging performance of the emulsion telescope in the 100 MeV energy region. Currently the flight data analysis is on going. In this paper, we checked the imaging performance of the emulsion telescope with an external calibration source. In the energy range above 100 MeV, we verified that the emulsion telescope in the flight had the expected angular resolution.