Gamma-ray Imaging Performance of Nuclear Emulsion Telescope in GRAINE-2018 Balloon Experiment

Abstract

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 the 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 0.01-100 GeV energy region. On April 26, 2018, the 3rd balloon experiment of the GRAINE project was conducted in Alice Springs, Australia, to detect celestial gamma-ray sources and to demonstrate the overall performance of the middle-sized emulsion telescope (aperture area of 0.4 m 2). The balloon floated at the altitude of 36-38 km for 15 h, and the telescope observed the target object, Vela pulsar, for 6 h. Following the recovery and the photofinishing, the data acquisition by the emulsion scanning system were completed, and then the flight data analysis has been performed using reconstructed gamma-ray events. In this presentation, the in-flight performance focusing on the converter part of the emulsion telescope employed in the GRAINE 2018 balloon experiment is reported.