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Observations of cosmic gamma rays are important to promote an understanding of such high-energy objects and phenomena in the universe. Since 2008, the Large Area Telescope on the Fermi satellite has surveyed the whole gamma-ray sky in the sub-GeV/GeV energy region, and accumulated a large amount of data. However, observations at low galactic latitudes remain difficult because of a lack of angular resolution, an increase of background flux originating from galactic diffuse gamma rays, etc. The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) is a gamma-ray observation project with a new balloon-borne emulsion gamma-ray telescope. Nuclear emulsion is a high-resolution 3D tracking device. It determines the incident angle with 0.1$^{¥circ}$ resolution for 1 GeV gamma rays (1.0$^{¥circ}$ for 100 MeV), and has polarization sensitivity. The goal of the GRAINE is to achieve precise observations of gamma-ray sources, especially in the galactic plane, by repeating long-duration balloon flights with large-aperture-area (10 m$^{2}$) high-resolution emulsion telescopes. In May 2015, we performed a balloon-borne experiment in Alice Springs, Australia, in order to demonstrate the imaging performance of our telescope. The emulsion telescope that has an aperture area of 0.4 m$^{2}$ was employed in this experiment. It observed the Vela pulsar (the brightest gamma-ray source in the GeV sky) at an altitude of 37 km for 6 hours out of the flight duration of 14 hours. In this paper, we report the data analysis, especially evaluations of the detector performance in the GRAINE 2015 experiment by observing an external calibration source.

GRAINE balloon-borne experiment in 2015 : Observations with a high angular resolution gamma-ray telescope