Abstract
Observations of cosmic gamma rays are important for studying high energy 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 accumurated a large amount of data. However, observations at the low galactic latitude remains difficult because of a lack of angular resolution, 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 ∘ resolution for 1 GeV gamma rays (1.0 ∘ for 100 MeV), and has linear polarization sensitivity. GRAINE aims at precise observation 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 presentation, we will report the latest results and the status of the GRAINE project.
Highlights
Observations at the low galactic latitude 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) project aims at precise observations of gammaray sources by a balloon-borne gamma-ray telescope
In the first balloon experiment, GRAINE 2011, the technical feasibility was demonstrated by a small-scale emulsion telescope and a star camera system [9]
Summary
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. AGILE [1] launched in 2007 and the Large Area Telescope on the Fermi Gamma-ray Space Telescope (Fermi-LAT) [2] launched in 2008 have surveyed the sub-GeV/GeV gamma-ray sky. They have achieved good results and contributed to the development of gammaray astronomy: detection of more than 3000 gamma-ray sources [3]; discovery of cosmic-ray proton acceleration in super nova remnants [4, 5]. Unassociated sources are still included in the high rate in gamma-ray sources detected in the galactic plane [3]. An unexpected gamma-ray excess in the galactic center region was reported [6]
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