High-Precision Observation of Cosmic Gamma-ray Sources with the GRAINE Telescope

Gamma rays represent the most “energetic” electromagnetic waves in the electromagnetic spectrum, and are generated by some of the most violent events in the universe, such as supernova explosions. Additionally, gamma rays are emitted by energetic objects such as pulsars (spinning neutron stars) and quasars (luminous objects powered by supermassive black holes). Observing gamma rays can, therefore, provide key insights into the evolution of our universe.

Since gamma rays are absorbed by the atmosphere, they can only be observed by telescopes aboard space satellites or high-altitude balloons. The large area telescope on the Fermi Gamma-ray Space Telescope (Fermi-LAT) launched in 2008 was the latest telescope to detect gamma rays in the sub-GeV/GeV energy range. However, the angular resolution of the Fermi-LAT is not high enough to clearly distinguish the multiple gamma-ray sources observed at the Galactic Center (the rotational center of the Milky Way galaxy).

The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) is a high-resolution gamma-ray telescope that aims to surmount this issue and make precise observations of gamma-ray sources. This balloon-borne telescope developed by Kobe University, Nagoya University, Okayama University of science, Aichi University of Education and Gifu University in Japan uses nuclear emulsion films (a type of photographic plate) and a large aperture area (10 m²) to detect gamma rays in the energy range of 10 MeV-100 GeV.

When charged particles encounter a nuclear emulsion film, they leave behind tracks that can be examined under a microscope, allowing for high-resolution observations. In the case of GRAINE, the nuclear emulsion films detect gamma rays by tracking the position of electrons and positrons generated in pair production.

In 2018, researchers involved in the GRAINE project used the telescope to observe the Vela pulsar, the brightest known gamma-ray source, in collaboration with the Japan Aerospace Exploration Agency (JAXA), who launched the balloon. Along with gamma rays, protons and helium nuclei passed through the emulsion film during the observations. The hadronic interaction between these particles produced short-lived π⁰ particles that decayed into gamma rays.

In a new study, researchers developed a method to identify these interactions, which could be used to not only detect gamma rays but also calibrate their arrival direction, energy, polarization, and efficiency. Additionally, they developed a high-precision measurement system to automatically record the particle tracks captured on the films.

The proposed methods are expected to improve the imaging resolution of the telescope by two orders of magnitude, and could be implemented during the next balloon experiments with GRAINE to be launched by JAXA on 2023.