GRAINE2023: Sub-GeV/GeV gamma-ray observation with a high angular resolution by the balloon borne emulsion telescope

We have been running the GRAINE project, a cosmic gamma-ray observation project covering the energy range from 10 MeV to 100 GeV, using nuclear emulsion detectors characterized with a high-angular resolution, polarization sensitivity and large aperture area. We conducted the fourth balloon experiment, GRAINE2023, in Australia with an aperture area of 2.5 m$^2$, for which we developed a new large-area telescope for scientific observations. We detected the Vela pulsar, thereby verifying the performance of the newly developed telescope. The measured angular resolution is $0.59^{+0.1}_{-0.07}$ $^\circ$ for energies above 75 MeV, primarily in the 100--500 MeV range. We then measured the gamma-ray spatial distribution near the Galactic Center for energies above 75 MeV, and the obtained results are consistent with the expected models derived from Fermi-LAT observations. Because the angular resolution of Fermi-LAT is about 1$^\circ$ at 1 GeV, obtaining the flux within 1$^\circ$ of the Galactic Center in the sub-GeV energy range requires assumptions based on observational results at energies above 1 GeV. By contrast, we achieved a direct measurement of the flux within 1$^\circ$ of the Galactic Center, enabled by the high angular resolution of GRAINE. This represents a unique feature of the present study.