MeV Gamma-ray imaging spectroscopic observation for Galactic Center and Cosmic Background MeV gammas by SMILE-2+ Balloon Experiment

Abstract: Recently, there appears lots of papers on the possibility of light Dark Matter (DM) in MeV and sub-GeV scale. Until now, only INTEGRAL and COMPTEL provided experimental data of 511keV of galactic center, and two spectra of Galactic Diffuse MeV gammas (GDMG), and COMPTEL provided the Cosmic Background MeV gammas (CBMG) for wide sky for indirect detection of light DMs. However except 511keV, those spectra for diffuse gammas included large statistical and systematic errors in spite of 10 years observation, since both two instruments suffered from severe background radiation in space. In 2018 April, we (SMILE-project in Comic-ray Group of Kyoto University) have observed MeV gamma rays for whole southern sky by Electron Tracking Compton Camera (ETCC) using JAXA balloon at Australia during one-day. (SMILE2+ Project) By measuring all parameters of Compton scattering in every gamma, ETCC has achieved for the first time to obtain the complete direction of MeV gammas as same as optical telescopes, and also to distinguish signal gammas from huge background gammas in space clearly. In this observation, ETCC with a large Field of View of 3sr observed MeV gammas from 3/5 of all sky including galactic centre, a half disk, crab, and most of CBMG By reconstructing the Compton process, we successfully obtained pure comic gammas by reducing background by more 2 orders, which is clearly certificated by the clear enhancement of detected gamma flux with ~30% during galactic center passing through the Field of View, which is consistent with the ratio of CBMG and GDMG. Now 511keV gammas GDMG are preliminarily detected with ~5 and >10 sigma respectively around Galactic Centre. Also we have obtained near 105events of CBMG in with quite low background of only a few 10% in total CBMG events. Thus we obtained good data for both with high statistics and very low systematics even one day observation.