First observation of MeV gamma-ray universe with bijective imaging spectroscopy using the Electron-Tracking Compton Telescope aboard SMILE-2+

Understand

MeV gamma-rays provide a unique window for the direct measurement of line emissions from radioisotopes, but observations have made little significant progress after COMPTEL/{\it CGRO}.

• To observe celestial objects in this band, we are developing an electron-tracking Compton camera (ETCC), which realizes both bijective imaging spectroscopy and efficient background reduction gleaned from the recoil electron track information.
• The energy spectrum of the observation target can then be obtained by a simple ON-OFF method using a correctly defined point spread function on the celestial sphere.
• The performance of celestial object observations was validated on the second balloon SMILE-2+ installed with an ETCC having a gaseous electron tracker with a volume of 30$\times$30$\times$30 cm$^3$.