Yoshitaka Mizumura

The SMILE-2+ balloon experiment, launched from Australia in 2018, successfully demonstrated the capabilities of the Electron-Tracking Compton Camera (ETCC) for MeV gamma-ray astronomy. The SMILE-2+ one-day flight achieved a 4.0 sigma detection of gamma rays from the Crab Nebula in the 0.15–2.1 MeV range and revealed an enhancement of gamma-ray events from the Galactic center region. These results validate bijective imaging spectroscopy and background modeling, marking a significant step toward opening the MeV window with high precision. In the era of multi-messenger astronomy, MeV observations provide a crucial link between GeV–TeV measurements and PeV discoveries by EAS arrays, offering complementary insights into particle acceleration and nucleosynthesis. Building on the success of SMILE-2+, the SMILE-3 project is now in progress, targeting the next balloon flight in Australia with an upgraded instrument to improve sensitivity and resolution, with the goal of enabling more detailed studies of particle acceleration sites and their possible connection to high-energy phenomena.

The center region of our Galaxy has an unresolved emission with a large spatial distribution of tens of degrees order, and its emission mechanism is still a puzzle. We do not understand any kind objects bright in MeV band, while the convolution of unresolved objects are the efficient candidates. Other hands, the Hawking radiation from the primordial black holes (PBHs) with the masses of 1016-17 g or the annihilation of the light weakly interacting massive particles (WIMPs) with the masses of tens of MeV are also the important candidates, because they have the electron-positron annihilation line as a secondary emission which are detected in the Galactic Center region. To reveal the emission mechanism, we need a detailed energy spectrum and an accurate spatial distribution of the diffuse galactic gamma-ray emission, which requires the low-noise and high-sensitivity observations with a true imaging detector having a large field of view. We are developing an electron-tracking Compton camera (ETCC) as such a telescope and have demonstrated its capabilities with two balloon experiments in 2006 and 2018. We are now preparing the next balloon flight (SMILE-3) to observe the Galactic Center region. In this presentation, we report on the current status of the component development and the expectations for SMILE-3.