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The SMILE-3 balloon mission aims to address long-standing questions in MeV gamma-ray astrophysics, including the origin of the Galactic Center diffuse emission and the “MeV excess.” Its key capability is the electron-tracking Compton camera (ETCC), which performs true event-by-event bijective imaging, determining the incident gamma-ray direction uniquely rather than as a Compton circle. Building on the successful SMILE-2+ detection of the Crab and Galactic Center, SMILE-3 ETCC incorporates major upgrades including a 3-atm CF_4 TPC, MPPC-based scintillator arrays, and a redesigned trigger logic. A first flight is planned for early 2028 to enable wide-area MeV surveys with unprecedented accuracy.

MeV gamma-ray observations remain far less explored than the X-ray, GeV, and TeV bands, creating the well-known MeV gap. A major challenge in this energy range is improving the point spread function (PSF) of MeV gamma-ray telescopes. The electron-tracking Compton camera (ETCC) is one of the most promising instruments for overcoming this gap, and its PSF strongly depends on the accuracy of the reconstructed electron-recoil direction. To address this issue, we developed a deep-learning–based reconstruction method using two-dimensional optical track images and one-dimensional waveform data. In simulations, the angular resolution for recoil electrons reached 44° in the 40–50keV range, surpassing our previous approach. In addition, the half power radius of the PSF, defined in geometrical optics, reached 8.5° for 511 keV gamma rays. The proposed approach demonstrates the feasibility of developing a CCD-based gaseous ETCC and improving the PSF of future MeV gamma-ray telescopes.

Observations in the MeV energy band are a key step to investigate the origin of the diverse spectral lags in gamma-ray bursts (GRBs). The SMILE-3 experiment is designed to survey this energy range using an Electron-Tracking Compton Camera (ETCC). We have developed a lightweight, compact balloon-borne anticoincidence scintillation detector capable of both rejecting cosmic-ray events and detecting low-energy GRBs. The detector employs a plastic scintillator, silicon photomultipliers (SiPMs), and readout electronics. We present the performance of the developed electronics and its GRB detection capability evaluated by sensitivity calculations.

Gamma-ray bursts (GRBs) are among the most energetic explosions in the universe, and their prompt optical flashes provide a unique probe of the radiation mechanism. In a slow-cooling synchrotron scenario, the locations of the cooling and self-absorption break frequencies critically determine the spectrum that we observe. However, optical flashes typically occur and fade within a few seconds after the GRB onset, so conventional follow-up observations triggered by satellite alerts rarely capture this earliest phase. KaGErOFU (Kanazawa University Gamma-ray Burst Explorer for Optical Flash Understanding) is a dual-platform (ground-based and balloon-borne) project designed to provide pre-planned, simultaneous optical coverage of GRBs by continuously monitoring the fields of view of satellites such as Swift/BAT and Fermi/GBM. Assuming that both the cooling and self-absorption frequencies lie below the optical band, a two smoothly broken power-law (2SBPL) model fitted to typical Swift/BAT spectra predicts an optical brightness of about 12.5 mag. KaGErOFU employs 135- mm f/1.4 lenses combined with back-illuminated full-frame CMOS sensors, tiled to cover a total field of view of approximately 3000 deg2 . This configuration achieves a theoretical 5 s limiting magnitude of about 13.1, about 1.8 mag deeper than the previous WIDGET experiment. Detections, marginal detections, and non-detections of optical flashes at this sensitivity will indicate that the cooling and self-absorption breaks lie below, around, or above the optical band, thereby tightening constraints on synchrotron model parameters in the prompt phase. In this presentation, we focus on the development and ground testing of the KaGErOFU detector system. We describe the overall system concept and architecture, including continuous-exposure, continuous-readout operation for wide-field GRB monitoring. We also present results from field-tracking tests using a prototype consisting of an alt-azimuth mount and a single camera–lens unit, demonstrating stable pointing and image quality suitable for future multi-camera deployment.

MeV gamma-ray observations are a probe for various physical phenomena, including searches for dark matter and primordial black holes, and nucleosynthesis in the Universe. In order to achieve high sensitivity in this band, we are developing an electron-tracking Compton camera (ETCC), which combines a gaseous time projection chamber (TPC) and pixelated GSO(Ce) scintillator arrays (PSAs). The TPC induces Compton scattering and tracks recoil electrons. The PSAs measure the absorption position and energy of the scattered gamma rays. From these measurements, the ETCC reconstructs the Compton kinematics and uniquely determines the gamma-ray arrival direction. Now, we are planning the Sub-MeV/MeV gamma-ray Imaging Loaded-on-balloon Experiment 3 (SMILE-3), which will use an ETCC with improved dynamic range and effective area from the previous balloon experiment SMILE-2+. The first one-day flight of SMILE-3 is scheduled for 2028 in Australia. The improved PSAs for SMILE-3 require higher energy resolution and a wider dynamic range than SMILE-2+, which is expected to improve both the angular resolution and the dynamic range of the ETCC. For this purpose, we replaced the photomultiplier tubes with Multi-Pixel Photon Counters, which have higher quantum efficiency at the GSO(Ce) emission wavelength, and designed two amplifiers with different gains to extend the dynamic range. We also implemented modifications to the sampling rate and the trigger scheme. We have developed PSAs incorporating these improvements and fabricated flight units for SMILE-3. In this work, we present an overview of the developed PSAs, including their system configuration, and evaluate their performance.