Yoshitaka Mizumura

Gamma-Ray Bursts (GRBs) are the most luminous explosions in the universe since the Big Bang, emitting intense gamma rays with rapid temporal variability over durations ranging from seconds to hundreds of seconds. In GRB prompt emission, a ”positive spectral lag,” where soft X-ray photons arrive later than hard X-ray photons, is generally observed. However, some GRBs exhibit the opposite behavior. Notably, observations by the Fermi Large Area Telescope (LAT) have revealed that high-energy photons are often characterized by a delayed onset. Furthermore, data analysis using the Fermi Gamma-ray Burst Monitor (GBM) and LAT Low Energy (LLE) techniques has shown diverse lag behaviors between the LLE band (30-100 MeV) and the GBM band (10 ‒ 100 keV). To elucidate the detailed origin of negative spectral lags, observations in the energy band corresponding to the gap between these two bands are essential. The SMILE-3 project possesses high sensitivity to gamma rays in this few-MeV range through the use of an Electron-Tracking Compton Camera (ETCC). The SMILE-3 project requires an anti-scintillation counter to prevent increased dead time in signal readout caused by charged particle events from secondary cosmic rays. Therefore, we are developing a lightweight, compact, balloon-borne anti-scintillation counter that possesses not only the primary function of charged particle rejection but also the capability to detect low-energy GRBs photons. We employed a detector system combining a plastic scintillator and MPPC (Multi-Pixel Photon Counter). By designing a readout circuit that integrates a preamplifier and a high speed shaping amplifier utilizing a second-order low-pass filter, we successfully miniaturized the signal readout system with a shaping time of 60 nsec. In this presentation, we will report on the performance evaluation of the developed electronic board and the assessment of its GRB detection capabilities.

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.

The SMILE project develops balloon-borne MeV gamma-ray telescopes based on an electron-tracking Compton camera (ETCC). An ETCC measures the three-dimensional track of the recoil electron in a gaseous time projection chamber (TPC), together with the energy and interaction position of the scattered gamma ray in pixelized scintillator arrays (PSAs). This information determines the incident gamma-ray direction event by event, rather than leaving each event on a Compton circle, and provides a localized point spread function for quantitative imaging in the MeV band. SMILE-2+ demonstrated the astronomical capability of the ETCC by detecting the Crab Nebula and diffuse emission from the Galactic Center region. SMILE-3 is the next balloon experiment and is designed to improve the effective area, angular resolution, energy resolution, and usable energy range. The flight-model detector consists of a 30 × 30 × 30 cm3 gaseous TPC designed for operation at 3 atm with a CF4-based gas mixture, a finer-pitch µ-PIC readout, and GSO(Ce) pixel scintillator arrays with MPPC readout. This paper reports the current status of the SMILE-3 flight-model ETCC. Previously demonstrated component performance is briefly summarized, together with the fabrication and integration status of the detector. The PSA readout has been operated through the common trigger-control system, and waveform data have been acquired. A partial-ETCC configuration combining the flight-model TPC, its readout electronics, and three PSA modules has also been installed at the UVSOR gamma-ray beamline for a 6-MeV Compton-event acquisition test. These developments are milestones toward full-ETCC verification and a one-day balloon flight in Australia planned for 2028. Diffuse emission from the Galactic Center is one of the primary scientific targets.

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.