水村 好貴

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.

遠方銀河からの放射が重なって形成される系外拡散MeVガンマ線の起源は未解明であり、AGNコロナでの粒子加速、Blazar放射、ダークマター起源などが候補として挙げられている。本研究では、従来の検出器とは異なり到来ガンマ線方向を一意に決定可能なETCC（Electron Tracking Compton Camera）を搭載した軟MeVガンマ線観測気球実験SMILE-2+のデータを用いた。検出器由来雑音をシミュレーションと実データにより気球上昇中の各高度で評価・除去し、検出器応答を解くことでガンマ線成分の抽出を行った。さらに高度依存性を利用して大気ガンマ線成分の切り分けを試みた。本講演ではその解析結果と現状を報告する。

JAXA宇宙科学研究所では毎年，宇宙科学研究のための成層圏気球（大気球）を利用した実験を公募により提供しています。JAXAの大学共同利用システムに基づき，全国の大学・研究機関等から多くの研究者・大学院生が大気球実験に参加し，多様な宇宙科学研究を実施しています。 大気球実験は，人工衛星や観測ロケットといった他の飛翔体による研究と比べ，提案から最短一年程度の短期間でも実施でき，相対的に厳しくない制約条件のもと，最先端の科学成果を生み出すとともに，新たに宇宙科学分野に参画しようとする多くの研究者の入口となってきました。 また，大気球実験は比較的小規模な実験であることが多いため，参加する若手研究者や大学院生が実験全体を理解，把握して，プロジェクトを実現することを学ぶ人材育成の場としても活かされています。 このポスターでは，大気球実験の概要と宇宙教育現場としての魅力と成果をお伝えします。

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.