水村 好貴

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.

This study addresses the challenge of slit-like hole generation due to impact damage in super-pressure balloons covered by nets, with a specific focus on the NPB2-3 model. Despite its advanced and lightweight design optimized for high-altitude flights, the launching process revealed a significant vulnerability: rapid contact between the net and the balloon film upon the spooler's release, leading to numerous slit-like holes and characteristic film damage. To tackle this issue, a quasi-static launch method was developed and evaluated to minimize stress on the balloon film. This method is characterized by a technical innovation that involves setting an additional retention point, apart from the tail, to maintain the collar position during gas filling. Results from a series of experiments, including a simulated launch test using the NPB2-4 model, demonstrated a significant reduction in damage, ultimately achieving complete prevention of slit-like holes. This paper presents the methodology, experimental setup, and results, and discusses the application of this method to the upcoming NPB2-5 model launch in 2024, as well as its potential extension to other balloon launches.

Although the MeV gamma-ray band is a promising energy-band window in astrophysics, the current situation of MeV gamma-ray astronomy significantly lags behind those of the other energy bands in angular resolution and sensitivity. An electron-tracking Compton camera (ETCC), a next-generation MeV detector, is expected to revolutionize the situation. However, the energy band observable with ETCC has been limited to < 2 MeV. Here, we study ETCC events in which the Compton-recoil electrons do not deposit all energies to the electron tracker but escape and hit the surrounding pixel scintillator array (PSA). We developed an analysis method for this untapped class of events and applied it to laboratory and simulation data. We also evaluated the detector performance using the simulation data and found that this new method has enabled us to extend the observable energy range in the previous studies with the ETCC to the higher energy.

MeV gamma rays from celestial objects provide unique information about nucleosynthesis in supernovae or neutron star mergers, the diffusion of matter in the galaxy, the existence of low-energy cosmic rays, and so on. However, the detection sensitivity in this band is not yet sufficient to discuss astrophysical phenomena because of the huge background. For future observations, we are developing an electron-tracking Compton camera (ETCC) with powerful background rejection tools based on the Compton recoil electron tracks. In 2018, our second balloon experiment was conducted to demonstrate the detection of bright sources, and it successfully detected the Crab Nebula and the Galactic Center region with the designed sensitivity. Therefore, we are planning some scientific observations using the ETCCs loaded on long-duration balloons to reveal the origin of galactic diffuse gamma rays and to discover new MeV gamma-ray sources. In this paper, we will present the scientific motivation of SMILE-3 and the preparations for the next flight.