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.

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.

Since the discovery of possible bacterial particles in the stratosphere in the 1930s, bioaerosol particles in the stratosphere have been studied to understand how far the biosphere extends, what types of bioaerosol particles and how they exist in the extreme stratospheric environment and how these bioaerosol particles can reach above the troposphere. Although little is known about the ecology in the stratosphere, not much work has been done in the stratosphere due to its limited accessibility. The physical and biological dynamics of bioaerosol particles in the stratosphere are closely related to aerobiological and astrobiological research topics such as global-scale long-range transport of bioaerosol particles, planetary protection, and the emerging and evolution of life. Thus, the development of the new experimental methodologies in the stratosphere will lead to the acquisition of the new high-altitude aerobiological research opportunities. In the presentation, we will present the details of our experimental platforms for aerobiological research projects in the stratosphere that we have established using scientific balloons, and the research objectives of each research project.

MeV gamma-ray observations provide unique information about nucleosynthesis, diffusion in our galaxy, low-energy cosmic rays, particle acceleration, and other phenomena. However, the detection sensitivity in this band is significantly lower than that in other bands due to a large background contamination. To address this issue, we are developing an electron-tracking Compton camera (ETCC) with powerful background rejection tools based on Compton recoil electron tracks. This will enable future observations to be conducted with greater sensitivity. We have successfully demonstrated the detection technology and performance of the ETCC with two balloon experiments. We are preparing for the next balloon flight, SMILE-3, to observe galactic diffusion gamma rays and some bright celestial objects.