Research

Hyperaccretion Disks as the Central Engine of Gamma-Ray Bursts

A gamma-ray burst (GRB) is the brightest astronomical phenomenon in the universe, emitting an amount of energy in just a few seconds that the Sun would emit over its entire lifetime. The central engine is thought to be a black hole (BH) formed after the gravitational collapse of a massive star or the merger of binary neutron stars, surrounded by a massive accretion disk. We analyzed the structure and stability of this massive accretion disk by considering detailed physics such as the equation of state of high-density matter, neutrino radiation, and magnetohydrodynamic effects (NK & Mineshige 2007 etc.). Based on this model, we analytically estimated the power emitted through magnetic field lines from the accretion disk and found that it very accurately reproduces the observed luminosity of GRBs (NK, Piran & Krolik 2013). Furthermore, we discovered that under certain conditions, the massive accretion disk becomes dynamically unstable and proposed a scenario where the resulting unsteady mass accretion produces the intense temporal variations observed in GRBs (Figure 2; NK, Mineshige & Piran 2013 etc.). This series of studies continues to be cited not only in the context of GRBs but also for its implications for supernova explosions, nucleosynthesis of heavy elements within them, and energy and mass ejection processes from compact binary mergers.

Hunting Galactic Black Holes

It is estimated that there are about 100 million black holes (BHs) in the Milky Way galaxy. However, only a few dozen BHs have been identified so far. Collecting diverse samples of BHs in the Milky Way and investigating their physical properties, as well as their positional and velocity distributions, is crucial for understanding the formation processes of BHs. We have proposed methods for discovering non-X-ray binary BHs in the Milky Way, categorized by type (Figure 3), and estimated the number of BHs that could be detected by future observations for each method. For binary BHs without mass accretion from stars [Figure 3(b)], we focused on the possibility of identifying BHs by accurately observing the orbital motion of stars and estimated that the astrometric satellite Gaia could detect 200 to 1,000 BH binaries. We also pointed out for the first time that information about the progenitor stars and their evolutionary processes could be obtained from this data (Yamaguchi, NK et al. 2018 etc.). Additionally, for solitary BHs, we estimated that up to about 700 could be detected in future observations if they accrete gas in high-density regions such as molecular clouds [Figure 3(a)] (Tsuna, NK & Totani 2018; Tsuna & NK 2019). We also studied the velocity distribution of solitary BHs obtained from gravitational microlensing observations [Figure 3(c)] (Koshimoto, NK & Tsuna 2024). By continuing this research, we aim to achieve a 'census of BHs' in the Milky Way, thereby probing the formation processes of BHs.

Origin of Galactic Cosmic-Rays

In outer space, there are high-energy particles such as protons, nuclei, and electrons traveling at speeds close to the speed of light. Physicists collectively refer to these particles as cosmic-rays (CRs). Since the energy distribution of CRs can be represented by a nearly single power law from ~ 1 GeV to ~ 3 PeV, it is believed that a single type of astronomical objects is the source for at least this energy range, with supernova remnants being the leading candidates. Since 2008, several CR observations have reported that the amounts of CR electrons and positrons exceed previous theoretical predictions. This has sparked a debate, involving researchers from both astrophysics and particle physics, between those who suggest that unknown particles constituting dark matter are the source and those who propose astronomical objects as the source. We pointed out that nearby pulsars and/or other celestial objects can naturally explain the observed results for cosmic ray electrons and positrons, and we proposed methods to verify this scenario (NK et al. 2010 etc.). This research led me to join the CALET project, where I am responsible for the theoretical interpretation of the observational results. Additionally, observations by CALET and other instruments have reported that the spectra of CR protons and nuclei deviate from the predictions of standard models. We proposed new types of CR sources, such as superbubbles created by star clusters and special supernovae, which can coherently explain the CR proton and nuclei spectra, and we also suggested methods for observational verification (NK & Yanagita 2018 etc.).