## High Energy Astrophysics Group

### 謎は極高エネルギー粒子に限らない Particle Acceleration

この図は$10^{8}$電子ボルトか$3\times10^{20}$電子ボルトまで（横軸）、 12桁を越えるエネルギー範囲に渡る宇宙線粒子の流量（単位時間・単位面積・単位立体角・単位エネルギー幅あたりに到来する粒子の個数） を示しています。ところどころに凸凹があるものの、流量はほぼエネルギーの逆三乗に比例していることが見られます。 何故そうなのか？それはまだ完全には解明されていません。

これらの粒子のうち、$10^{15}$電子ボルト程度までは超新星残骸に伴う衝撃波での加速に起源を持つとするのが定説であり、 急速に進展しつつある高エネルギーγ線観測によって証明されると期待されていました。 しかし、現在までのγ線観測結果は定説を裏付けず、むしろ謎が深まっていると言えます。

こうして、これまで当然視されていた衝撃波加速過程の見直しを始め、磁気リコネクション、磁気乱流に伴う加速など、 別種の加速過程も含めた理論的再検討が必要とされています。こうした研究の現状は、 衝撃波加速ばかりでなく多種多彩な加速過程の研究に実績のある我が高エネルギー天体グループにとって、活躍のチャンスであるといえます。

### 論文リスト Our Papers

Katsuaki Asano, and Sho To
"Subsequent Nonthermal Emission due to the Kilonova Ejecta in GW170817", ApJ 852, 105(5pp) (2018)

B. P. Abbott, et al.
"Multi-messenger Observations of a Binary Neutron Star Merger", ApJ 848, L12(59pp) (2017)

O. Adriani, Y. Akaike, K. Asano, et al.
"Energy Spectrum of Cosmic-ray Electron and Positron from 10 GeV to 3 TeV Observed with the Calorimetric Electron Telescope on the International Space Station", PRL 119, 181101(6pp) (2017)

Akinobu Miyamoto, Tomoya Kinugawa, Takashi Nakamura, and Nobuyuki Kanda
"How to Confirm the Existence of Population Ⅲ Stars by Observations of Gravitational Waves", PRD 96, 064025(10pp) (2017)

### High-Energy Astrophysical Objects

Examples of high-energy astrophysical phenomena are supernovae, pulsars, giant flares from magnetars, jets launched from supermassive black holes in the center of galaxies, starburst galaxies, gamma-ray bursts, and non-thermal emission from clusters of galaxies. Our research subjects are physical mechanisms for jet formation, acceleration of relativistic particles, photon emissions (radio, optical, X and gamma-ray) from such particles and so on. In order to obtain hints to reveal those high-energy phenomena, we also study nearby phenomena such as solar wind and solar flare, which give us more detailed data than those for extrasolar objects. The research methods are not only thoretical study including numerical simulations but also analysis of observational data (electromagnetic field, plasma particle, gamma-rays, etc.) from satellites. We have frequently obtained surprising results by analyses starting from the physical principle of the detection technic.

### Ultra-High-Energy Cosmic Ray

Cosmic rays are charged particles, photons, and neutrinos with relativistic energies, originating outside the Solar System. When cosmic rays penetrate the Earth's atmosphere, they collide with an atomic nucleus (nitrogen, oxygen etc.) and produce secondary particles such as pions and photons. The chain reaction initiated by this collision is developed into a shower of secondary particles as shown in this figure. Ultra-high-energy cosmic rays have a kinetic energy larger than $10^{20}$ eV, which is $10^{8}$ times the energy a particle accelerator can achieve. Why such high-energy particles exist? This is one of the most mysterious problems in astrophysics. We are challenging to find sources of ultra-high-energy cosmic rays in high-energy astrophysical phenomena.

### Particle Acceleration

This figure shows the spectrum of the cosmic ray flux per unit time, surface, solid angle, and energy from $10^{8}$ eV to $3\times10^{20}$ eV. Although the spectral curve is slightly fluctuated, we can see that the flux is inversely proportional to the third power of the energy for 12 orders of magnitude. This energy distribution has not been explained. The cosmic rays below $10^{15}$ eV have been considered to be produced in shock waves of supernova remanants so that we have expected that high-energy gamma-ray observatories detect signatures of cosmic-ray acceleration. However, the recent gamma-ray observations seem to show a lower maximum energy of cosmic rays than $10^{15}$ eV. The conventional shock acceleration mechanism is therefore needed to be reexamined. We also need to consider other types of acceleration mechanisms, for example, acceleration due to magnetic reconnection and magnetohydrodynamic instability.

### Our Papers

Recent papers：

Katsuaki Asano, and Sho To
"Subsequent Nonthermal Emission due to the Kilonova Ejecta in GW170817", ApJ 852, 105(5pp) (2018)

B. P. Abbott, et al.
"Multi-messenger Observations of a Binary Neutron Star Merger", ApJ 848, L12(59pp) (2017)

O. Adriani, Y. Akaike, K. Asano, et al.
"Energy Spectrum of Cosmic-ray Electron and Positron from 10 GeV to 3 TeV Observed with the Calorimetric Electron Telescope on the International Space Station", PRL 119, 181101(6pp) (2017)

Akinobu Miyamoto, Tomoya Kinugawa, Takashi Nakamura, and Nobuyuki Kanda
"How to Confirm the Existence of Population Ⅲ Stars by Observations of Gravitational Waves", PRD 96, 064025(10pp) (2017)

Previous papers are here.