## High Energy Astrophysics Group

### 中性子星連星合体 Neutron Star Binary Mergers

アインシュタインによって提唱された一般相対論では、重力の正体は時空の歪みであると考えられており、 時空の歪みが波として伝搬する現象（＝重力波）が存在すると予言されていました。重力波は、中性子星やブラックホールからなる連星のように、 質量が大きく密度が高い物体が加速度運動する系から効率的に放出されます。こうした連星は重力波によって次第に軌道エネルギーを失い、 やがて合体すると考えられており、その重力波の直接検出は長年の課題でした。 そしてついに、2015年9月14日のアメリカのAdvanced LIGOによる検出を皮切りに、数多くの重力波イベントが観測されはじめました。

2017年8月17日には2つの中性子星からなる連星合体の際に放出された重力波が観測され、 電波からガンマ線まで幅広い波長域の電磁波現象が同時に観測され大きな話題となりました。 今後もこうしたイベントは数多く観測されると期待されており、 我々のグループでは、こうした重力波と突発的高エネルギー天体現象の観測から極限の物理に迫るべく、日々研究を進めています。

### 論文リスト Our Papers

N. Cannady, Y. Asaoka, F. Satoh, et al.
"Characteristics and Performance of the CALorimetric Electron Telescope (CALET) Calorimeter for Gamma-Ray Observations", ApJS 238, 5(16pp) (2018)

Yutaro Tachibana, Makoto Arimoto, Katsuaki Asano, et al.
"Late Engine Activity of GRB 161017A Revealed by Early Optical Observations", Accepted for PASJ, arXiv:1807.08923

O. Adriani, Y. Akaike, K. Asano, et al.
"Search for GeV Gamma-ray Counterparts of Gravitational Wave Events by CALET", ApJ 863, 160(9pp) (2018)

G. Abdellaoui, et al.
"EUSO–TA — First Results from a Ground-based EUSO Telescope", APh 102, 98-111 (2018)

### 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 (radio, optical, X and gamma-ray) and neutrino emissions from such particles and so on. Moreover, we study merger of binary stars composed of neutron stars and/or black holes based on hydrodynamical simulations with general-relativistic effects. Multi-messenger astronomy –astronomy via collaborating observations of electromagnetic waves, cosmic rays, neutrinos, and gravitational waves– will be drastically developed in this century. Therefore, interpretation of observed data and prediction of new astrophysical phenomena from various perspectives are also important themes in our study.

### Particle Acceleration

The theory of relativity tells us that the energy of a particle is expressed as $E=\gamma mc^2$, where the Lorentz factor is defined as $\gamma \equiv 1/\sqrt{1-(v/c)^2}$. If the velocity of a particle is close to the speed of light, the Lorentz factor becomes $\gamma \gg 1$ and such particles are called relativistic particles. Relativistic electrons can emit electromagnetic waves via synhcrotron or inverse Compton scattering. Cosmic rays, which are relativistic protons or nuclei, can also emit gamma-rays or high-energy neutrinos via collision with another particle or photon. The shock waves propagating interstellar medium after supernovae are sites where such relativistic particles are accelerated. As the left figure shows, emissions from radio to gamma-ray by electrons or protons have been observed. However, the maximum particle energy in supernova remnants is lower than $3 \times 10^{15}$eV, the maximum energy of galactic cosmic rays. The origin of cosmic rays is not fully revealed yet.

The IceCube Neutrino Observatory in Antarctica detected neutrinos whose energy is above $10^{15}$eV. Those neutrinos may be emitted from protons of $>10^{17}$eV, produced in other galaxies. Furthermore, Telescope Array and Pierre Auger Observatory detected ultra high-energy cosmic rays, whose energy is larger than $10^{20}$eV. The acceleration site and mechanism for such particles are also open problems.

### Relativistic Outflow

Some of supermassive black holes in galactic neclei launch collimated relativistic jets with $\gamma>10$. When a giant star ends its life and its core collapses into a black hole, relativistic jets with $\gamma>100$ are considered to be ejected and emit gamma-ray flash, which is called gamma-ray burst. The jet launching and acceleration mechanisms are not revealed yet. The gravitational energy released when gas falls onto a black hole or spin energy of a black hole are candidates of the energy source of relativistic jets. Jets may be magnetically driven, or alternatively radiation pressure may play a role in the acceleration.

The right figure shows an X-ray image of electron–positron plasma outflowing with $\gamma>10^5$ from a pulsar, fast rotating neutron star. The outflow energy is injected from the spin energy of the pulsar via magnetic field. The acceleration mechanism of the pulsar wind is an unsolved problem. From objects with relativistic outflows violently variable emissions have been frequently observed, which implies that high-energy particles are accelerated there.

### Neutron Star Binary Mergers

In the framework of general relativity, gravitation is phenomenological consequence of geometrical property in space and time, and the existence of gravitational waves –ripples of space-time curvature propagating with the speed of light– has been theoretically predicted. Gravitational waves are efficiently emitted from a system which contains massive and compact objects in accelerated motion, such as a binary system composed of neutron stars and/or black holes. In such a binary system, the orbital separation shrinks gradually via the gravitational-radiation reaction, and eventually the two objects merge. A direct detection of gravitational waves had been a difficult and challenging issue for a long time. But now, many detections of gravitational-wave events have been reported after advanced-LIGO –a ground-based detector in USA– achieved the first detection on 14th of September 2015.

Binary mergers including a neutron star are in particular of interest, since they also cause high-energy astrophysical phenomena in electromagnetic waves. For example, a black-hole accretion disk formed after the merger may be a central engine of gamma-ray burst. Extreme environment, for example density of $\sim 10^{14}$g/cc and temperature of $\sim 10^{12}$K, is realized in a neutron star binary merger. The observation of binary mergers via gravitational waves, electromagnetic waves, and neutrinos enable us to study physics in such an extreme environment.

The first neutron star binary merger was detected on 17th of August 2017 with advanced-LIGO, and simultaneous electromagnetic-wave observations were also achieved for this event. More and more events are expected to be observed in near future, and our group is aiming to extract the physical information in the extreme environment from observations of both gravitational waves and high-energy astrophysical phenomena.

### Our Papers

Recent papers：

N. Cannady, Y. Asaoka, F. Satoh, et al.
"Characteristics and Performance of the CALorimetric Electron Telescope (CALET) Calorimeter for Gamma-Ray Observations", ApJS 238, 5(16pp) (2018)

Yutaro Tachibana, Makoto Arimoto, Katsuaki Asano, et al.
"Late Engine Activity of GRB 161017A Revealed by Early Optical Observations", Accepted for PASJ, arXiv:1807.08923

O. Adriani, Y. Akaike, K. Asano, et al.
"Search for GeV Gamma-ray Counterparts of Gravitational Wave Events by CALET", ApJ 863, 160(9pp) (2018)

G. Abdellaoui, et al.
"EUSO–TA — First Results from a Ground-based EUSO Telescope", APh 102, 98-111 (2018)

Previous papers are here.