## 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。

### 重力波天文学 Gravitational-Wave Astronomy

ブラックホールや中性子星のもととなる大質量星は数が少なく、その進化の過程や、星が崩壊する際の超新星爆発についても謎が多く残されています。さらに重力波によって連星が合体するのには非常に長い時間がかかるため、宇宙初期に形成されたブラックホール連星が最近になって合体し、現在稼働している重力波検出器で観測されることもあるでしょう。重力波によってどのようなブラックホールや中性子星が宇宙に存在するかがわかれば、宇宙初期から現在までの大質量星形成の歴史に迫れるかもしれません。実際に重力波で観測された太陽の約30倍の重さのブラックホールは、宇宙最初期の星である初代星起源ではないかと注目を集めています。

また、現在までに報告されている近傍のブラックホールや中性子星からなる連星の観測以外にも、より遠方での連星合体や、白色矮星連星の合体などを観測する将来計画もあり、重力波を用いた新たな天文学が行えると期待されています。こうした将来計画で、何が見えて何がわかるのかを予測するような研究も我々のグループでは行っています。

### 論文リスト Our Papers

C. B. Adams, W. Benbow, A. Brill, et al.
"Observation of the Gamma-Ray Binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes", arXiv:2109.11894, accepted for ApJ

Reetika Dudi, Ananya Adhikari, Bernd Brügmann, Tim Dietrich, Kota Hayashi, Kyohei Kawaguchi, Kenta Kiuchi, Koutarou Kyutoku, Masaru Shibata, and Wolfgang Tichy
"Investigating GW190425 with Numerical-Relativity Simulations", arXiv:2109.04063

V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, et al.
"Search for Very High-Energy Emission from the Millisecond Pulsar PSR J0218+4232", arXiv:2108.11373, accepted for ApJ

Tomohisa Kawashima, Ken Ohsuga, and Hiroyuki R. Takahashi
"RAIKOU: A General Relativistic, Multi-Wavelength Radiative Transfer Code", arXiv:2108.05131, submitted to ApJ

### 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 nuclei 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 a 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 bursts. 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.

### Gravitational-Wave Astronomy

Now we have opened a new window to observe stars by gravitational-wave observatories. This makes it possible to observe "invisible" objects such as black holes. In the near future, current ground-based gravitational-wave observatories such as KAGRA, LIGO, and VIRGO can drastically increase the detection number of binary black holes and other types of binary mergers. We are now at the dawn of gravitational-wave astronomy.

Massive stars, which are progenitors of black holes and neutron stars, are relatively rare objects in the universe. There remain many mysteries in the massive star evolution and the supernova explosions triggered by the core-collapse, in which a black hole or a neutron star is formed. Furthermore, it takes a very long time for binary stars to coalesce via gravitational-wave emission, so that present-day instruments can detect even the binary mergers originated from massive stars formed in the early universe. Gravitational-wave astronomy will tell us the formation history of massive stars from the early universe to the present time. We already have observed a merger of black holes, which are about 30 times heavier than the Sun. This event is remarkable because they were probably born as the first-generation stars in the very early universe.

In addition to the nearby binary mergers discovered by the current instruments, gravitational-wave astronomy is expected to open up new fields in astronomy, such as more distant binary mergers and white dwarf binary mergers. We also study to predict what we can see and what we can learn from future observations with new techniques optimized for such targets.

### Our Papers

Recent papers：

C. B. Adams, W. Benbow, A. Brill, et al.
"Observation of the Gamma-Ray Binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes", arXiv:2109.11894, accepted for ApJ

Reetika Dudi, Ananya Adhikari, Bernd Brügmann, Tim Dietrich, Kota Hayashi, Kyohei Kawaguchi, Kenta Kiuchi, Koutarou Kyutoku, Masaru Shibata, and Wolfgang Tichy
"Investigating GW190425 with Numerical-Relativity Simulations", arXiv:2109.04063

V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, et al.
"Search for Very High-Energy Emission from the Millisecond Pulsar PSR J0218+4232", arXiv:2108.11373, accepted for ApJ

Tomohisa Kawashima, Ken Ohsuga, and Hiroyuki R. Takahashi
"RAIKOU: A General Relativistic, Multi-Wavelength Radiative Transfer Code", arXiv:2108.05131, submitted to ApJ

Previous papers are here.