Department of Physics, School of Science
An enthusiastic Professor Nobuyuki Kawai states, "In September 2015, the first detection of gravitational waves produced during a merger of black holes1 opened the door to the field of gravitational wave astronomy. As someone involved in research on gamma-ray bursts, I want to spend the rest of my career in gravitational wave astronomy."
Gravitational waves are propagations of the distortion of space-time2 caused by the presence of mass. Albert Einstein predicted their existence some 100 years ago. Strong gravitational waves are thought to be produced from the merging of binary neutron stars3 or binary black holes, or other events such as supernovae4. A gamma-ray burst is a flash of strong gamma rays5 from a point in the sky. In the 1960s, gamma-ray bursts were first observed by US surveillance satellites to detect gamma radiation pulses emitted by nuclear tests. However, the source and mechanism of the emissions remained unclear for 30 years. The biggest reason for this difficulty was that the majority of gamma-ray bursts last just tens of seconds, and their location is unpredictable.
In August 2017, however, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves associated with the merger of neutron stars. The strange, weak gamma-ray bursts that were detected gave rise to a new stage in research.
Kawai explains, "I was originally working in the field of X-ray astronomy. In the 1980s, I was involved in the observation of neutron stars that emit X-rays, including X-ray pulsars and X-ray bursters. At that time, it was generally presumed that gamma-ray bursts also originated from neutron stars. This led me to start working on gamma-ray bursts as an extension of my studies on X-ray astronomy. Gamma-ray bursts were an obscure phenomenon at the time, and some colleagues told me that I would be wasting my career."
A significant turning point came in 1997, when an Italian-Dutch X-ray astronomy satellite first recorded the afterglow of gamma-ray bursts. Interestingly, X-ray afterglow and visible light sources were detected at the same place. Unlike gamma-ray bursts, which last just a few seconds, the afterglow was observed to last over a longer period of time. This allowed for accurate measurement of direction, distance from the Earth, and amount of energy.
The analysis yielded remarkable results, revealing that most of the gamma-ray bursts were generated billions of light years away from the Earth. Such a tremendous amount of energy covering vast distances in such a short amount of time could only be produced by one of two phenomena. One is a star6 with a mass of several tens of that of the Sun, undergoing gravitational collapse at the end of its short life to produce a black hole. The other is when extremely compact neutron stars crash into each other. Based on the evidence, it was concluded that gamma-ray bursts with durations longer than a few seconds, which accounts for the majority, were caused by the former.
Meanwhile, it was also found that short gamma-ray bursts lasting less than one second can occur in a galaxy where massive stars do not exist, a finding that supported the theory that this class of gamma-ray bursts were produced in the formation of black holes as a result of the merger of binary neutron stars.
According to Kawai, "Although the detection of gravitational waves and gamma-ray bursts in 2017 seemed to prove the theory, they differ from short gamma-ray bursts. This means that we have to observe more to see if we can clarify the details. Combining the observation of gravitational and electromagnetic waves, such as gamma rays, X-rays, and visible rays, has shown us a path that may lead to significant progress in understanding the origin and composition of the universe."
"Gamma-ray bursts are extremely interesting to me," says Kawai. "We know that gamma-ray bursts are emitted as high-speed jet flows traveling very close to the speed of light; however, we still do not know how black holes or neutron stars produce them. I'm also attracted by the thrill of exploring the mystery of the origin of the Universe. Gamma-ray bursts are the key to finding out how many black holes there are in the Universe, and when, where, and how they were produced. In addition, observing the Universe when it was formed requires us to look out to its very edge. It is, however, hard to observe stars in such distant locations because they are very dim. Gamma-ray bursts, on the other hand, are very bright, so they can be observed with satellites and telescopes, even those produced when the first star in the Universe was formed. Another reason for my interest in gamma-ray bursts is my desire to understand the mystery of r-process elements7, a group of elements that are heavier than iron. We already know that hydrogen and helium were created during the Big Bang 13.8 billion years ago, and that the elements up to and including iron were created through nuclear fusion in the interior of stars. However, we still do not know where and how r-process elements, which include gold and platinum, were created. It was once speculated that these elements were created in supernovae; however, the theory getting more favor now is that they were created during the merger of binary neutron stars and scattered into space. Gravitational wave astronomy is key to understanding the origin of these elements."
Kawai has developed instruments to observe gamma-ray bursts. The first was the High Energy Transient Explorer 2 (HETE-2) satellite developed in a collaboration of American, French, and Japanese teams while he was working at RIKEN. HETE-2 gathered critical evidence proving that the gravitational collapse of massive stars produced long-life gamma-ray bursts.
Because gamma-ray bursts must be observed immediately after detection, the Gamma-ray Coordinates Network (GCN) at the National Aeronautics and Space Administration (NASA) transmits information on gamma-ray bursts to observers around the world via the internet as soon as they are detected. Kawai began development of robotic telescopes around 2001. He designed a system that responds automatically to gamma-ray burst information received from the GCN. With support from the National Astronomical Observatory of Japan (NAOJ) and the Institute for Cosmic Ray Research (ICRR) at the University of Tokyo, Kawai has placed robotic telescopes with a diameter of 50 cm in Asakuchi, Okayama Prefecture and Hokuto, Yamanashi Prefecture to observe the afterglow of gamma-ray bursts.
"The most impressive observation for me was the afterglow of gamma-ray bursts caught by the Subaru Telescope on Hawaii Island in 2005. Analysis of the spectrum8 showed that these bursts occurred 12.9 billion light years away, a mere 0.9 billion light years after the Big Bang. This was the most distant gamma-ray burst ever observed. In addition, the information showed that although electrons and protons recombined about 10,000 light years after the Big Bang and that the Universe was neutralized, re-ionization had already occurred by the time of these gamma-ray bursts. This was an extremely important achievement for clarification of the ancient state of the Universe and the original formation of stars and galaxies," says Kawai.
For his achievements utilizing HETE-2 and the Subaru Telescope, Kawai was awarded a Commendation for Science and Technology (Research Category) from the Minister of Education, Culture, Sports, Science and Technology in 2007 and the 2010 Chushiro Hayashi Prize from the Astronomical Society of Japan in 2011.
When the gravitational waves were detected in August 2017, some 70 telescopes in Japan and other countries around the world chased the electromagnetic waves associated with the merger of neutron stars. Analysis of the data revealed that the merger of neutron stars occurred in a location that was approximately 0.13 billion light years away from the Earth.
"My research team worked to detect X-rays associated with the gravitational waves utilizing the Monitor of All-sky X-ray Image (MAXI) mounted on the International Space Station's Kibo module. Unfortunately we were unable to get the results we were hoping for because the gravitational wave event occurred in a direction that could not be observed. However, when the sensitivity of LIGO and Virgo are improved and the Japanese gravitational wave telescope KAGRA starts operation, it will be possible to observe the merger of neutron stars more than 10 times a year. It will be possible to observe electromagnetic radiation from neutron stars, including light, X-rays, and possibly short gamma-ray bursts, to examine what happens in the merger of neutron stars," says Kawai with excitement.
Professor Kawai then described the real pleasure of his research. "First, gamma-ray bursts were a mysterious phenomenon. Then in 1997, an afterglow was discovered. Later in 2017, research advanced significantly through the detection of a merger of neutron stars with the gravitational wave detector. Looking back on these events, I can say that the real pleasure for me is seeing my field expand more than I could ever have imagined, with unexpected discoveries and encounters with mystery. Every day I feel that nature is rich and plentiful beyond human imagination, and I am delighted that being a researcher has allowed me to experience this. I would like for young people who are thinking about going into research to first follow their curiosity rather than stick to research goals that make immediate contributions to society. In addition, the experience to solve mysteries for which nobody knows the answers will help them to address new problems in our evolving society."
When an object is smaller than a specific size determined by its mass, its gravitational field becomes so strong that not even light can escape from its surface. To better understand this concept, imagine, for example, the mass of a star ten times larger than our Sun being stuffed into a volume the size of New York City. When viewed from a distance, this gives the appearance of a black hole that only has gravitational pull. We think that black holes with masses approximately ten times that of the sun are created when a massive star collapses into its center as it reaches the end of its life; however, the origin of super massive black holes found at the centers of galaxies is still the subject of speculation. The first gravitational waves detected were generated by the merger of two black holes each with masses some 30 times that of our Sun, but astronomers are still debating their origin.
According to the theory of relativity, time and space cannot be separated. When an observer moves, time and space are fused together. Therefore, they are called space-time.
Different from normal stars, neutron stars are composed mainly of neutrons like giant atomic nuclei. The mass of a neutron star is almost the same as the Sun, but its radius is only 10 km (the radius of the Sun is 700,000 km). They are extremely dense, which produces enormous gravity on the surface. A highly magnetized rotating neutron star is observed as a pulsar that emits electromagnetic waves periodically.
Supernovae are enormous explosions of stars that radiate intense light for months. They occur in the Milky Way galaxy approximately once every 100 years. The old supernovae that could be seen with the naked eye were referred to as "guest stars" by ancient Chinese astronomers. There are two major types of supernovae. The first type of supernova occurs when the core of a massive star collapses at the end of its life, blowing away part or all of the star and sometimes leaving a black hole or a neutron star as the remnant. The second type of supernova happens when an old, dense white dwarf explodes as a result of thermonuclear runaway reaction.
Electromagnetic waves, propagations of variations of electric and magnetic fields, exert different influence on matter and, therefore, have different names depending on their wavelengths. For example, electromagnetic waves with wavelengths larger than 1 mm are called radio waves, and those measuring between 0.4 and 0.7 μm are visible light. Waves smaller than 0.01 nm behave more like particles and are called gamma rays. Gamma rays emitted from the universe react with the atmosphere and do not reach the Earth's surface. Therefore, it is necessary to search outside the atmosphere to directly detect gamma rays emitted by astrophysical sources.
Stars are spherical masses of high-temperature gases held together by their own gravity with an internal heat source, like the sun. Their heat source is the nuclear fusion at the core. Most of the "stars" seen in the night sky are these stars. The greater the mass of a star, the faster the fusion reaction at its core occurs, making stars shine brighter, burn hotter, and die faster.
Among the elements in the Universe, those from carbon through iron, as ordered by the atomic number, are synthesized at the cores of stars by thermonucliear fusion. Elements heavier than iron are considered to be created by adding neutrons to atomic nuclei in special environments. In particular, some elements, such as neodymium and other rare-earth elements, silver, gold, platinum, and uranium, are synthesized by capturing numerous neutrons on short timescales ("rapid process") and therefore called r-process elements.
Electromagnetic waves emitted from celestial bodies have components with a wide range of wavelengths. As sunlight can be separated into rainbow colors through a prism, so can light (or electromagnetic waves in general) from celestial bodies be separated according to wavelength. This decomposition is called a spectrum. When a certain element or ions emit or absorb electromagnetic waves, they appear in the spectrum at a specific wavelength. Movement of a light source can be detected through the shift in wavelength caused by the Doppler Effect. The Universe is expanding such that more distant celestial bodies appear to be moving at greater speeds. Thus shifts in wavelength make it possible to estimate the distances to distant celestial bodies.
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Published: May 2018