Unveiling the black hole

RIKEN has had a long history of space research, and many groundbreaking discoveries are continuing to be announced.

Kazuo Makishima

Chief Scientist
Director of the Cosmic Radiation Laboratory
Advanced Science Institute

“Today I would like to talk about black holes.” Kazuo Makishima, chief scientist at the Cosmic Radiation Laboratory in the RIKEN Advanced Science Institute, begins his talk. One might ask, “Is space research also going on at RIKEN?” Makishima continues, “To my regret, RIKEN’s space research activities are not well known. Mine is one of the laboratories for which the origin can be traced back to the laboratory of Yoshio Nishina (1890–1951), the father of modern physics in Japan, who made remarkable achievements at RIKEN. Since his work on cosmic rays, RIKEN has had a long history of space research, and many groundbreaking discoveries are continuing to be announced.” The Cosmic Radiation Laboratory used the small satellite High Energy Transient Explorer-2 (HETE-2) to clarify the nature of gamma ray bursts, often referred to as “a mysterious cosmic phenomenon.” The laboratory is also exploring the evolution of black holes by means of the cosmic X-ray satellite Suzaku, and is about to install the Monitor of All-sky X-ray Image (MAXI) instrument in the International Space Station to search for instances of merging black holes.

Profound relationship between RIKEN and black holes

A black hole is a celestial body with a gravitational field so strong that everything is absorbed by it. Not even light can escape. “Initially, black holes were thought to be imaginary objects predicted by Einstein’s theory of general relativity, and no one believed in their existence,” explains Makishima.

Advances in research into stellar evolution showed that fixed stars heavier than ten solar masses undergo gravitational collapse and supernova explosion at the end of their lives. In the middle of the twentieth century, researchers realized that this event might lead to an extremely high density at the center, and hence the formation of a black hole. In 1962, Riccardo Giacconi of Italy made the first discovery of a mysterious celestial body that was emitting X-rays (work for which he received the 2002 Nobel Prize in Physics). X-rays are emitted by celestial bodies that are at ultrahigh temperatures of several ten million to several hundred million degrees, such as supernova remnants and black holes. In Japan there is also one person who must always be mentioned in connection with the relationship between X-rays and black holes.

Makishima says, “That person is Minoru Oda, President of RIKEN between 1988 and 1993.” He continues, “Dr Oda took note of a very strong X-ray source known as Cygnus X-1. After discovering that the intensity of its X-rays is fluctuating rapidly more than once per second, he wrote, ‘Cygnus X-1 must be a unique celestial body with a very small size. Its X-ray emission may be a result of the accretion of gases onto a black hole.’ This was the first article in the world to correlate a real celestial body with a black hole.” The observations made all over the world during the subsequent several years demonstrated that Cygnus X-1 is a binary system consisting of an ordinary star and a black hole of about ten solar masses. Hence, the intense X-rays were being emitted as the star’s gases were absorbed by the black hole.

The existence of black holes became generally recognized in the 1980s, but scientists are sceptical about everything. Do black holes really exist? If so, what are their masses, and where are they? The Cosmic Radiation Laboratory has been working on black holes under the supervision of two generations of chief scientists.

The nature of gamma-ray bursts, a mysterious cosmic phenomenon

Makishima assumed his current position in April 2001. “We are implementing three projects on black holes. Two of them continue work by former chief scientist Masaru Matsuoka (now research director at the Japan Aerospace Exploration Agency), and I instituted the other.”

The first is the HETE-2 project, in succession to Matsuoka’s work. This is an astronomical observatory satellite to monitor gamma-ray bursts, developed jointly by RIKEN, the US, and France.

In 1967, gamma-ray bursts were first detected in the world by US military satellites that were monitoring nuclear tests by the USSR. Although the purpose of its operation was to capture gamma rays produced as a result of a nuclear explosion, it strangely sometimes detected intense gamma ray flashes coming from various directions in the sky, rather than from the ground. “It seems that, in those days, this finding was kept strictly confidential by the US military. It was later shown to be a celestial phenomenon, but when and where it occurred was totally unpredictable. Additionally, the intense gamma rays disappeared in several seconds to several minutes. If one examined the direction from which the gamma rays came, one could not find any likely celestial candidate. HETE-2 became the first observatory to make a dedicated observation of gamma-ray bursts, at that time called ‘a mysterious cosmic phenomenon.’”

On capturing a gamma-ray burst, its position in the sky is automatically determined by HETE-2, which immediately reports it to ground stations. This positional information is transmitted instantaneously throughout the world via the Internet to astronomical observatories and amateur astronomers, who then direct their telescopes to the indicated position in an attempt to watch optical afterglows of the gamma-ray burst. “For the GRB030329 gamma-ray burst of March 29, 2003, the explorer and ground observations showed the best match,” says Makishima. The positional information was transmitted everywhere in the world 73 minutes after the onset of the event. The 20 cm automated telescope installed on the roof of the main research building at RIKEN’s Wako campus then succeeded in capturing the afterglow of the gamma-ray burst, the earliest in the world. A celestial body of magnitude 13 appeared at a position where nothing had been present (Fig. 1). As a result, the position of the event in the sky was determined with much greater accuracy than was available with HETE-2.

GRB030329 is about 1.8 billion light years from Earth. Being the second nearest of the gamma-ray bursts that had allowed their distance to be determined, GRB030329 permitted extensive observations for a long time, resulting in groundbreaking findings. “Astronomers used their large telescopes to examine the afterglow at various wavelengths, and the data obtained at early stages of the fading of the afterglow were typical of all other gamma-ray bursts that had ever been observed. Some 10 days later, however, the situation changed to provide data that agreed well with those observed so far from an extreme class of supernova explosion known as ‘hypernova.’ We eventually unveiled the gamma-ray burst. It is thought that when an extremely heavy star collapses owing to gravity and undergoes a hypernova explosion at the end of its life, gamma-ray bursts occur with a certain probability.” In addition to clarifying the source of gamma-ray bursts, this discovery had another major significance. “A hypernova explosion is considered to be followed by the formation of a black hole. We were watching the instant of birth of a black hole.”

There are two types of gamma-ray bursts, depending on their duration: those shorter than about 2 seconds, and longer ones. GRB030329 falls into the latter category. The longer-duration type was shown to represent an event that accompanies a hypernova explosion. In contrast, observations of other gamma-ray bursts through the use of HETE-2 and other observatories suggest that the shorter-duration type is likely to be associated with the collision and merger of neutron stars. A neutron star is a celestial body formed as a result of the collapse of the central portion of a star of about ten solar masses after a supernova explosion, and its mass is less than three solar masses.

Having accomplished its objectives, HETE-2 completed its operation at the end of March 2007. Gamma-ray bursts are no longer mysterious celestial phenomena.

Are there any black holes in the midst of growth?

In addition to black holes of about ten solar masses as components of binary systems such as Cygnus X-1, supermassive black holes of about one million to one billion solar masses are known to exist in the centers of galaxies. A supermassive black hole is also present in the center of our Galaxy. “There is now a consensus that ‘each galaxy has one supermassive black hole,’ but how they were formed remains elusive.”

In fact, a particular type of celestial body has provided a key to answering this question. “Since the 1980s it has been known that X-ray sources 100 to 1,000 times more luminous than Cygnus X-1 (Fig. 2) exist in the arm regions of a number of nearby spiral galaxies. Their existence had been annoying researchers,” says Makishima. He used Japan’s X-ray astronomy satellite ASCA, NASA’s Chandra X-ray observatory, and others to demonstrate that those extraordinarily luminous X-ray sources might be ‘intermediate-mass black holes’ of about several tens to several hundreds of solar masses, thus arousing international disputes. It was Makishima who named them ultraluminous X-ray sources (ULXs).

Stimulated by these observations, chief scientist Toshikazu Ebisuzaki at the Computational Astrophysics Laboratory in RIKEN wrote a scenario for the formation of a supermassive black hole by means of numerical simulations. “In his scenario, a supermassive star is first formed in a star cluster, and it then collapses to produce a heavy black hole. In this process, gamma-ray bursts may occur,” explains Makishima. “The black hole swallows the gases of nearby stars, one after another, and becomes larger and larger while shining as a ULX. It gradually sinks toward the center of the galaxy, during which process such black holes repeatedly merge together and finally produce a single supermassive black hole.” (Fig. 3).

To identify the nature of ULX, the key to this scenario, Makishima instituted his second project using Japan’s Suzaku X-ray observatory. Launched in July 2005, Suzaku is equipped with a hard X-ray detector (HXD) developed jointly by his laboratory, the Japan Aerospace Exploration Agency, the University of Tokyo, and others. Being capable of high-sensitivity observations covering high-energy X-rays (hard X-rays), the Suzaku observatory makes it possible to obtain extensive data on the behavior of the gases being absorbed in black holes.

“Observations made with Suzaku have revealed very similar properties between black-hole binaries, ULXs, and supermassive black holes. More evidence is being obtained for the likelihood that ULXs are intermediate-mass black holes that link stellar black holes and supermassive black holes. In combination with the performance of Suzaku, observational and theoretical investigations at RIKEN will elucidate the identities of mysterious ULXs and their relation to supermassive black holes.”

Revealing the merging of black holes

The third point of note is the MAXI project. If a supermassive black hole is a result of a merger of smaller black holes, there should be black holes that are about to merge. Although this scenario was unknown in 1997, when former chief scientist Matsuoka proposed the MAXI project, soon after assuming his current position Makishima noted that MAXI was ideal for visualizing the merger of black holes.

Black holes that are about to merge should be absorbing the gases from the environment while revolving around each other. If this is true, the intensity of X-rays emitted will change periodically because the amount of gases absorbed varies with the relative positions of the two black holes. MAXI will be used to monitor about 100 X-ray-emitting supermassive black holes for X-ray sources in the entire sky, and celestial bodies showing periodical changes in X-ray brightness will be sought. “Any object with a period of several months is likely to be a black hole about to merge with another.”

In June 2008 an international workshop for MAXI was held at RIKEN, with an attendance of about 180 people, including 33 from abroad; this is evidence of the high expectations that are held for the opportunity. MAXI has already been assembled, and it is awaiting transportation to NASA in the US this autumn. In May 2009 it will be launched on a space shuttle and installed on Japan’s experimental module, Kibo, on the International Space Station (Fig. 4).

The Cosmic Radiation Laboratory is performing three satellite projects: HETE-2, Suzaku and MAXI. Many other projects are also ongoing. They include high-energy neutron monitoring in Tibet; the Wide-field Telescope for GRB Early Timing (WIDGET), to monitor one-tenth of the sky continuously for sudden cosmic events; research into past supernova explosions by means of Antarctic ice cores excavated by Japan’s National Institute of Polar Research; and the ground detection of gamma rays generated by thunderclouds. “Because artificial satellite projects involve long time spans, they have difficulties because new ideas go out of date. I advise young researchers to engage in small projects concurrently with their major ones. By doing so, they are able to refresh their minds and are trained in drawing up and implementing new projects.”

At present, the Cosmic Radiation Laboratory has about 30 members, including significant numbers of special postdoctoral researchers and graduate students, making the mean age of the laboratory’s members relatively young among the laboratories in RIKEN. “I believe it is my mission to vitalize space research at RIKEN, which has a long history starting with Dr Nishina’s work, along with young researchers,” says Makishima, standing beside framed calligraphy that was done by Nishina soon after the end of the war.

About the researcher

Kazuo Makishima was born in Tokyo, Japan, in 1949. He graduated from the Faculty of Science, University of Tokyo, in 1974, and left the graduate school of the same university in 1978. He worked as a Junior Lecturer at the Institute of Space and Astronautical Science, where he participated in the cosmic and solar X-ray satellite projects Hakucho (launched in 1979), Hinotori (1981), and Tenma (1983). Promoted in 1986 to Associate Professor of the University of Tokyo, he obtained his delayed PhD from that department. There he continued X-ray astrophysics studies by participating in Ginga (1987), Yohkoh (1991), and ASCA (1993) satellites, and became Full Professor of the same department in 1995. In 2001 he was jointly appointed Chef Scientist of the Cosmic Radiation Laboratory of RIKEN. The latest satellite he has participated in is Suzaku, launched in 2005. His favorite research subjects are mass-accreting black holes, strongly magnetized neutron stars, and clusters of galaxies.