The boy who caught the M87 signal in the mountain … 20 years later, the ‘second black hole’ protagonist

EHT’s international joint research team has captured, for the first time ever, a supermassive black hole located in the center of the Milky Way, ‘Sagittarius A’. [사진=천문연]
An image of the Sagittarius A black hole observed this time.  The black part at the center is the black hole (event horizon) and the shadow containing the black hole, and the shiny part of the ring is light bent by the black hole's gravity. [사진=천문연 제공]
An image of the Sagittarius A black hole observed this time. The black part at the center is the black hole (event horizon) and the shadow containing the black hole, and the shiny part of the ring is light bent by the black hole’s gravity. [사진=천문연 제공]

An international joint research team at the Event Horizon Telescope (EHT) for the first time ever captured a supermassive black hole in the center of our galaxy and released the image at 22:07 on the 12th. This is a Sagittarius A black hole. This is the second black hole discovered by mankind after M87, which was first detected in 2019, and the Korea Astronomy and Space Science Institute also listed it.

The Sagittarius A black hole, located at the center of the Milky Way, is about 27,000 light-years from Earth and has a mass of about 4 million times that of the Sun. Compared to the M87 black hole, the distance from the solar system is close to 1/2000, making it a strong target for black hole research. However, since it has a mass 1500 times smaller than that of M87, the gas flow around the black hole changes rapidly, and the image suffers from severe scattering, making it difficult to observe compared to M87.

The joint research team succeeded in observing black holes using EHT. EHT is an attempt to capture black hole images by connecting radio telescopes scattered around the world to create a virtual telescope the size of the Earth. 80 institutions around the world, 300 researchers, and 8 radio telescopes were mobilized for 5 years.

Through the 3rd Korea Space Radio Observation Network (KVN) (Yonsei University in Seoul, Ulsan University, and Tamna Radio Observatory in Seogwipo, Jeju), the Astronomical Research Institute confirmed that the structure of the Sagittarius A black hole is close to a ‘circle’ with an international research team. This means that the disk of attachment created when surrounding gases are rotated and attracted by the black hole’s gravity has a circular shape, suggesting that the black hole is oriented toward the Earth.

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“The Sagittarius A black hole and the M87 black hole have very similar shapes, according to Einstein’s theory of general relativity,” said Sarah Markov, co-chair of the EHT Science Council.

Keiichi Asada, Ph.D., Astrophysics Researcher at the Taiwan Central Academy of Sciences, said, “Through this study, we compared and analyzed images of the largest supermassive black hole, the M87 black hole and the smallest Sagittarius A black hole, to see how gravity works in extremely different situations, more than ever before. We can test it in more detail.”

“The Sagittarius A black hole is the closest black hole among the black holes directly observed by human beings with collective intelligence,” said Bong-won Son, PhD, from the Radio Astronomy Headquarters, who participated in this study. We are preparing to participate directly in the event.”

◆ The boy who caught the M87 signal in the mountains…to a full-fledged scientist

Bong-Won Son, Ph.D., Radio Astronomy Headquarters, participated in this study.  He majored in radio astronomy and worked as a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Germany.  He was then involved in the study of capturing signals from black holes near the event horizon. [사진=이유진 기자]
Bong-Won Son, Ph.D., Radio Astronomy Headquarters, participated in this study. He majored in radio astronomy and worked as a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Germany. He was then involved in the study of capturing signals from black holes near the event horizon. [사진=이유진 기자]

Dr. Son was a member of that research team when humans first caught the signal from a black hole. He received a master’s degree in radio astronomy from Yonsei University, followed by a doctoral course at the University of Bonn, Germany. For reference, Bonn was the center of black hole research at the time.

According to him, there was a boom in black hole research at the time of the lab. Beyond the conventional observation of jet emission from active galaxies, attempts have been made to capture black hole signals near the event horizon with high-resolution telescopes. By chance, he joined the research team, and he took charge of observing the event horizon. The signal successfully caught at this time is the starting point of the black hole M87, which was first captured by mankind.

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☞ Event horizon: A wide boundary zone connecting the inside and outside of a black hole. When a substance passes through the event horizon and is sucked into a black hole, some of it is released as energy, so if you use high-resolution observation equipment, you can see the edge of the event horizon.

According to Dr. Son, the observations of the Very Long Baseline Interferometry (VLBI) were recorded on video tape at the time. Synthesizing signals from dark celestial bodies requires very precise timing, which is not suitable for Taperon. A hard disk was required to observe the event horizon level. At that time, a hard disk for radio interferometer was developed, and the research team confirmed its possibility with the first prototype. The result was a huge success, able to catch a black hole signal near the event horizon.

☞Very Long Baseline Interferometry (VLBI): Several radio telescopes located hundreds to thousands of kilometers away simultaneously observe the same celestial body and implement a huge virtual telescope with an aperture corresponding to the distance between the radio telescopes to achieve resolution (lower resolution). A technique that increases the ability to distinguish between two objects.

Dr. Son said, “At the time, I was the youngest member of the team with a postdoc, so I used to climb the mountain with the first prototype and observe it for 12 hours. Everyone was not sure if it would or not, but the observation went well. It was a record moment in the study of black holes.”

◆ Latecomer Korea’s performance? “The result of tenacity”

Korea Space Radio Observation Network (KVN) Tamna Radio Observatory located in Seogwipo, Jeju Island.  In addition, in Korea, Yonsei University in Seoul and Ulsan University are located respectively.  Unit 4 is currently under construction at Seoul National University's Pyeongchang campus. [사진=대덕넷DB]
Korea Space Radio Observation Network (KVN) Tamna Radio Observatory located in Seogwipo, Jeju Island. In addition, in Korea, Yonsei University in Seoul and Ulsan University are located respectively. Unit 4 is currently under construction at Seoul National University’s Pyeongchang campus. [사진=대덕넷DB]

Korea completed the KVN in 2008 and began to observe space phenomena in earnest. At that time, leading countries such as Japan were also skeptical of Korea at first. This is because of the concern that signal acquisition and synthesis take a significant amount of time. However, contrary to expectations, KVN produced the same results as Japan in the first observation to verify the telescope’s performance. When the confirmed signals were synthesized, the correct image came out.

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Since then, KVN has been used for research since 2010. At that time, international event horizon research was being conducted centered on the United States, Germany, and Spain, and Korea, China, and Japan made observations that could complement M87 and the black hole centered on the Milky Way as their main research topics. Then, in 2017, a campaign was launched in which 8 telescopes in 6 regions could participate at the same time, and Korea was invited to participate. It was the result of previous research.

Dr. Son said, “The existing telescope had a drawback in that it was difficult to observe due to water vapor. But I was not sure, so I couldn’t try. Then Korea tried and succeeded in KVN for the first time in the world.”

He added, “The KVN can simultaneously observe multiple frequencies and create images. It is a great innovation as one telescope is performing multiple roles.”

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