A gamma-ray burst that occurs once in a thousand years! But does it bring more cosmic mysteries?

In a disk galaxy 2.4 billion light-years away from the Milky Way, a huge explosion occurred. The material ejected by the explosion formed an extremely narrow jet that moved at close to the speed of light. This caused the energy it released to shine in a specific direction like a laser beam, and our Earth happened to be in this direction. At 1:17 pm on October 9, 2022, Universal Time, this flash of light visited the Earth in the form of a gamma-ray burst (GRB), and was immediately captured by many satellites and ground-based gamma-ray observatories. As usual, it was named according to the date and type of discovery: GRB 2210009A (abbreviated as GRB 09A). But soon, a more resounding name spread throughout the astronomical community: the brightest GRB in history. Its brightness was so unprecedented that most of the high-energy telescopes in space that observed it were overexposed and could not be reliably measured. The Insight-HXMT and Insight-HXMT space telescopes developed by the Institute of High Energy Physics of the Chinese Academy of Sciences (IHEP) have successfully provided accurate measurements of this gamma-ray burst due to their unique design. Their discovery and accurate measurements will refresh astronomers' understanding of the origin and physical mechanism of gamma-ray bursts, and promote a better understanding of basic physical laws and the history of cosmic evolution.

Violent explosions throughout the universe

The discovery of gamma-ray bursts is a legendary story. In 1963, the United States, the Soviet Union and the United Kingdom signed the Partial Nuclear Test Ban Treaty, which banned nuclear weapons testing including in the atmosphere, underwater and space. In order to monitor other countries, the United States launched the Vela series of satellites in the late 1960s, equipped with the most advanced gamma-ray detectors at the time to detect gamma-ray flashes produced by nuclear explosions. On July 2, 1967, Vela series 2 and 3 satellites detected gamma-ray flashes that were different from any known nuclear weapon explosions; a total of 16 similar gamma-ray flash events were soon discovered. However, based on their rough positioning information, the research team ruled out the possibility that these flashes originated from artificial objects or solar activity.

These discoveries were published in the academic journal The Astrophysical Journal in 1973 under the title of "Observations of Gamma-ray Bursts of Cosmic Origin". Since then, mankind has widely known the existence of such violent and mysterious explosions in the universe. Since 1991, the Compton Gamma-ray Space Observatory in the United States has discovered more than 2,700 gamma-ray bursts. They are almost evenly distributed in all directions of the sky, rather than concentrated in the narrow band of the Milky Way. Therefore, astronomers immediately reached a consensus: gamma-ray bursts originate from the depths of the universe outside the Milky Way. In the terms of astronomers, "gamma-ray bursts have a cosmological origin." After knowing that gamma-ray bursts originate from extremely distant distances, based on their observed brightness, scientists can conclude that the energy released by these gamma-ray bursts is astonishing, even greater than the energy released by the entire Milky Way through radiation in a year. Therefore, gamma-ray bursts are also called "the most violent explosion phenomenon in the universe since the Big Bang."

Since the discovery of gamma-ray bursts, scientists have been speculating on what exactly causes such violent explosions in various parts of the universe. Soon, a variety of theoretical conjectures filled various academic journals. At that time, there was a joke in the astronomical community: "There are more theoretical models for gamma-ray bursts than gamma-ray bursts."

As the observational data of gamma-ray bursts became more and more detailed and complete, most of the conjectures failed to withstand the test of observation. After the great waves of elimination, two conjectures gradually became widely accepted gamma-ray burst theories: collapsing stars and neutron star mergers. Astronomers now believe that these two cosmic catastrophic events correspond to the two types of gamma-ray bursts observed so far - long gamma-ray bursts and short gamma-ray bursts.

The collapsing star model proposes that when a massive star evolves to the end of its life, the nuclear fusion fuel in its core is exhausted. The star's own huge gravity loses the thermal pressure support that has long been in balance with it, and its core quickly collapses inward to become a rapidly rotating black hole. As the outer layer of the star continues to fall towards the newly born black hole, under the combined action of the magnetic field and the effects of general relativity, two beams of plasma will be ejected along the direction of the black hole's rotation axis at a speed close to the speed of light, thereby releasing the gravitational potential energy of the collapsed star. In the jet close to the speed of light, the ejected matter collides with each other or with the surrounding interstellar medium, converting part of their kinetic energy into electromagnetic radiation, thus forming the instantaneous radiation and afterglow of the long gamma-ray burst we observe on Earth.

In the neutron star merger model, a neutron star will form a binary system with another compact object. When the orbital energy of the binary system continues to decay due to gravitational wave radiation, the neutron star will get closer and closer to its compact object companion until they collide. This merger process will tear the neutron star into two parts, one part will form a rapidly rotating black hole with the compact object companion, and the other part will then fall into the black hole, and then, like in the collapsing star model, two high-speed plasma jets will be ejected, forming the short gamma-ray burst we observed. Unlike the collapsing star model, gravitational waves will be released during the neutron star merger process, so short gamma-ray bursts are also considered to be multi-messenger sources that can be observed jointly with gravitational waves. The 2017 gravitational wave event GW170817 is such an example. It was found to occur at the same time and location as the short gamma-ray burst GRB 170817A, becoming the strongest evidence for the neutron star merger model.

China has completely and accurately measured the strongest gamma-ray burst

In the first few decades of gamma-ray burst astronomy research, the name of Chinese detectors was absent. This situation did not change until the successful launch of the Shenzhou-2 spacecraft carrying a gamma-ray burst detector in 2001. In 2017, the launch of the Insight-HXMT satellite allowed us to show our prowess in the field of observation and research of gamma-ray burst celestial bodies. The Insight-HXMT satellite is China's first X-ray astronomical satellite developed by the Institute of High Energy Physics. The high-energy X-ray telescope on board is a powerful tool for searching and measuring gamma-ray bursts. Four years after its launch, the Insight-HXMT scientific team released the first batch of 322 gamma-ray bursts observed by the Insight-HXMT satellite. In 2020 and 2022, my country successively launched three satellites developed by the Institute of High Energy Physics specifically for gamma-ray burst detection - "Jimu" A, B, and C. At this point, the "Insight" satellite and the "Jimu" satellite, together with the world's predecessor gamma-ray burst detection satellites such as Fermi, Swift, INTEGRAL, Konus, etc., have formed a skynet to capture and measure gamma-ray bursts.

Not long after this big net was woven, the most powerful gamma-ray burst in history, 09A, visited the Earth.

GRB 09A is a long GRB, so it is speculated that it originated from the collapse of a massive star. The galaxy where it occurred is 2.4 billion light-years away from Earth. This sounds very far away, but compared with other GRBs, this distance is still very close. In addition, the jet angle of this GRB is very small, so compared with other GRBs, it can radiate energy more concentratedly in the direction of the Earth. These two unique features make GRB 09A the brightest GRB in observational history: its brightness is thousands of times higher than that of a typical bright GRB.

This unexpected brightness has also caused a lot of trouble for the international mainstream gamma-ray burst detectors, causing problems similar to camera overexposure. These detectors with problems cannot provide complete and reliable detection data, and therefore cannot accurately answer how bright the gamma-ray burst 09A is.

When GRB 09A was at its brightest, the Insight-HXMT satellite and the JIMU-C satellite were in orbit and working condition to observe it. What was particularly coincidental was that the JIMU-C satellite was flying over the high latitudes of the Earth at that time. Because there was strong interference from charged particles in that area, researchers designed a special working mode for the JIMU-C satellite - a low-sensitivity working mode. This is like a person wearing sunglasses in advance when he is about to go out into the bright outdoors.

It is precisely because of this special working mode, coupled with the design of "Jimu" specifically for detecting bright sources, that "Jimu" C and "Hitomi" jointly gave the most complete and accurate measurement of Gamma-ray Burst 09A.

More cosmic mysteries to be solved

Based on statistics of previously observed gamma-ray bursts, astronomers have concluded that a gamma-ray burst as bright as 09A is only detected once every few thousand or even ten thousand years. It is surprising that such a gamma-ray burst was discovered only half a century after humans acquired the ability to observe gamma-ray bursts. Some astronomers speculate that 09A may have an unknown burst mechanism that is different from other gamma-ray bursts.

But our analysis shows that it is the extremely small jet angle that causes the observed energy difference. In the papers published by the Insight-HXMT team and the Extreme Vision team, by measuring the time-varying curve of the afterglow of GRB 09A, it was inferred that the jet angle of the GRB is less than 1 degree. Because of this, the energy of GRB 09A is highly concentrated in one direction, making the brightness seen by observers about 10,000 times higher than that of non-directional radiation. This also explains why 09A has such a unique isotropic energy. After taking into account the correction of the jet angle, the actual energy released by GRB 09A is similar to that of ordinary long GRBs.

But this did not completely answer the mystery of "Why does GRB 09A look so special?" Instead, it led to a series of new mysteries:

What physical mechanism can make the energy of a gamma-ray burst be released in such a collimated and directed manner? In the short history of gamma-ray burst observations, what is the probability that the Earth is aligned with such a collimated and directed radiation energy in the nearby universe? In addition, with the brightness of GRB 09A, it can be detected by human detectors even if it is located at a much farther distance than the current distance. So the question is: why is 09A, a nearby gamma-ray burst "only" 2.4 billion light-years away from the Earth, detected first, rather than other similar gamma-ray bursts in more distant places? You know, the more distant universe has a wider space, and the number of massive stars undergoing collapse is far greater than that of the nearby universe, so there should be more opportunities to detect gamma-ray bursts like 09A.

Scientists are not afraid of puzzles, but rather love them. Because every puzzle is connected to a potential new discovery. The radiation of gamma-ray bursts comes from the complex radiation transfer process in the jets that are close to the speed of light, and the plasma in the jets is accelerated near the rotating black hole and collimated by complex magnetic field dynamics. Therefore, the energy spectrum and light curve of each gamma-ray burst contain the codes of particle physics, shock wave physics, relativity, black hole physics and magnetohydrodynamics. At the same time, since we now generally believe that the progenitors of long gamma-ray bursts are dying massive stars, and the progenitors of short gamma-ray bursts are neutron star/compact star binaries, the former can be regarded as a beacon of the history and distribution of massive stars in the universe, while the latter is a beacon of the history of neutron star/compact star mergers. Scientists currently believe that heavy elements with atomic numbers greater than zinc in the universe were born in these two cosmic catastrophe events. Therefore, the occurrence rate and redshift distribution of gamma-ray bursts also contain the answer to the mystery of the origin of the elements that make up our world.

Gamma-ray bursts are born in the most extreme environment in the universe, where there is the most extreme curved space-time, the densest matter, the strongest electromagnetic field, the highest temperature and the fastest movement speed... Gamma-ray photons carry the code of the most profound laws in the universe. More than 50 years ago, it made mankind turn their attention to the vast and eternal space; more than 50 years later, China's "Hui Yan" and "Jimu" accurately portrayed this "brightest stroke", making the gamma-ray burst, a once dazzling star in the history of world astronomy, shine again in the era of multi-messenger astronomy. Today, China's high-energy astronomical telescopes play an increasingly important role in observing high-energy astrophysical phenomena, and Chinese astronomers will also make more contributions to mankind's understanding of the universe.