How do we “see” the brightest “fireworks” in the universe?

On October 9, 2022, there was a very shocking and beautiful "fireworks" explosion in the universe.

Scientists specifically gave this "cosmic firework" a name "BOAT", which means "brightest of all time".

This is the brightest gamma-ray burst discovered so far - GRB 221009A.

An image of the brightest gamma-ray burst (GRB 221009A) discovered so far, courtesy of Institute of High Energy Physics, Chinese Academy of Sciences

Gamma-ray bursts of this brightness are extremely rare, with an occurrence rate of about once in 10,000 years. (Some conservative researchers say it's at least once in 1,000 years.)

However, this magnificent "cosmic fireworks" with such a low probability of occurrence was "seen" by our country's scientific researchers.

How do they "see"?

Gamma-ray bursts are produced in extremely extreme physical environments.

What exactly is a gamma-ray burst?

Gamma-ray burst, abbreviated as GRB, refers to the phenomenon that the intensity of gamma rays from a certain direction in the sky suddenly increases in a short period of time and then weakens.
Gamma-ray bursts are the most violent celestial explosions since the Big Bang. The energy it releases in one second is equivalent to the total energy released by the sun in about 5 billion years. The energy it radiates in a few seconds is equivalent to the total energy radiated by the sun in 10 billion years. Therefore, it is called the most powerful event in the universe.

There are two types of gamma-ray bursts: one is short burst, which lasts less than 2 seconds and usually comes from areas where few stars are formed; the other is called long burst, which lasts longer than 2 seconds, some lasting from tens of seconds to several minutes, or even longer. The gamma-ray burst discovered this time belongs to this category.

Scientists generally believe that long bursts are produced by the core collapse explosion of very massive stars and are related to the formation of neutron stars or black holes. This is why many people speculate that GRB 221009A may have formed a newborn black hole.

Since the first gamma-ray burst was discovered in 1967, nearly 10,000 gamma-ray bursts have been detected. After half a century of research, people have come to realize that gamma-ray bursts are produced in extremely extreme physical environments, such as extremely high magnetic fields, extremely strong gravity, and extremely fast speeds. Gamma-ray bursts have thus become an extreme physics laboratory favored by astrophysics and even basic physics.

Scientists have not yet observed gamma-ray bursts in the Milky Way. These powerful rays come from the depths of the distant universe. What are they really like? Humans still know very little about them. Scientists hope to use gamma-ray bursts to study major issues such as the evolution of the universe, the origin of heavy elements, and the correctness of relativity. However, before this, they must have a deep understanding of the physical origin of gamma-ray bursts themselves. This is an important reason why many researchers in astrophysics and cosmology are paying close attention to gamma-ray bursts.

The gamma-ray burst observed on October 9, 2022, was an unprecedented feast for the scientific community. At 21:16:59.59 Beijing time on that day, NASA's Fermi Gamma-ray Space Telescope first detected an unusually bright gamma-ray burst, named GRB221009A according to international convention. Subsequently, many astronomical facilities around the world detected GRB 221009A.

The gamma-ray burst numbered GRB 221009A originated from the sky in the direction of the Sagitta constellation. Image source @International Gemini Observatory/NOIRLab/NSF/AURA/B.O'Connor (UMD/GWU) & J. Rastinejad & W Fong (Northwestern Univ)

In so many astronomical facilities, Chinese scientific research teams have conducted measurements with the highest international precision.

Heaven and Earth Collaborate to "Watch Fireworks"

If you want to do your work well, you must first sharpen your tools. In many scientific fields, as the threshold for research equipment becomes higher and higher, this saying is becoming more and more true, especially in the cutting-edge field of space exploration.

For a long time, due to the backwardness of observation and research equipment, the level of cutting-edge scientific and technological achievements achieved by my country in this area is far behind that of developed countries, and some of the research carried out can only be follow-up research of major scientific research results (the value and significance of such research results are greatly reduced in most cases).

However, this situation is changing as China builds more and more major scientific and technological infrastructure.

In the detection of this brightest gamma-ray burst, the "Laso", the High Energy Burst Explorer (HEBS) satellite and the Insight-HXMT satellite, all built under the leadership of the Institute of High Energy Physics of the Chinese Academy of Sciences, also participated. They detected this gamma-ray burst at the same time, achieving coordinated ground-ground observations across a wide energy range of 11 orders of magnitude.

Together, they carried out the highest international precision measurements of the radiation properties of the gamma-ray burst at each key stage, from the main burst to flares and early afterglow.

Our research found that this gamma-ray burst is 50 times brighter than the brightest gamma-ray burst ever observed by humans. (Foreign radio data show that GRB 221009A is 70 times brighter than any gamma-ray burst ever observed.)

Schematic diagram of the brightest gamma-ray burst discovered by a Chinese satellite. Photo courtesy of Institute of High Energy Physics, Chinese Academy of Sciences

The Insight-HXMT satellite observed a gamma-ray burst (GRB 221009A). Before and after the gamma-ray burst, the Insight-HXMT satellite was scanning the sky. The green dotted line represents the scanning trajectory pointed by the telescope. Image courtesy of Institute of High Energy Physics, Chinese Academy of Sciences

my country's High Altitude Cosmic Ray Observatory (LHAASO) played an important role. Through it, researchers collected more than 60,000 gamma photons in more than 20 minutes, and for the first time completely recorded the entire process of the tera-electron-volt gamma-ray burst, which is an unprecedented feat in human history. It can be said that the research team led by Chinese scientists has reached the forefront of astrophysics research using it.

"If the selection conditions are reduced to the minimum, the number of photons can reach 100,000!" said Cha Min, one of the corresponding authors of the paper and a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences. Compared with other experimental devices in the same energy range, and even equipment specially designed to track gamma-ray bursts, the number of photons they measured was only below a thousand, and they only measured the afterglow more than 60 seconds after the explosion (what is afterglow will be explained below).

Is it a coincidence that the detection results of "Laso" are so good?

Why is "Laso" so great? Why did it win the first prize this time? This is a question that many people will immediately ask.

Now let’s answer this question carefully.

The "Laso" is a major national scientific and technological infrastructure with cosmic ray observation and research as its core. It is located in Haizi Mountain, Daocheng County, Sichuan Province, with an average altitude of 4,410 meters. It was independently designed by Chinese scientists, construction began in 2017, and it was fully put into operation in July 2021.

This is a large-scale cosmic ray detection array: the 1-square-kilometer ground-based particle shower detector array (KM2A) composed of 5,216 electromagnetic particle detectors and 1,188 muon detectors, the 78,000-square-meter Water Cherenkov Detector Array (WCDA) composed of 3,120 detection units, and the wide-angle Cherenkov telescope array composed of 18 telescopes, covering an area of ​​about 1.36 square kilometers.

The "Lazo" is currently the world's highest, largest and most sensitive gamma-ray detector . Its mission is to catch cosmic rays from outer space heading to Earth, allowing scientists to discover the unknown world deep in the universe and solve the mystery of the origin of cosmic rays.

Why was such a high altitude chosen for the construction of the "Lasoe"? Because high-energy cosmic rays and gamma rays are easily absorbed by the atmosphere when they enter it, and the air is thin at high altitudes, so it is easier to detect cosmic rays and gamma rays by building an observatory. Moreover, detection here is completely unaffected by weather and climate conditions, and can be carried out continuously 24 hours a day.

Aerial photo of "Cable" provided by Institute of High Energy Physics, Chinese Academy of Sciences

Many people will also ask, how come the best detection results of GRB 221009A's afterglow happened to be captured by "Lasor"? Isn't this too coincidental?

This is not a coincidence. The fact that Laso did not miss this scientific research feast that was delivered to its doorstep is also largely related to the powerful observation capabilities of Laso itself.

When GRB 221009A occurred, the excessive photon flux saturated the detectors of multiple international experiments. Therefore, although many other space-based and ground-based observation equipment on Earth also observed GRB 221009A, they did not obtain the same observation effect as "Lasso". Some low-altitude observatories are even more difficult to observe because of the influence of the atmosphere. "Lasso" is at a high altitude and is a newly built cosmic ray observation device. Compared with similar devices in the world, it has a strong detection capability.

Because of its huge size, Lasso has an extremely wide "field of view" when detecting cosmic rays. Cao Zhen, chief scientist of the Lasso project and researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, said, "It can see 1/6 of the sky when it opens its eyes." The probability of Lasso capturing information from the depths of the universe is not too low. Conventional gamma-ray telescopes can only observe one point or one star at a time, so the probability of hitting the jackpot is naturally much lower.

Because the Lasso has a very high sensitivity, it can capture even a single photon as long as it enters its field of view. Before it, humans did not have such a high-sensitivity ground-based equipment to carry out gamma-ray burst detection, and foreign observation equipment in the same energy range has not yet been able to fully record the entire process of gamma-ray bursts. This is also the special feature of the high-quality detection results achieved by the Lasso this time.

It is reported that the observation results published in this paper are mainly provided by the water Cherenkov detector array. The detector uses 360,000 tons of pure water as a medium and measures the secondary products of the movement and action of gamma rays or cosmic rays in the atmosphere through 6,240 photosensitive probes of different sizes placed on the bottom of the water, such as low-energy gamma photons, positrons and electrons, which will generate Cherenkov light signals in the water.

The array's observation energy range for gamma rays spans two orders of magnitude, with precise measurement capabilities between 100 billion electron volts and 10 trillion electron volts. It also has wide field of view and all-weather characteristics, giving it outstanding advantages in capturing observations of sudden celestial phenomena such as gamma-ray bursts.

When gamma rays enter the atmosphere, they produce many secondary particles, like a particle "shower". The water Cherenkov detector array of "Laso" is like a "rain collecting" device that works all year round and can capture them.

"Laso" observed the gamma-ray burst GRB 221009A at a high significance level of more than 250 times the standard deviation. Image source: Institute of High Energy Physics, Chinese Academy of Sciences

Although Lasso was not the first to discover GRB 221009A, it had the best "hardware" and achieved the best observation results. It accurately recorded the gamma-ray burst that lasted for several hundred seconds from the beginning and obtained complete and accurate observation data. So much so that the head of a gamma-ray burst observatory in Europe joked to Cao Zhen, saying "God is too partial!"

A simulated image of the moment a gamma-ray burst arrives at a high-altitude cosmic ray observatory. Photo courtesy of Liang Yan's team at the University of Science and Technology of China.

"Lasor" reveals the mystery of GRB 221009A, the brightest star in history

GRB 221009A is the most unique gamma-ray burst detected so far. Compared with typical gamma-ray bursts discovered before, it is about 2.4 billion light-years away from the Earth, much closer than other gamma-ray bursts. In the scale of the universe, such a distance is not too far from the Earth. However, its brightness is extremely high, completely breaking the previous observation record.

Its "brightest" status probably has a lot to do with its relatively close distance to the earth. However, Chinese scientists have discovered that there are other reasons behind this.

When GRB 221009A occurs, it devours surrounding matter through extremely strong gravity and ejects matter from its poles at nearly the speed of light, forming a pair of jets in opposite directions. Only when the jets happen to be aimed at the Earth can humans have the opportunity to detect these radiations.

Through research, scientists believe that the gamma photons observed by Lasso come from the aftermath of the main explosion of a massive star. The main explosion of a gamma-ray burst event, that is, the shock wave or magnetic field annihilation (magnetic reconnection) inside the jet accelerates the charged particles to produce gamma-ray radiation, also known as transient radiation. It is a huge explosion in the initial stage and will manifest as strong low-energy gamma-ray radiation. Immediately afterwards, the jet formed by the initial explosion, which expands at a speed close to the speed of light, collides with the surrounding interstellar medium to produce a aftermath explosion, which is also called an afterglow.

Lasso accurately observed the entire process of the GRB 221009A event for the first time, and made very important and groundbreaking observations of the earliest afterglow of the gamma-ray burst in the TeV energy region. It discovered the rapid growth and rapid decay of the afterglow radiation process of GRB 221009A, and recorded the entire stage of the enhancement and decay of the tera-electron-volt gamma-ray flux.

Among them, there is an obvious inflection in the flow attenuation stage, and the flow decreases rapidly. It can be inferred that the ejecta of this explosion is a jet-like structure with an abnormally narrow jet angle. It seems to be a needle-like beam emitted from the explosion. When the radiation angle expands to the edge of the jet, the brightness drops rapidly.

The jet angle is calculated to be only 0.8 degrees, which is the smallest jet angle known so far. Just like a water pipe, the thinner the water flow, the farther it sprays, and the jet directly points to the earth.

The position of GRB 221009A in the field of view of "Laso" at the time of its outburst. Photo courtesy of Institute of High Energy Physics, Chinese Academy of Sciences

During the previous flow enhancement stage, "Lasso" also observed the phenomenon of extremely rapid enhancement for the first time. In the initial less than two seconds, the flow increased by more than 100 times, which exceeded the expectations of previous theoretical models.

The observation results of "Laso" this time also show that the brightness of the high-energy radiation suddenly and rapidly decreased at some point less than 10 minutes after the detonation. Since this brightness transition occurred very early, it means that "Laso" actually observed the brightest core of a typical jet that is bright inside and dark outside.

"It is precisely because the observer happened to be facing the brightest core of the jet that naturally explains why this gamma-ray burst is the brightest in history, and also explains why such events are extremely rare," said Dai Zigao, professor at the University of Science and Technology of China and one of the corresponding authors of the paper.

Gao He, a professor at the Department of Astronomy of Beijing Normal University, commented, "For the first time, Lasso has given a complete light curve and energy spectrum of a gamma-ray burst at TeV energy. This observation has greatly enhanced our understanding of the radiation mechanism and jet structure of gamma-ray bursts."

Yao Zhiguo, one of the corresponding authors of the paper and a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, said that the observation results published this time will trigger in-depth discussions in the scientific community on the mechanisms of gamma-ray burst energy injection, photon absorption, and particle acceleration.

In addition, during the 10 minutes of this gamma-ray burst event, the number of photons recorded by Lasso also exceeded the accumulation of observations of the "standard candle" Crab Nebula in the past few years.

Composition of gamma-ray bursts, image source: Institute of High Energy Physics, Chinese Academy of Sciences

Astronomers believe that the newly discovered GRB 221009A likely represents the birth of a new black hole. When a black hole forms, it drives powerful particle jets. These particle jets are accelerated to nearly the speed of light and then pass through the remains of the progenitor star, emitting X-rays and gamma rays as they flow into space. When the direction of these jets is roughly pointed at the Earth, they can be detected in the form of bright X-ray and gamma-ray flashes.

From the perspective of scientific exploration, the observations by Lasso are the first opportunity for humans to accurately measure the entire process of high-energy gamma-ray bursts produced at the moment of death of massive stars. Cao Zhen said that this is the first time that humans have fully recorded the entire process of a trillion-electron-volt gamma-ray burst at the moment of death of a massive star, which will greatly enhance human understanding of the radiation mechanism and jet structure of gamma-ray bursts.

On May 10 this year, after eight years of preliminary research and four years of construction, the "LaCe" passed the national acceptance and was officially put into operation. Before it was officially put into operation, the "LaCe" had such a major observation breakthrough, which fully proved the excellent performance of the "LaCe".

The acceptance committee believes that the completion and operation of "Lasso" has made it one of the three major international particle astrophysics experimental facilities, which is of great significance in promoting major original breakthroughs in this field, driving the development of cutting-edge interdisciplinary disciplines and international cooperation.

As one of the largest experimental devices for studying cosmic rays in the past 110 years, Lasso has brought gamma astronomy research into a new band that humans have never observed before. In fact, before it was fully built, Lasso had already discovered ultra-high-energy cosmic particles exceeding 1PeV (petaelectronvolt, P is 1015). In the next decade or so, it will collaborate with other earth-space exploration equipment in my country and abroad to help scientists unravel the mystery of the origin of cosmic rays.

There are still more secrets to be uncovered about gamma-ray bursts

The GRB 221009A explosion is also a rich mine of research, and there are still many questions about the event that are leaving researchers around the world scratching their heads.

Based on the observations of tens of thousands of gamma-ray bursts, scientists have established a theoretical model that seems perfect, and even believe in it. GRB 221009A made scientists realize that the previously established theoretical model of gamma-ray bursts is still incomplete.

Some foreign researchers were surprised to find that radio data showed that over a longer period of time, the GRB 221009A jet evolved smoothly and fairly slowly over time, gradually disappearing over time, which was incompatible with the existing model showing rapid energy jumps and could not be explained.

Why is the jet so narrow after the GRB 221009A event? There has been no reliable explanation. Kate Alexander, an astronomer at the University of Arizona in Tucson, said, "The afterglow model that has been working for 25 years cannot fully explain this jet."

Is there really a new black hole born?

After the GRB 221009A event, scientists believe that it may form a supernova, but so far no trace of this supernova has been found, and astronomers are still searching. Astronomers also need more gamma-ray burst and supernova association systems to fully explore the generation mechanism of gamma-ray bursts and supernovas.

Some researchers believe that if the merger of neutron stars (an unlikely option) can be ruled out, then the most likely scenario is that the newly formed black hole directly swallowed up the debris produced by the explosion, causing the supernova to die in the womb, but such a conjecture still requires more reliable evidence to support it.

Astronomers say that it is human luck to detect GRB 221009A. The most spectacular part of this gamma-ray burst lasted only a few hundred seconds. For the universe, which is measured in billions of years, this is an extremely short moment. It is the most beautiful "fireworks" in the universe . Although our naked eyes cannot see its existence, it happened to be captured by the clearest "camera" of human beings. I hope that studying it can reveal more secrets of the universe for us.

References:

https://svs.gsfc.nasa.gov/14227

https://www.nasa.gov/feature/goddard/2022/nasa-s-swift-fermi-missions-detect-exceptional-cosmic-blast

http://www.ihep.cas.cn/xwdt2022/cmsm/2023/202303/t20230330_6720319.html

http://www.hnskxy.com/info/1006/4364.htm

http://www.ihep.cas.cn/lhaaso/zyxw/202306/t20230609_6775760.html

http://www.ihep.cas.cn/lhaaso/zyxw/202210/t20221018_6533180.html

Author: Li Peng

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