Rare! New discovery by Chinese observatory! Received photons from 2 billion years ago!

On the evening of October 9, 2022, Beijing time, the Fermi satellite of the United States detected a gamma-ray burst (hereinafter referred to as GRB) event from outer space, named GRB 221009A. The huge radiation flux of this GRB caused the saturation of the detection capabilities of most international space satellite experimental detectors, resulting in temporary instrument failure or data accumulation.

Subsequently, after comparative analysis of multiple detectors around the world, astronomers unanimously agreed that this was the brightest gamma-ray burst recorded in human history , about 50 times brighter than the second, and thus named it BOAT (Brightest of All Time).

During this outburst, China's High Altitude Cosmic Ray Observatory (LHAASO, abbreviated as "LASO" in Chinese) performed outstandingly and obtained extremely high-quality trillion-electron-volt observation data. It not only depicted the luminosity rise phase of the trillion-electron-volt afterglow of the gamma-ray burst for the first time, but also discovered the light variation and breaking phenomenon of the very early afterglow .

High Altitude Cosmic Ray Observatory. Image source: Institute of High Energy Physics, Chinese Academy of Sciences

01

Gamma-ray bursts, the swan song of star death

A gamma-ray burst is a sudden burst of gamma rays in the sky.

The origin of gamma-ray bursts has puzzled astronomers since they were first discovered in 1967. Gamma rays on the ground generally come from radioactive elements, so gamma-ray bursts on Earth are often associated with violent nuclear explosions. However, the nuclear reactions of stars in the sky are relatively stable and there will be no sudden explosions.

What celestial bodies do the gamma-ray bursts in the sky come from? Are they in our galaxy or in the distant universe? Why do these gamma rays appear suddenly and disappear quickly?

The relevant questions did not have clear answers until the 1990s. In 1997, thanks to the good positioning of X-ray afterglows provided by the BeppoSAX satellite, gamma-ray bursts were confirmed to come from distant galaxies, and the relativistic fireball model of gamma-ray bursts was also confirmed in many aspects.

The secret of gamma-ray bursts has finally been revealed: gamma-ray bursts originate from a "fireball" moving at a speed close to the speed of light . The "fireball" contains a large number of gamma photons and positive and negative electron pairs. Due to the strong magnetic field environment in the "fireball", high-energy electrons can produce gamma rays through mechanisms such as synchrotron radiation.

This time, the process of "Laso" receiving gamma rays was as follows:

About 2 billion years ago, a massive star ran out of nuclear fusion fuel and collapsed due to gravity, ejecting an extremely narrow cone-shaped "fireball". A large number of high-energy gamma photons were generated in the "fireball", which happened to fly in the direction of the earth, crossing a distance of 2.4 billion light years to reach the earth . When these high-energy photons reached the earth, they had already spread to the range of thousands of galaxies, and only a very small number of gamma photons could hit the earth in the end.

When a gamma photon enters the atmosphere, it collides with the nuclei of atoms in the Earth's atmosphere and breaks into a pair of positrons and electrons. The positrons and electrons collide with the nuclei of atoms in the atmosphere, producing photons of slightly lower energy. These photons continue to break up, and so on. The original gamma photon quickly turns into a cluster, which grows stronger and stronger, and the energy of the individual particles becomes lower and lower until the energy is dispersed and exhausted and no longer has any effect.

At 21:21 on October 9, 2022, Beijing time, gamma photons arrived at the Earth one after another, entered the atmosphere, and air showers bombarded the surface. Unfortunately, the human eye cannot distinguish these spectacular showers. "Lasso" witnessed everything, and humans used "Lasso" to travel through 2 billion years to decipher the secrets of the star 2.4 billion light-years away.

A high-energy gamma event hits the "Laser", an artistic rendering. Image credit: Institute of High Energy Physics, Chinese Academy of Sciences, produced by the Center for Art and Science, University of Science and Technology of China

02

Afterglow, an excellent probe for deciphering gamma-ray bursts

How did Lasso decipher the secrets of that star? It all started with the afterglow of the gamma-ray burst.

When the "fireball" ejected by a gamma-ray burst collides with the interstellar medium (such as dust and gas), it will accelerate particles including electrons and atomic nuclei. Among them, high-energy electrons will radiate electromagnetic waves of various bands including radio, infrared, visible light, ultraviolet, X-rays, and gamma rays, which will last for several hours, days or even months in the sky like a rainbow. This phenomenon is called "afterglow."

The afterglow produced by the interaction of the cone-shaped "fireball" with the interstellar medium. Image source: Institute of High Energy Physics, Chinese Academy of Sciences, produced by the Center for Art and Science Research, University of Science and Technology of China

At the beginning of the afterglow process, gamma photons will continue to be produced at an increasingly faster rate, and the number of photons produced will reach a maximum in a very short period of time. During this process, the energy of the "fireball" is gradually lost, and the increase in the number of radiated photons is gradually slowing down.

In the "eye" of the "Lazo", the sky is suddenly lit up, and then gradually dims. The "Lazo" can "feel" the brightness changes of gamma-ray bursts . The process of gamma photons hitting the earth is like a summer rainstorm, which comes quickly and goes away slowly. Scientists use "light curves" to describe the process of afterglow brightness changing over time, which includes two main stages: brightness rise and brightness fall.

The discovery and observation of afterglows have brought breakthrough progress to the study of gamma-ray bursts. Compared with the radiation produced by the interaction within the gamma-ray burst fireball, which only lasts for a short time (called transient radiation), afterglows last longer and have a richer wavelength band, which can provide humans with more comprehensive information about gamma-ray burst "fireballs".

Over the past few decades, astronomers have observed nearly 10,000 gamma-ray burst afterglows in multiple bands, including radio, optical, X-ray, and gamma-ray, and have basically perfected the image of the afterglow. However, astronomers have not observed much about the initial process of the afterglow, that is, the stage when the brightness gradually increases.

This is because the duration of the brightness increase phase is very short, only about a few tens of seconds, and requires rapid aiming and tracking observations by instruments. In addition, since the radiation flux decreases rapidly with the increase of energy, astronomers have never observed an increase in the brightness of the afterglow in the high-energy range.

Does this unobserved band contain new secrets?

03

"Lasso"

First complete observation of high-energy afterglow radiation

Since 2018, two atmospheric Cherenkov telescopes have observed afterglows of three gamma-ray bursts in the tera-electron-volt (TeV) energy range. However, since it takes a certain amount of time for the telescopes to turn to the direction of the gamma-ray burst, they only saw the descent phase of the afterglow and did not depict the complete time evolution of the high-energy afterglow.

But Lasso is different. Lasso has three advantages, giving us more opportunities to achieve better observations of the afterglow of gamma-ray bursts.

First, it has an ultra-high sensitivity in the energy range from 100 billion to 10 trillion electron volts. The total coverage area of ​​the water Cherenkov detector array of "Laso" is as high as 78,000 square meters, which is 4 times that of similar detectors in the world, and can observe very weak gamma-ray burst signals.

The second is the wide field of view. The Lasso can simultaneously observe 1/6 of the space range, which is hundreds of times larger than a telescope, and can capture more gamma-ray bursts without rotating.

The third is the all-weather working status.

Whether it is day or night, sunny or rainy, "Laso" can continue to observe, and does not have to work only on clear nights like a telescope.

Schematic diagram of the "Laso" water Cherenkov detector array. Image source: Institute of High Energy Physics, Chinese Academy of Sciences

The observation of gamma-ray bursts is one of the important scientific goals of Lasso. The high-energy radiation signal of gamma-ray burst 221009A arrived at the field of view of Lasso at 21:20:50 on the evening of October 9, 2022. In less than an hour, more than 60,000 gamma photons were collected by Lasso at an excellent observation angle, with a significance of up to 250 times the standard deviation.

After several months of analysis, scientists finally unveiled the mystery of the explosion, and the research paper was published online in Science on June 9, 2023. Relying on the "Lasso", humans finally "saw" the entire evolution of the high-energy afterglow radiation of the gamma-ray burst from rising to falling for the first time, completing the missing piece of the puzzle on the afterglow light curve.

The light curve of the complete tera-electron-volt energy range observed by Lasso. Image source: Institute of High Energy Physics, Chinese Academy of Sciences

So, what is the significance of observing the stage when the afterglow brightness rises?

"Lasso" monitored the situation within a few seconds after the afterglow occurred, and calculated the starting time of the gamma-ray burst afterglow radiation based on the light curve.

Some of the theoretical model's speculations about the very early afterglow can also be verified. In previous theoretical analysis, the first ray of afterglow should appear at the peak stage of the gamma-ray burst's transient radiation burst, which is basically consistent with the peak moment of the gamma-ray burst observed by other detectors.

Lasso detected the rising phase of the afterglow light curve and gave the time interval from the beginning to the peak. Theorists can infer the speed of the GRB221009A "fireball" based on the model. In comparison, the speed of this gamma-ray burst "fireball" is faster, with a Lorentz factor of 440, which is only three millionths different from the speed of light in a vacuum!

04

Light changes and breaks, revealing the secrets of BOAT

In addition to the above discoveries, "LASO" directly observed a sudden change in brightness starting 700 seconds after the afterglow appeared. There was a broken structure in the light curve. This phenomenon was believed to be the edge of the "fireball" jet.

Where does the "fireball" of a gamma-ray burst come from? Current theories believe that it comes from a relativistic jet generated by a central celestial body. The central celestial body can be a rapidly rotating neutron star or a black hole, which uses the extremely violent electromagnetic field formed by rapid rotation to throw out a part of the gravitationally collapsed recoil matter along the rotation axis, forming a jet like a firework. If the ejection speed of these substances is close to the speed of light, there will be a relativistic beam effect, which gathers light together like a spotlight.

If the size of the "spotlight beam" is smaller than the jet itself, and the observer is in the beam, then we will not be able to distinguish the shape of the jet, because the light we see at this time is all focused.

However, the jet cannot always maintain the same speed. When the speed decreases, its light-gathering ability will weaken, and we can see the edge of the jet. Its average visual brightness will also be lower, which is manifested as a sudden increase in the speed of the decline in brightness, with an obvious inflection, or break, which is the evidence of the shape and size of the jet.

Many experiments have observed the light break of gamma-ray bursts before, but these phenomena usually appear several hours after the afterglow appears. The results of the Lasso observations are the first time that people have seen the light break of the afterglow within hundreds of seconds, which is the earliest in history. This is of great help to our understanding of jets and their generation mechanism.

This is because only by seeing the light break can we determine the size of the jet itself and then infer the conditions required to produce such a "fireball".

From the observation data of Lasso, scientists inferred that the half-angle of this jet is only 0.8 degrees. This is the smallest jet angle known so far, which means that the observed photons actually come from the brightest core of a typical jet. 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 why such events are rare.

Schematic diagram of the cause of light distortion and breakage. The afterglow has a strong collimation property. The observer initially only sees the middle area of ​​the conical "fireball". As the "fireball" slows down, the collimation property decreases, the field of view gradually expands, and finally the entire "fireball" can be seen. Image source: Reference [10]

05

Rapid rise, still unsolved puzzles

Thanks to its superior observation capabilities and excellent observation angles, Lasso made a perfect observation of GRB 221009A, the brightest gamma-ray burst ever recorded and only occurs once in a thousand years. It achieved the first complete measurement of the light curve of the high-energy afterglow radiation of a gamma-ray burst, directly observed the breaking phenomenon of the afterglow radiation, explained the cause of the brightest gamma-ray burst, and verified the classical theory of gamma-ray bursts. This observation result will leave a strong mark in the history of human gamma-ray burst observations.

But this is not the end.

The light curve includes a rapid rise phase, in which the gamma-ray burst radiation flux increased by more than 100 times in less than two seconds, and the subsequent slow growth behavior is consistent with the expected characteristics of the subsequent explosion. This is the first time in the world that the rapid increase in the photon flux in the afterglow radiation of a gamma-ray burst has been detected. Such a rapid increase exceeds the expectations of previous theoretical models.

What kind of mechanism exists here?

I believe 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, particle acceleration, etc. Scientists will continue to delve deeper into this field and reveal more mysteries of gamma-ray bursts for us.

References:

[1] Lesage S, Veres P, Briggs MS, et al. Fermi-GBM discovery of GRB 221009A: An extraordinarily bright GRB from onset to afterglow[J]. arXiv preprint arXiv:2303.14172, 2023.

[2] An ZH, Antier S, Bi XZ, et al. Insight-HXMT and GECAM-C observations of the brightest-of-all-time GRB 221009A[J]. arXiv preprint arXiv:2303.01203, 2023.

[3] Burns E, Svinkin D, Fenimore E, et al. GRB 221009A: The Boat[J]. The Astrophysical Journal Letters, 2023, 946(1): L31.

[4] LHAASO Collaboration. A tera–electron volt afterglow from a narrow jet in an extremely bright gamma-ray burst [J]. Science, 2023. DOI: 10.1126/science.adg9328

[5] Zhang B. The physics of gamma-ray bursts[M]. Cambridge University Press, 2018.

[6] MAGIC Collaboration. Teraelectronvolt emission from the γ-ray burst GRB 190114C[J]. Nature, 2019, 575(7783): 455-458.

[7] HESS Collaboration. Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow[J]. Science, 2021, 372(6546): 1081-1085.

[8] Bose D, Chitnis VR, Majumdar P, et al. Ground-based gamma-ray astronomy: history and development of techniques [J/OL]. Eur. Phys. J. ST, 2022, 231(1): 3-26. DOI: 10.1140/epjs/s11734-021-00396-3.

[9] arXiv:2305.17030v1 [astro-ph.HE]

[10] Woosley, S. Blinded by the light. Nature 414, 853–854 (2001).

Produced by: Science Popularization China

Author: Huang Yong, Institute of High Energy Physics, Chinese Academy of Sciences; Zheng Jianhe, Nanjing University

Producer: China Science Expo

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