"Laso" has made new discoveries! The world's first accurate observation of the entire afterglow of a once-in-a-millennium gamma-ray burst

Produced by: Science Popularization China

Author: Crossing the Stars Team

Producer: China Science Expo

On June 9, 2023, the latest observational research results of the High Altitude Cosmic Ray Observatory (LHAASO) on the gamma-ray burst (GRB) named GRB 221009A were published online in the journal Science, titled "A tera-electronvolt afterglow from a narrow jet in an extremely bright gamma-ray burst 221009A". The paper was completed by the LHAASO international collaboration.

High Altitude Cosmic Ray Observatory (LHAASO)

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

About 2 billion years ago, a "super sun" massive star that was more than 20 times heavier than the sun burned up its nuclear fusion fuel and collapsed instantly, triggering a huge explosion fireball, emitting a huge "cosmic fireworks" gamma-ray burst that lasted for hundreds of seconds. The large number of trillion electron volt high-energy gamma photons produced by the collision of the fireball and interstellar matter flew through the vast universe and flew straight to the earth, arriving at the field of view of "Laso" at 21:20:50 on October 9, 2022. More than 60,000 gamma photons were collected by "Laso". After several months of analysis, scientists finally unveiled the veil of this explosion.

What is a gamma-ray burst?

Gamma-ray bursts (also abbreviated as GRBs) are the most violent celestial explosions after the Big Bang. They refer to the sudden increase in the flash of gamma rays from a certain direction in the sky. Gamma-ray bursts can be as short as one thousandth of a second or as long as several hours. Short-duration GRBs are produced by the merger of two nearby compact stars (black holes or neutron stars), while long-duration GRBs are produced by the collapse and explosion of giant stars (super stars) when their fuel is exhausted.

At 21:16:59.59 Beijing time on October 9, 2022, the Fermi spacecraft first detected an unusually bright gamma-ray burst, named GRB221009A according to international convention. Dozens of space and ground detectors subsequently observed this burst. This gamma-ray burst is a long burst, and its brightness is dozens of times higher than the brightest gamma-ray burst in the past. The excessive photon flux has saturated the detectors of multiple international experiments. my country's High Altitude Cosmic Ray Observatory (LHAASO), High Energy Burst Explorer (HEBS) satellite and Insight-HXMT satellite simultaneously detected this gamma-ray burst, realizing coordinated observation of the sky and the earth over a wide energy range of 11 orders of magnitude.

Artist's impression of the Lasso detecting a gamma-ray burst (GRB 221009A).

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

First precise measurement of the entire process of a high-energy photon burst

The signal details collected by Lasso this time indicate that the detected photons come from the aftermath of the main explosion. The "main explosion" of a gamma-ray burst event, also known as transient radiation, is a huge explosion in the initial stage, manifested as strong low-energy gamma-ray radiation. The collision of the explosive at close to the speed of light with the surrounding gas produces a "aftermath", also known as afterglow. Lasso accurately observed the complete process of the "aftermath" for the first time, recording the entire stage of the enhancement and attenuation of the trillion electron volt gamma-ray flux.

With the observation of tens of thousands of gamma-ray bursts, scientists have established a seemingly perfect theoretical model and even firmly believe in it. "Laso" has achieved a textbook-like complete observation of the light variation process in the high-energy band that other experiments have not achieved, providing an experimental basis for the precise verification of the theoretical model. Given the rarity of this burst, which occurs only once in a thousand years, this observation result is expected to remain the best for decades or even hundreds of years to come.

The secret of the brightest gamma-ray burst in history has been discovered

Observations from the Lasso show that the brightness of the high-energy radiation suddenly and rapidly decreased at a certain moment less than 10 minutes after the detonation. This is mainly because the ejecta after the explosion are jet-like structures, and when the radiation angle expands to the edge of the jet, the brightness drops rapidly. Since this brightness transition occurs very early, the angle of the jet measured is also extremely small, only 0.8 degrees. This is the jet with the smallest angle known to date, which means that what is observed is actually 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 why such events are extremely rare.

Lasso observed the gamma-ray burst GRB 221009A with a high significance level of more than 250 standard deviations.

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

LHAASO: 4410 meters high, exploring the ultra-high energy limits of the universe

LHAASO 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 and covers an area of ​​about 1.36 square kilometers. LHAASO consists of three arrays: the 1 square kilometer ground shower particle 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 (WFCTA) composed of 18 telescopes. It can measure gamma rays and cosmic rays from high-energy celestial bodies in a wide band and multiple ways, and carry out research in astrophysics and other aspects.

The results of this observation were 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, such as low-energy gamma photons, positive and negative electrons, through 6,240 photosensitive probes of different sizes placed on the bottom of the water. They will generate Cherenkov light signals in the water. The array's observation energy range for gamma rays spans two orders of magnitude, from 100 billion electron volts to 10 trillion electron volts, and has the characteristics of a wide field of view and all-weather, which has outstanding advantages in capturing sudden astronomical phenomena such as gamma-ray bursts.

High Altitude Cosmic Ray Observatory (LHAASO)

(Image source: Institute of High Energy Physics, Chinese Academy of Sciences)

The high-statistical observations of Lasso at the highest energy level will reveal more mysteries. Scientists are still working tirelessly to dig deeper into the data of Lasso, trying to reveal more mysteries. We also look forward to more observational research results that will allow us to understand the universe more deeply.