Shulou(Shulou.com)11/24 Report--

At the end of last month (March 2023), more than 40 scientific institutions around the world, including the Institute of High Energy of the Chinese Academy of Sciences, jointly released a study on GRB 221009A, the brightest gamma-ray burst to date. Together with 11 papers published in the Astrophysical Journal KuaiBao, they analyzed and studied the incident in detail from many aspects based on probe data from around the world.

Today we're going to explore a cosmic high-energy event second only to the Big Bang-gamma ray bursts (GRB).

The discovery of gamma-ray bursts dates back to the sixties of last century. At that time, during the Cold War between the United States and the Soviet Union, in order to prevent a further nuclear arms race and prevent nuclear weapons tests from having an excessive impact on the earth's atmosphere, the United States, Britain, and the Soviet Union jointly signed a limitation treaty, the partial Nuclear Test Ban Treaty.

In order to monitor whether the Soviet Union violated the treaty, the United States then launched a series of satellites used to monitor gamma rays, the Vela.

At 14:19 on July 2, 1967, Villa's satellites 3 and 4 suddenly received an abnormal gamma-ray signal different from any known nuclear weapon. Then, as more and more similar inexplicable gamma-ray signals were discovered, the researchers published the findings entitled "observations of gamma-ray bursts originating in the universe" in the June 1973 issue of the Astrophysical Journal.

By monitoring satellite researchers captured a total of 16 brief photon bursts over a three-year period from 1969 to 1972 and confirmed that these bursts did not come from the earth and the sun, but from distant space. From then on, astronomers became interested in the mysterious cosmic event of gamma-ray bursts.

In the early days, many people thought that gamma-ray bursts should be a phenomenon that occurs in the Milky way and may be related to pulsars. So everyone rushed to create hundreds of theoretical models of gamma bursts based on neutron stars.

However, with the help of more and more high-precision detectors, people have gradually discovered that almost all gamma bursts are not emitted in the Milky way, but from more distant galaxies billions of light-years away. But the light from such a distance is still so strong that the energy is frighteningly high.

By calculation, the energy released by a typical gamma burst in a few seconds is equal to the sum of the energy released by the sun over a billion years! And this gamma burst released the equivalent of eight suns in a minute!

People are very curious about such a rapid outbreak, what happened at its source? However, the duration of gamma bursts is very short, some even only a few milliseconds, and the time and place of occurrence are very random, so it is very difficult to study them directly. In general, it is found that there is a gamma burst, and then quickly point the telescope there to see what corresponding celestial bodies are in the corresponding position.

At first it was thought that it might correspond to pulsars, supernovae, and even bright celestial bodies such as globular clusters and quasars. But maybe the timing was wrong, or the technology was limited, and in the decades that followed, astronomers didn't find anything to be expected.

The turnaround occurred on April 25, 1998, when a gamma-ray burst monitor on an X-ray satellite captured a three-second gamma burst signal (GRB 980425). Then a bright supernova (SN 1998bw) appeared at that location, and the event became the first strong evidence of a link between gamma bursts and the death of massive stars.

As more and more advanced monitors for gamma-ray bursts are put into use, it is now believed that most of the observed gamma-ray bursts should originate from supernova explosions of massive stars at the end of their evolution. On the other hand, a small number of "short bursts" with a duration of less than 2 seconds may be related to thousands of novae (that is, the merging of neutron stars mentioned last time), and the relevant evidence comes from gravitational wave signals captured at almost the same time as gamma bursts.

On the night of October 9, 2022, the brightest gamma burst in history, the Swift space telescope, which specializes in the study of gamma ray bursts, captured an unusually bright signal.

The system calculated the location of the outbreak within 5 seconds and distributed the relevant data to the ground team. The researchers then found the signal in data from the Fermi Gamma-Ray Telescope. Somewhat surprisingly, the Fermi telescope detected abnormal signals 55 minutes before Swift's discovery, but for some reason the data was not distributed. In any case, detectors around the world eventually picked up the signal one after another, and the event was soon officially classified as a gamma ray burst (GRB 221009A).

This gamma burst not only lasts for a long time, but also has a surprisingly high brightness, and has been considered to be the brightest gamma burst at present and even in the foreseeable future. Because in terms of probability, such an event happens only once every few centuries. In other words, we may no longer be able to observe such extremely bright events in our lifetime, so this is an extremely valuable research opportunity for human civilization as a whole.

Another advantage of the bright gamma burst is that its afterglow will be very long.

Although the brightest phase of a gamma burst is concentrated in the first short period of time, the interaction between the photon beam and the environmental medium produces shock waves, in which electrons are accelerated and synchrotron radiation is produced. This radiation runs through the entire electromagnetic wave band from gamma rays to radio, and those longer wavelengths can last for days or even years. So even in the next few years, it is still possible to make follow-up observations of the event through radio telescopes.

The cause of GRBs has just been mentioned, and it is generally believed that such long bursts should be related to the death of massive stars.

After a massive star collapses into a neutron star or black hole at the end of evolution, the accretion of surrounding matter or companion stars will cause it to form an accretion disk under the strong gravitational force. When the magnetic induction line of the accretion disk is twisted to a certain extent, a magnetic reconnection like the surface of a star will occur. The difference is that for stars, this may trigger surface matter ejections (SME), but for accretion disks, high-energy particles can be ejected from the poles at close to the speed of light, known as relativistic jets. This brief high-energy jet may be one of the sources of gamma-ray bursts.

But not all long GRBs are caused by the death of massive stars. For example, in this event (GRB 221009A), we did not observe any obvious supernova features in the spectrum. At present, it is thought that this may be the supernova's energy and jet overlap, or it may be that the supernova is not that bright at all.

But how does a dim supernova burst into such a bright gamma burst?

Through modeling and analysis of the gamma burst, the researchers seem to have found the reason: the high brightness this time may not be because of the energy of the burst, but because the direction of the jet is facing us. That's right, we're facing the jet!

Although facing the jet, the good news is that we are far enough away. By measuring the spectral redshift, the location of the explosion is more than 2 billion light-years away. In other words, this beam of light came to us as early as 2 billion years ago, and it is not until today that a few remaining photons have arrived on Earth and have been captured by us. If all this had happened in the Milky way, this might be another story.

Wait a minute

Since gamma ray bursts are electromagnetic waves that travel at the speed of light, that is to say. Before it reaches Earth, we have no way to predict it.

There are hundreds of billions of stars in the Milky way, and even massive stars are estimated to be tens of billions. Although the probability is very low, we cannot rule out the possibility that there is already a gamma ray burst coming straight to us, which we do not know for the time being. In the future, when it arrives on Earth, it may also be the end of human civilization. Is this irregular biological "cleaning" in the universe the legendary "big filter" to explain the Fermi paradox?

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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