2020-10-26

Crab Nebula-the lighthouse in the high-energy universe

By yqqlm yqqlm

About 7,500 years ago, due to the exhaustion of available energy, a massive star more than 6,000 light-years away from the earth died of life. The supernova exploded. The visible light produced by this outbreak reached the earth after nearly 6,500 years of propagation, and was recorded by people at the time. The eruption remains after another 1,000 years of evolution, forming the”Crab Nebula” we see today. The protagonist, the standard candlelight of high-energy astronomy, is also one of the celestial bodies widely studied by astronomers (Figure 1).

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Figure 1:Multi-band composite image of the Crab Nebula

Historical review

In the Northern Song Dynasty of our country, in the May of Renzong to He first year, Ji Chou (ie July 4, 1054 AD) Around the dawn of , observers from the Kaifeng Division Tianguan found that in the Eastern Tianguan (ie Taurusζ) A bright big star suddenly appeared nearby, its brightness is several times brighter than Venus, second only to the sun and the moon(Figure 2). After about 23 days, its brightness began to weaken and disappeared for about two years. Because of its presence near Tianguan, astronomers in the Song Dynasty called it the”Tianguan Guest Star”, and Yang Weide, an official of the Tianguan Supervisor, reported the incident to the court. At the same time, some historians also recorded it in the history books. At present, it can be found in”Song History·Astronomy Chronicles-Ninth”,”Song History·Renzong Benji”,”Song Huiyao”,”Xu Zi Zhi Tong Jian” Record. For example, it is recorded in”Song Hui Yao”:”In March of the first year of Jiayou, Si Tian Jian said:”The guest star is not there, the sign of the guest going away.” From the beginning to May of the first year, I left the east in the morning and guarded the sky. , In the daytime you see it as white, the horns are everywhere, the color is red and white, and you see it on the 23rd”(Picture 3).

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Figure 2:Night sky stars when the Tianguan guest star appears (simulated by Stellarium software)

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Figure 3:Records in Song History

the same During the period, astronomers in other countries and regions also observed this phenomenon. For example, the ancient books of Japan and Arabia clearly recorded this event. The aborigines of the southwestern United States may have recorded this strange astronomical phenomenon. On the second day of observation by the observers of the Chinese Tianjian, the newly discovered bright celestial body (also known as Crab Nebula) A crescent moon can be seen nearby. Historians believe that certain hieroglyphs found in Chaco Canyon, New Mexico, USA may describe observations of this event(Figure 4 ).

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Figure 4:Pictographs found in Chaco Canyon

Thanks to the invention of the astronomical telescope, after hundreds of years of disappearance, Tianguanke star changed to a new one The posture reappeared in people’s vision. In 1731, British doctor and astronomy enthusiast Bevis (Bevis) first took the position of”Tianguan Guest Star” A cloud of gas was found. In the following one or two hundred years, many astronomers and enthusiasts made a lot of observations on this gas cloud. For example, French astronomer and comet enthusiast Charles Messier (Charles Messier) also compiled it into his” In the table of nebulae and star clusters, it is numbered M1. In 1844, Rosse observed the gas with a telescope with an aperture of 0.9 meters and drew the following figure (Figure 5). Because this cloud of gas looks like a crab(Crab), Rosse calls it the Crab Nebula, which is also this The origin of the name Nebula.

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Figure 5:The Crab Nebula drawn by Rosse in 1844

At that time, no one had connected this gas cloud with the Tianguan guest star. Until 1928, the American astronomer Hubble (Hubble) proposed that the Crab Nebula was formed after a supernova explosion, and based on Predecessors’ observations have calculated that the Crab Nebula’s age is around 900 years2. This is very close to the time when the Tianguan Kexing appeared in 1054 AD, so that the Crab Nebula and Tianguan Kexing were connected by people. In 1942, the Dutch astronomer Oort (Oort) and his students published two papers, demonstrating the crab shape Nebula is formed after the supernova explosion in 1054. 3, 4.

Today, it is generally accepted that the Crab Nebula is the remnant of the Tianguan Guest Star, and it has become one of the remnants of 9 recorded supernovae in history. Because of this, coupled with its own strong radiation in various bands of electromagnetic waves, astronomers have conducted a large number of observations and studies on the Crab Nebula in each band(See Figure 1).

Formation of the Crab Nebula

Through modern astronomical observations and related theoretical studies, it has been determined that the Crab Nebula originated from the nuclear collapse type(ie type II) The supernova explodes, and its precursor star is a massive star with 8-12 times the mass of the sun. Usually when such massive stars explode, they will violently eject a large amount of matter into the surrounding interstellar space, with an initial speed of up to tens of thousands of kilometers per second. Consider that the speed of the first universe is only 7.9 kilometers per second, and the maximum speed that humans currently achieve for macroscopic objects, that is, the speed of the US Helios probe is only 70 kilometers per second. These projectiles carry huge kinetic energy, with a typical value of 1051 ergs. These materials interact with the surrounding interstellar medium to form a shell-like shock wave structure, and emit radiation at different wavelengths of electromagnetic waves, which is the so-called supernova remnant.

On the other hand, an unusually dense neutron star will remain in the center of the explosion (a larger mass star Collapsed into a black hole when it broke out). Some neutron stars rotate rapidly and produce directional radiation from the poles. Observationally, they appear as lighthouse-like pulsars. In the center of the Crab Nebula, there is a pulsar with a rotation period of 33 milliseconds (number PSR B0531+21), namely It can turn about 30 times a second. Pulsars usually have strong magnetic fields, and their rotational energy is gradually released by driving extremely relativistic stellar winds. These stellar winds, media, and projectiles can also generate electromagnetic radiation and form pulsar wind clouds. This kind of supernova remnant shell contains pulsar wind cloud system called hybrid supernova remnant(Figure 6).

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Figure 6:Left, examples of hybrid supernova; right, schematic diagram of hybrid supernova formation 5

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Actual observations show that the Crab Nebula has no obvious shell structure corresponding to shock waves. Therefore, it was historically classified as a solid supernova relic. The remnant of a supernova with a bright center and no shell structure is the pulsar wind cloud. The main source of radiant energy is the central pulsar, not the kinetic energy injected into the projectile when the supernova explodes. Of course, the Crab Nebula also has the characteristics of supernova remnants, and its optical waveband radiation mainly comes from ionized projectiles. But in general, treating the Crab Nebula as a pulsar wind cloud can better reflect its multi-band radiation characteristics.

The benchmark for gamma-ray astronomy

< p>The Crab Nebula is one of the first celestial bodies discovered to produce gamma rays. After long-term observations, it is found that the gamma-ray flux of the Crab Nebula is very stable, so it is used as a standard source to calibrate the detection efficiency of the gamma-ray telescope, and its flux is also used as the basic radiation flux of high-energy celestial bodies. unit. Observations from Fermi gamma-ray satellites indicate that there are glare phenomena in the GeV(109eV) energy range. span style=”color:888888; –tt-darkmode-color:888888;”>(Of course, when there is no flare, the flow is relatively stable) 6. During the flare, the current intensity changes are not periodic, and the current flow can increase dozens of times, which shows that the inside of the Crab Nebula is not static. The radiant energy spectrum during the flare shows that electrons can be quickly accelerated to the energy of PeV(1015eV)! The acceleration mechanism is still an unsolved mystery.

After entering the new century, thanks to the construction of a new generation of gamma-ray telescopes, a large number of new gamma-ray sources have been discovered, and gamma-ray astronomy has also developed rapidly. So far, in our Milky Way, the largest number of GeV gamma-ray sources certified as pulsars; and in TeV(1TeV=1000 GeV ) Energy segment, the most certified is Pulsar Fengyun. Both types of gamma-ray sources are related to the protagonist of this article. Not only that, the Crab Nebula also holds two records!

First of all, the pulsar at the center of the Crab Nebula has the highest energy pulse radiation. Like the earth, the pulsar’s magnetic axis and rotation axis are also deviated. Only when the magnetic axis is facing the earth, can you see the pulsar’s radiation(Figure 7). Therefore, the signal we monitor is like an electrocardiogram pulsation, with spikes one by one, and the spike interval is the rotation period of the pulsar. The energy of photons radiated by general pulsars is cut off at a few GeV, but pulsars in the Crab Nebula can radiate photons of TeV energy, which refreshes people’s understanding.

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Figure 7:Pulsar Art Image|

Secondly, in 2019, astronomers through the Sino-Japanese cooperation air shower(ASγ) The cosmic ray experiment detected photons above 100 TeV in the Crab Nebula for the first time, and the highest energy photon can reach 470 TeV. The ASγ array evenly arranges 597 plastic detectors in an area of ​​65,700 square meters to detect atmospheric showers produced by high-energy particles(Air Shower) Electromagnetic particles inside. The falseton detector array is underground and consists of 64 Cherenkov-type water detectors (Figure 8).

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Figure 8:Tibet Yangbajing ASγ experimental surface array

Because Muzi has the ability to penetrate the soil and enter underground detectors, people use this feature as a way to compare Muzi with other An effective method of particle separation. It is precisely by using this method of identification that ASγ can effectively distinguish the atmospheric shower produced by photons from the shower produced by charged nuclei(Figure 9:For a given energy, the number of muons produced by cosmic rays is much more than that in the photon Air Shower), realizing human detection of the highest-energy photons.

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Figure 9:Atmospheric showers produced by photons (left) and cosmic rays (right)

But this record should be broken soon. The new generation telescope array located in Daocheng, Sichuan—High Altitude Cosmic Ray Observation Station(Picture 10, 11:LHAASO)—is Construction is proceeding in an orderly manner. Half of the array has been constructed so far, and data has already been collected at 1/4 of the array. LHAASO has 5195 electromagnetic particle detectors and 1171 Muzi detectors. Compared with the ASγ electromagnetic signal collection area has been expanded 20 times, the detection area of ​​Muzi has also expanded 400 times. LHAASO therefore has better detection sensitivity, and the upper limit of the photon energy that can be detected is also greatly improved. I believe that LHAASO will bring us more surprises and deepen our understanding of high-energy astrophysics.

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Figure 10:Detection principle diagram of high altitude cosmic ray observatory

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Figure 11:Real shots of high altitude cosmic ray observatory|

About the author

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p >Zhang Xiao, Special Research Assistant, School of Astronomy and Space, Nanjing University, research direction:supernova relics.

Xie Yuxin, Phillips Academy Andover high school student in the United States, intern at the Purple Mountain Observatory, instructor:Researcher Liu Siming.