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A few days ago, NASA released a message saying that it would jointly with the world’s major observatories in Beijing time on the evening of October 16, 2020. As soon as the news of
broke out, netizens began to speculate on what to announce?
Three body appearance? Earth pill?
However, more professional astronomy netizens speculate that it may be necessary to release the latest information related to gravitational waves.
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What is NASA going to announce?
In fact, the content to be released has long been published in NASA’s microblog, which is the latest discovery about gravitational waves.
The following is the announcement at 22:00 Beijing time on October 16, 2020
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Telescopes around the world witness the merger of Gemini
David middot, executive director of LIGO; David Reitze announced that the laser interference gravitational wave observatory (LIGO) and the Virgo gravitational wave observatory (Virgo) discovered an unprecedented new type of gravitational wave event for the first time on August 17, 2020. It is generated by the merging of two neutron stars with mass of 1.15 and 1.6 solar masses. According to the detection date, the number gw170817 is 130 million light-years away from us.
In addition, the electromagnetic counterpart of the gravitational wave event,
, has been discovered through the concerted efforts of many astronomers and detection equipment all over the world
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The main findings are as follows: kilonovae, short gamma ray bursts, and merging of two neutron stars
These observations provide a comprehensive description of the catastrophic event from about 100 seconds before merging to a few weeks after merging, which finally confirms many scientists’ conjectures: the merging of two neutron stars in NGC 4993 Galaxy produces gravitational waves, short gamma storms and kilonovae
The process of two neutron stars approaching and finally merging to produce a kilonova
The optical images corresponding to the gravitational wave source were observed by several different telescopes at onan station (left). The images observed by the Hubble telescope at different times were
As mentioned in the press conference, the detected gravitational waves are generated by the merger of two neutron stars. The four previously published gravitational wave events are all generated by double black holes. The biggest difference between
is that the merger of two neutron stars will produce electromagnetic radiation, while for black holes, we usually don’t think it will. This has been verified by observation
Generally speaking, according to the theory of astrophysical radiation, in order to produce electromagnetic radiation, there must be gas around the celestial body For the black hole system, although there may be a lot of gas around the black hole at the initial generation, in the long evolution process, if there is no more gas sources, in the final stage of black hole merger, the gas has been consumed, so it can not generate electromagnetic radiation, only produce gravitational waves that disturb space-time, just as scientists have detected in the previous four times
However, during the merging process, some matter will be ejected at a speed close to or far below the speed of light, which will produce various electromagnetic phenomena we see; Short time scale gamma ray bursts (GRBs), gamma ray bursts (GRBs), gamma ray bursts (GRBs) and kilonovae (KA) were observed by short-time scale gamma ray bursts (GRBs), gamma ray bursts (GRBs) and kilonovae (QNS).
materials moving close to the speed of light produce GRBs as seen by Fermi satellites, while low-speed materials produce kilonovae, which are captured by many optical / infrared telescopes
Wait a minute. What are short-term gamma ray bursts, gamma ray bursts afterglow, and kilonovae? Let’s talk about them one by one.
In short, gamma ray bursts (GRBs) are the phenomena of sudden increase of gamma ray radiation in a certain direction in the sky, which can be said to be the most violent celestial explosion after the big bang in the universe In the early 1990s, Compton gamma ray observatory made a simple statistics after observing thousands of GRBs. According to their duration, they can be divided into two categories: one is the long time scale GRBs whose burst time is longer than 2 seconds, and the other is the short time scale GRBs whose burst time scale is shorter than 2 seconds After further study, it was found that the origin of these two kinds of GRBs was completely different
According to the current understanding, whether it is the long-term GRBs formed by the collapse of massive stars or the short-term GRBs produced by double compact stars, although the central objects may be different (either black holes or extremely fast rotating magnetic stars), the generation mechanism of GRBs and their subsequent evolution can be called “fireball”;
in this theory, the central sky will produce relatively persistent extreme relativistic jets over a period of time, which means that the ejected matter will move outward along the rotation axis of the celestial body at the speed of light Because there is a small difference in the velocity between the ejected materials, they collide with each other, convert the kinetic energy of their own motion into the thermal energy of gas particles, and then generate the high-energy radiation we see under the action of magnetic field, which is the early gamma ray, which explains the gamma ray bursts we see The jet time produced by massive stars is longer than that by the merging of two neutron stars, which leads to the difference in our observations
There are interstellar gas medium around these stars. After stopping colliding with each other, the jet material will continue to move outward, interact with the surrounding gas medium, and transfer the energy of its own motion to the surrounding interstellar gas. The interstellar gas is heated to produce strong radiation, which is called gamma ray burst afterglow
to a certain extent, the intensity of the afterglow is related to the density of the surrounding interstellar gas. The higher the density, the brighter the afterglow
The GRB associated with gravitational waves belongs to short-term GRBs, because the burst time scale observed by Fermi satellite is 0.7 seconds. In addition, the results of gravitational wave and electromagnetic wave fitting are consistent with the expectation of merging two neutron stars For example, the fitting of gravitational wave waveform tells us that the mass of neutron star is consistent with the mass range of neutron star
In the process of merging two neutron stars, about 1 / 1000 to 1 / 100 of the mass of the sun is ejected in all directions, which is similar to a sphere These elements are unstable and can decay rapidly to produce radiation heating projectiles, which can emit bright visible light and near-infrared radiation. The brightness of these elements usually reaches 1000 times the level of Nova, so they are called “kilonovae”;
because this kilonova is very close to the earth, it is very bright. It is one tenth of the distance of the short-term scale GRBs detected before
The gravitational wave phenomenon occurred in the southern constellation of Hydra, which is difficult to see in the northern telescope. Therefore, most Chinese telescopes have failed to observe it, such as the newly built fast and many optical telescopes (2.4m telescope in Lijiang, Yunnan Province, and 2.16m optical telescope at Xinglong Observatory of National Astronomical Observatory, etc.).
Fortunately, China has two telescopes involved in the observation. One is the 50 cm Antarctic optical Survey Telescope (ast3) located in Dome A, Antarctica. The project leader is researcher Wang Lifan of Zijinshan observatory. About one day after the information of gravitational wave source was released, ast3 telescope carried out observation on this target source.
At that time, the winter in Antarctica was just over, and the altitude of the target object was low. Due to the limitation of the sun, the observation time of the target object was about two hours every day
Another participant in the observation is the hard X-ray modulated Space Telescope (also known as the comet).
when the observation news was released, the event happened to be within its observation range. Unfortunately, although the comet is the most sensitive observation equipment in this energy range, it can not be within 0.2-5 Any electromagnetic signal is detected in the energy band of MeV, which is probably related to the fact that the gamma ray bursts are not completely facing us
This is the first time in human history that gravitational waves and their electromagnetic counterparts have been detected at the same time, which will become another very important milestone in gravitational wave astronomy.
this exploration has solved some doubts for us, but also raised more questions. Like all astronomical discoveries in history, it is a victory and a new starting point of human curiosity After the curtain of the age of multi messenger gravitational wave astronomy is opened, we believe that with the power of human unity and cooperation, more mysteries of the universe will be revealed one by one
It is understood that there are dozens of astronomical institutions around the world, including scientists from about 70 observatories, including Nanjing Zijinshan Observatory in China, LIGO working group of Tsinghua University, European Southern Observatory and ozgraf team of Australia
Two press conferences will be held at the National Press Club in Washington, D.C., to announce new discoveries and details in gravitational wave detection
In fact, NASA also learned to see you on Monday.
, but scientific discoveries are always exciting. Let’s take a look at what amazing discoveries NASA will announce.