On the afternoon of June 16, Beijing time 10, a two-hour press conference was held at the National Press Club in Washington. David Reitze, executive director of LIGO, announced that LIGO and Virgo will interfere with the gravitational wave observatory with lasers in 2065438. It is formed by merging two double neutron stars with solar masses of 1. 15 and 1.6 respectively. According to the detection date, the number is GW 1708 17, which is 654.38+300 million light years away. In addition, with the joint efforts of many astronomers and detection equipment around the world, the electromagnetic counterpart of gravitational wave events has also been discovered.
At the beginning of 20 16, David Leeds stood in the same place and announced that human beings had detected gravitational waves for the first time-at that time, we said that a new era of multi-messenger astronomy was about to open. In this exploration of GW 1708 17, human beings have detected gravitational waves and their electromagnetic counterparts at the same time for the first time, which can be regarded as the real beginning of the era of gravitational wave multi-messenger astronomy and has epoch-making significance in the history of astronomical development. On the other hand, the merger of two neutron stars is usually regarded as a source of gamma ray bursts, which will produce many different observation phenomena. Therefore, we can get a more detailed understanding of neutron star, a mysterious celestial body, by integrating gravitational waves, electromagnetic waves and other observation methods.
Figure 1: Gravitational waves and corresponding electromagnetic signals merged by two neutron stars were first detected by humans.
Figure 2: Comparison of the duration of gravitational wave signals generated by the merger of neutron stars and those generated by previous black holes. This time, the double neutron star lasted about 100 second, and it only showed more than 50 seconds.
Figure 3: It can be seen that the difference between the end time of LIGO gravitational wave signal and the start time of gamma burst is about 2 seconds.
Just as we directly detected the gravitational wave of a black hole for the first time, the detection of the gravitational wave of a double neutron star was a complete accident and came a little early. Previously, based on the understanding of double neutron stars and the analysis and comparison of LIGO detection sensitivity, scientists estimated that VIRGO would not detect the merger of double neutron stars until at least 20 19 years before LIGO was further upgraded and reached the expected sensitivity. Humans successfully detected the merger of two neutron stars two years in advance, which is a wonderful surprise. If the reason is investigated, in addition to the proximity of the detected system to us, cooperation in many aspects is an important factor contributing to the success of detection.
1, global cooperation, targeted.
The detection process of GW 1708 17 is inspiring and noteworthy, even more exciting than Interpol's transcontinental pursuit of fugitives.
On August 7, 2065438, astronomers all over the world got a message that LIGO and Virgo detector detected a new gravitational wave signal, which lasted about 100 second, and its form was consistent with the merger of two neutron stars. About 1.7 seconds after the arrival of the gravitational wave signal, the weak short-time scale gamma-ray burst was detected by the Gamma-ray Burst Monitor (GBM) carried by Fermi satellite of NASA and the SPI-ACS detector carried by the European Integral Telescope, and it was named GRB 1708 17A. Because of the consistency of time and space, they are considered to be related to gravitational wave events ("correlation" means that the two phenomena are related).
After learning this news, telescopes all over the world began to observe busily. In less than 1 1 hour, the Swope Supernova Survey (SSS) telescope in Chile observed a bright light source in the galaxy NGC4993 for the first time, which was initially confirmed as its optical counterpart, numbered 20 17gfo/SSS 17a. After that, several other teams independently tested the light source and confirmed it.
In the next few weeks, astronomers used some of the most advanced telescopes in the world, such as Chandra X-ray Telescope, Hubble Space Telescope, the very large telescope with a diameter of 8.4 meters in Chile, and ALMA, the Atacama large millimeter wave array with the highest sensitivity in submillimeter band, to make intensive observations in this area. These observations provide a comprehensive description of this catastrophic event from about 100 seconds before the merger to several weeks after the merger, and finally confirm many conjectures of scientists: the merger of two neutron stars in NGC4993 galaxy produced gravitational waves, short gamma bursts and thousands of new stars.
Figure 4: (left) Optical images of gravitational wave sources seen by several different telescopes in Ounan Station. (right) Images observed by Hubble telescope at different times.
This kind of exploration is the perfect embodiment of global cooperation. However, as David Leeds said at the press conference, the detection of gamma-ray burst signal from Fermi satellite of NASA made this LIGO exploration shine. Although the gravitational wave signal is generated before the gamma ray signal, it is interesting that the detection signal from Fermi satellite of NASA is earlier than that of LIGO team. The reason is that GRB 1708 17A was detected by GRB monitor of Fermi satellite of NASA, and the relevant alarm was automatically sent to GCN system. However, the automatic data analysis of LIGO takes about 6 minutes-scientists first found a gravitational wave event candidate GW 1708 17 in the almost simultaneous data of LIGO hanford Observatory, and found that this gravitational wave occurred two seconds earlier than GRB 1708 17A, and the Virgo rapid response team then manually checked it. Later, scientists further confirmed the existence of gamma-ray burst signals in the observation data of European integration satellites. The original unremarkable gamma-ray burst signal, because it coexists with a strong gravitational wave candidate, has aroused the observation interest of the whole astronomical community at once, and this celestial region has also become a hot observation object.
At the Fourth Gravitational Wave Conference at the end of September, the belated Virgo has reduced the spatial positioning range of LIGO detector from 1 160 square degree to 100 square degree, and the cooperation between them has greatly improved the accuracy of spatial position. If all possible parameters are further estimated by Bayesian statistical method, the spatial position will be further reduced to 60 square degrees. In this way, the spatial positioning has been comprehensively improved by nearly 20 times. In this double neutron star event, the three detectors finally located the source within 28 square degrees. It is precisely because the accuracy of spatial positioning has been greatly improved that the spatial confirmation of electromagnetic wave profile detection is possible.
Fig. 5: Comparison of the spatial positions of five gravitational waves detected so far. Yellow is the area where the latest gravitational wave GW 1708 17 determines the gravitational wave source.
Another important significance of joint observation is rapid response. Both the gamma-ray bursts observed by Fermi and the gravitational waves observed by VIRGO have a very short duration, so other observatories and observers need to follow up the possible areas immediately, which requires a system to inform the possible location information immediately.
As for gamma-ray bursts, the network appeared during the on-orbit operation of BeppoSAX satellite at the end of last century, and NASA established the mail system of Gamma-ray Coordination Network (GCN). Once the satellite detects the gamma-ray burst signal, it will send the location information of the gamma-ray burst to the system as quickly as possible, and anyone who subscribes to the mail system can immediately receive a prompt for possible observation. This Fermi observation used this system to inform many organizations around the world at the fastest speed, and then many telescopes joined the observation. Of course, for VIRGO organizations, in order to ensure their possible follow-up observation, they have signed memorandum contracts with nearly 70 observation organizations around the world (there are nearly 10 organizations in China), and once gravitational wave signals are detected, they will also transmit relevant information through their own unique channels.
2. The merger of two neutron stars is better than the merger of two black holes.
It was mentioned at the press conference that the gravitational wave detected this time was produced by the merger of two neutron stars. The four previously announced gravitational wave events were all caused by double black holes. The biggest difference between them is that the merger of two neutron stars will produce electromagnetic radiation, but for black holes, we usually think it won't, which has also been verified by observation.
What caused this difference? Generally speaking, according to the theoretical requirements of astrophysical radiation, 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 beginning, in the long evolution process, if there are no more gas sources, the gas has been exhausted in the final stage of black hole merger, so electromagnetic radiation cannot be generated, and only gravitational waves that disturb time and space can be generated, just like the four times that scientists have detected before.
Before the two neutron stars merged, the surrounding gas was probably consumed. However, in the process of merger, some materials will be thrown at a speed close to the speed of light or far below the speed of light, resulting in all kinds of electromagnetic phenomena we see-short-time-scale gamma-ray bursts, afterglow of gamma-ray bursts and thousands of new stars. Matter moving near the speed of light produces gamma ray bursts seen by Fermi satellites, while matter moving at a low speed produces thousands of new stars, which are captured by many optical/infrared telescopes.
Wait, what is a short-time scale gamma burst, gamma burst afterglow, and millions of new stars? Let's talk about it one by one.
Simply put, gamma-ray bursts are the phenomenon that gamma-ray radiation suddenly brightens in a certain direction in the sky, which can be said to be the most violent celestial explosion since BIGBANG. In the early 1990s, Compton Gamma-ray Observatory made a simple statistics after observing thousands of gamma-ray bursts, and divided them into two categories according to their duration: one is long-time scale gamma-ray bursts with an explosion time longer than 2 seconds, and the other is short-time scale gamma-ray bursts with an explosion time shorter than 2 seconds. After in-depth study, it is found that the origins of these two kinds of gamma bursts are completely different.
According to the current understanding, whether it is a long-time scale gamma-ray burst caused by the collapse of a massive star or a short-time scale gamma-ray burst caused by a double compact star, although the central celestial body is different (either a black hole or a rapidly rotating magnetic star), the generation mechanism and subsequent evolution of the gamma-ray burst can be explained by a theory called "fireball model". In this theory, the central celestial body will produce a relatively continuous jet of extreme relativity for a period of time, which means that these ejected substances will move outward along the axis of the celestial body at a speed close to the speed of light. Because there is a slight speed difference between the ejected substances, they collide with each other, transforming their kinetic energy into the heat energy of gas particles, and then generating the high-energy radiation we see under the action of magnetic field, that is, the early gamma rays, which explains the gamma bursts we see well. The jet time produced by a massive star is long, and the jet time produced by the merger of two neutron stars is short, which leads to the difference in our observations.
These stars are surrounded by interstellar gas medium. After the jet material stops colliding with each other, it will continue to move outward, interact with the surrounding gas medium and transfer the energy of its own movement to the surrounding interstellar gas. The interstellar gas is heated to produce strong radiation, which is called gamma ray burst afterglow. Its energy spectrum band will extend from X-ray to radio band. To some extent, the intensity of afterglow is related to the density of surrounding interstellar gas. The higher the density, the brighter the afterglow.
This gamma-ray burst related to gravitational waves belongs to a short-time scale gamma-ray burst, because the explosion time scale observed by Fermi satellite is 0.7 seconds. In addition, both the results of gravitational waves and the observation and fitting results of electromagnetic waves are consistent with the expectation of the merger of double 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.
During the merger of the two neutron stars, about110,000 to110,000 solar-mass substances were thrown in all directions, which were shaped like spheres. These ejected substances produce a large number of heavy elements through the fast neutron capture process. These elements are unstable and can decay rapidly, generating radiation to heat the projectile, thus making it emit bright visible light and near infrared radiation, and its brightness usually reaches the nova level of thousands of times, so it is called "thousand nova". Because this thousand nova is very close to the earth, it is very bright, which is one tenth of the distance of the short-time scale gamma storm detected before.
Figure 6: The process of two neutron stars rotating together and finally merging to produce thousands of new stars.
Because the celestial bodies that produce gravitational waves are completely different, the gravitational wave waveforms we observe will be very different. Compared with black holes, the mass of neutron stars is much smaller, and the disturbance and deformation of space-time are weaker in the process of merger. Therefore, under the condition that the sensitivity of the detector is determined at present, we can only detect the gravitational wave signal nearby. This gravitational wave source is1.300 million light years away, which is the latest example of all gravitational wave sources detected so far. Through waveform fitting, scientists determined that the masses of the two neutron stars are about 1. 15 and 1.6 solar masses, respectively. The combined celestial body mass is about 2.74 solar masses, and only 0.0 1 solar mass is ejected.
3. Solved mysteries and unsolved mysteries
Previously, we still had many difficult questions to answer, whether it was the neutron star itself or the gamma ray burst produced by the merger of two neutron stars. After the merger of two neutron stars, is it a neutron star or a black hole with faster rotation? How much material will the explosion throw? What is the mechanism and angle of injection? We are not sure yet.
In addition, so far, scientists are not particularly clear about the composition and structure of neutron stars. When two neutron stars are close to each other but do not merge, the two neutron stars will be seriously deformed by each other's tidal force, which will eventually affect the spin speed and gravitational wave waveform. Therefore, scientists hope that the joint observation of gravitational waves and electromagnetic waves can provide some precious answers to these questions.
Unfortunately, due to the sensitivity of the current gravitational wave detection equipment, the gravitational wave signal curve is not very good, so the question about the internal structure has not been answered. However, we have a preliminary answer to the question of how much material has been thrown after the partial merger. To be proud, this answer was given by a China telescope that participated in the observation. (The answer will be announced soon)
Did the merger of two neutron stars produce a neutron star or a black hole? We're not sure yet. Because by fitting the gravitational wave waveform, the combined mass is about 2.74 solar masses. Theoretically, if the mass of a celestial body exceeds three solar masses, it is usually considered as a black hole. But the maximum allowable value of neutron stars is not clear. If the neutron star is composed of neutrons, it is impossible to reach 2.74 solar masses by considering the equation of state and the rotational speed. However, if the interior is composed of other foreign substances (such as quarks), under certain conditions, a celestial body of this quality has certain possibilities, and this celestial body should be called a "quark buster". But at present, all the observations fail to give the critical mass of neutron stars and black holes, and of course, they fail to give evidence of quarks. From the observation point of view, the heaviest neutron star we observed is about 2 solar masses, and the smallest black hole mass is 5 solar masses; Between the two, there is a blank, and the mass of any dense celestial body is not found to belong to this range. Therefore, although we are not sure what the 2.74 solar mass object produced by the merger of two neutron stars is, this discovery fills the gap between black holes and neutron stars and opens the curtain for more astronomical discoveries in the future.
Figure 7: The mass distribution of black holes and neutron stars detected so far shows that there is a big gap between them. This exploration is the first celestial body to fill this gap.
Although scientists didn't see the information inside the neutron star and didn't know what the final merger was, many electromagnetic observations later told us some uncertain information. For example, the spectral observation of the Very Large Telescope (VLT) confirms the sources of heavy metals (such as gold and silver, which we are familiar with), and most of them are produced during the merger of neutron stars.
Figure 8: Element Origin Table. Yellow represents the elements produced by the merger of neutron stars, and our common gold and silver are produced through this process.
Previously, scientists have detected three cases of suspected thousands of new stars in short-time scale gamma bursts, but only a few data points were seen in the light curve of afterglow. Because the distance is very close, the afterglow of gamma-ray bursts is very weak, which completely confirms the existence of thousands of new stars. In addition, by fitting the evolution of its light curve, it can be inferred that about 1% of the substances were thrown out during the merger.
Besides, what is the significance of the combination of electromagnetic signal and gravitational wave signal to astronomical theory itself? On the one hand, scientists can test Einstein's weak equivalence principle through the time difference of arrival of these two signals, which is the cornerstone of Einstein's general theory of relativity and other theories of gravity, and Einstein's theory has passed the test again.
In addition, the combination of gravitational wave signal and electromagnetic signal can limit some basic parameters of cosmology, such as Hubble constant, which is used to describe the expansion speed of the universe. Through the amplitude comparison of gravitational waves, we can infer the photometric distance from the system to us, and through the spectral analysis of electromagnetic waves, we can know the red shift of this system. Given these two, we can calculate the value of Hubble constant:
Compared with the value of Planck satellite:
Obviously, the numerical error given by gravitational waves is very large. However, it can be predicted that with the improvement of detection accuracy (except VIRGO, the arm length of KAGRA detector is 3 kilometers, and India and many third-generation gravitational wave detectors are planned) and the increase of the number of gravitational wave sources detected, this error will soon be improved.
Gravitational waves occur in Ophiuchus in the south, which is difficult to be seen by telescopes in the north, so most telescopes in China have failed to observe them, such as the newly-built FAST and many optical telescopes (2.4m telescope in Lijiang, Yunnan, 2. 16m optical telescope in Xinglong Observatory of National Astronomical Observatory, etc. ).
Fortunately, however, China has two telescopes to participate in this observation. One is the 50cm Antarctic Optical Sky Survey Telescope (AST3) located in Dome A, Antarctica. The project leader is researcher Wang Lifan from Purple Mountain Observatory. About one day after the release of gravitational wave source information, AST3 telescope observed this target source. At that time, the Antarctic winter had just passed, and the horizon height of the target celestial body was low. Due to the limitation of the sun, there are almost 2 hours of observation time every day. The telescope finally observed 10 days, and finally got the light curve of the target celestial body, which was highly consistent with the theoretical prediction of the giant nova.
Another participant in the observation is the hard X-ray modulated space telescope (also known as the eye). When the observation news was released, the event happened to be within its observation range. Unfortunately, although Yan Hui is the most sensitive observation equipment in this energy band, it failed to detect any electromagnetic signal in the energy band of 0.2-5 MeV, which is probably related to the fact that this gamma storm is not completely suitable for us.
This is the first time in human history that gravitational waves and their electromagnetic counterparts have been detected simultaneously, which will become another very important milestone in gravitational wave astronomy. This exploration answers some doubts for us, but it also raises more questions. Like all astronomical discoveries in history, it is a victory and a new starting point for human curiosity. After the curtain of the era of multi-messenger gravitational wave astronomy is opened, we believe that with the strength of human solidarity and cooperation, more mysteries of the universe will be revealed one by one.