Scientists recognize the new gravitational wave event optical counterpart

[ Instrument Network Instrument Development ] An international cooperation team consisting of the Purple Mountain Observatory of the Chinese Academy of Sciences, the Italian National Institute of Astrophysics, and the University of Guizhou newly discovered a candidate for a thousand stars in the short-time gamma ray burst (short storm) GRB 070809 This work has been published online by Nature Astronomy.
The neutron star merger process not only produces strong gravitational waves, but also electromagnetic radiation counterparts such as gamma-ray bursts and thousand-star stars (also known as giant stars). The gamma-ray bursts generally come from the extreme relativistic jets ejected by the merged black holes, so the rate of convergence with the gravitational wave signals is low; and the thousand-stars are super-irons that are ejected by the neutron star merge process (that is, than iron) Infrared, optical temporary sources produced by the radioactive decay of heavy elements, whose radiation is approximately isotropic, and the rate of convergence of gravitational wave signals is high. Therefore, the new star is the most promising gravitational wave event electromagnetic radiation counterpart, which helps to solve the major scientific problems in the field of modern aerophysics in the "origin of super-iron elements". The full-sensitivity advanced LIGO/Virgo detector detects red-shifting of neutron star convergence events that produce electromagnetic radiation signals no more than 0.2, while the detection distance of the new star can be farther, thus revealing some higher redshifts. The origin of the neutron star merger will play a key role.
The first international case, and the only case of gravitational wave and thousand stars in the past, is GW170817/AT2017gfo. Since then, the enthusiasm for the electromagnetic counterparts of gravitational wave events, especially the thousand stars, has never been higher. For example, for the neutron star black hole merge event GW190814, the gravitational wave detector gives an accurate positioning of 23 square degrees but claims strong evidence that there is no material ejection for this merger. The international 8-10 meter telescope VLT, Keck, GTC The 4-meter telescope DES, WHT, etc. all resolutely invested a lot of time in searching for electromagnetic counterparts. Unfortunately, LIGO/Virgo's O3 has been running for more than half of the time, but has not yet found a case with the gravitational wave event/thousand stars. Taking into account the gap between domestic and international companies in the detection of kilonovae, an international team of Chinese and Italian scientists has taken a different approach to prove the optical counterpart of gravitational wave events through systematic analysis of some “historically available historical data of short bursts”. .
The team's latest research object is the short-lived GRB 070809. There is less data on this storm, so no related papers have been published in the past 12 years. However, Jin Zhiping, a researcher at the Purple Mountain Observatory, found that the X-ray radiance spectrum of this storm is very hard and hardens over time. Normally, the X-ray afterglow spectrum of a gamma-ray burst is soft and softens over time. In addition, the optical radiant energy spectrum of GRB 070809 is very red. Under the gamma-ray standard afterglow model, the optical afterglow spectrum is not softer than the X-ray spectrum unless it suffers from severe dust extinction. Since the extinction of the Milky Way is small in this direction, strong extinction can only be contributed by the host galaxies. However, the Hubble telescope's deep exposure observations indicate that GRB 070809 occurs outside of its host galaxies, so there is no strong extinction. In addition, X-ray data analysis shows that the hydrogen column density in this direction is highly consistent with the contribution of the Milky Way, which means that there is not a lot of dust around the source. Based on this information, Jin Zhiping pointed out that GRB 070809 is a short storm with strange optics and X-ray radiance energy spectrum that occurs outside the host galaxies. Compared to other short bursts, GRB 070809 has the softest optical radiation and the hardest X-ray spectrum (Figure 1) and cannot be explained in the forward shock persistence model. The authors believe that the peculiar spectral behavior of X-rays may reflect the long-term activity of the central engine, while the extremely soft optical radiation can naturally be interpreted as the blackbody spectrum of the thousand stars. If these interpretations are correct, all X and optical radiation from GRB 070809 is not from external shock radiation from gamma-ray bursts, which is consistent with the extrapolation of the storm's medium density outside the host galaxies.
Jin Zhiping also counted the nature of the thousand-star (and candidate) with red-shift measurements at this stage. It is found that the temperature and velocity of the early thousand-star radiation zone may have a wide distribution range. AT2017gfo may not be typical (see Figure 2) and the thousand star radiation may have subclasses. Considering the observations of LIGO/Virgo, the neutron star coincidence rate is as high as ~1000Gpc-3yr-1, and after 2022, the positioning accuracy of the gravitational wave event will be greatly improved due to the addition of LIGO-India, so people will have a larger one in a few years. A sample of the thousand stars to gain a more complete understanding of the relevant physical properties.
The research was funded by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, and the Key Science Research Project of the Chinese Academy of Sciences and the Strategic Pilot Special Project B.

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