Space "cows" make scientists intrigued. Several theoretical explanations are flaws.
Newspaper In June of last year, an exceptionally bright light suddenly appeared in the sky, which left astronomers crazy. After a few months of research, they are still not sure what flash is – officially called "AT2018cow", but it has been nicknamed "cows" all over the world. Today, scientists presented ideas at the American Astronomical Society meeting in Seattle on January 11. However, whatever, Liliana Rivera Sandoval, an astronomer at Texas Tech University in Lubok, said, "It's very strange."
The cow first appeared in telescope observations on June 16, 2018, and later proved to be a small galaxy about 200 million light-years away. It is very bright and was not there the day before. This rapid appearance seems to rule out the possibility of supernovae because the brilliance of such a stellar explosion usually grows more slowly. Daniel Perley, an astronomer at John Moores University in Liverpool, UK, said: "When we see this phenomenon, we think, let's begin."
Astronomers initially believed that "dairy cows" were an event closer to us, probably in the Milky Way, much less than the disaster caused by a supernova. One possibility is for a white dwarf to devour a companion star and occasionally burn in the process. Such events are common in the Milky Way. But the spectral analysis of the AT2018 curve quickly showed that the celestial body was far from Earth to be in other galaxies – such a distance would never see a bright white dwarf.
Perley is one of the leaders in the global network of rapid response telescopes called GROWTH, and several telescopes on the network quickly attacked the "cows." These include the Liverpool Telescope in La Palma, Canary Islands, Spain, and the Palomar Observatory in California. Perley said, "In the first two weeks, we gave up everything and watched seven times a night."
Initial observations confirmed that this "cow" was really strange. It does not show the change in the light emitted by the supernova, its brightness continues to increase, and the brightness and the heat last for almost 3 weeks. "These are things supernovas do not usually do," Perley said.
Sandoval said that when she and her colleagues learned in 2018 that "dairy cows" were really far from us, they asked the NASA Neil Gaines Swift Observatory to conduct their activities under ultraviolet rays and X-rays. Observed Observations of the orbiter show that the celestial body is very bright in both parts of the spectrum. Although the brightness of the X-ray has fluctuated during the first few weeks, "there is no change in the spectrum, which is very unusual." Sandoval said. After 3 weeks, the X-ray signal began to float drastically, and the brightness also began to decrease.
Many astronomers agree that this event lasts long and is stable, which means that it has been fed by some form of central engine after the initial outbreak. But what this engine can be is far from clear. Some people think this may be an unusual supernova whose nucleus collapses inside after a stellar explosion. Others believe that this is a tidal destruction event – the star is torn by a black hole. But this usually requires a supermassive black hole in the center of the galaxy, and the "cow" is located on the spiral arm of the galaxy. Therefore, it has been said that this may be a tidal disturbance caused by a medium mass black hole, although the evidence for the existence of such a small black hole is still controversial. "All the explanations are problematic," Sandoval said.
Four days after the discovery of the "cow," Anna Ho of the California Institute of Technology in Pasadena observed her at a Mamma Array telescope in Mauna Kea, Hawaii. At the short end of the radio spectrum, millimeter waves are generally not used to observe such erupting objects because their signals decay very rapidly and the telescope can not capture them. And this "cow" is different. "After a few days, it's still very bright," Ho said. "This is the first time we have seen a light source."
Ho believes that this signal shows that the initial explosion of shock waves hit the dense clouds of gas and dust around. When this happens, the cloud gets hot and the gas emits light of different wavelengths. At the same time, the radiation continues as the shock wave travels through the clouds. A sudden decrease in the submillimetric signal over time may indicate that the shock wave has reached the outer boundary of the gas cloud.
Ho pointed out that if astronomers can find similar sources of light in the future, studying shock waves in this way will provide valuable data about their size, velocity, total energy and the structure of the environment around the star. Bob Kirshner, an astronomer at the Palo Alto Gordon Foundation and Betty Moore, Calif., Said, "That will tell us what the star did before the explosion."
As is often the case, researchers need more data. "I hope there are more dairy cows," said Sandoval. (Zhao Xixi)
Chinese Journal of Science and Technology (2019-01-14 2nd Edition International)