When stars like our sun run out of fuel, they shrink to form white dwarfs. The dead star sometimes revives in a super hot explosion and produces a fireball of X-ray radiation A research team from several German institutions, including the University of Tubingen, under the leadership of Friedrich Alexander Nuremberg University (Fau), observed such an X-ray explosion for the first time
"It's a lucky coincidence in a way, really," said ole K ö nig of the Fau astronomical Institute. "These X-ray flashes last only a few hours and are almost impossible to predict, but the observation instrument must be directly aimed at the explosion at the right time." Together with Professor J ö RN Wilms and research teams from the Max Planck Institute of extraterrestrial physics, the University of Tubingen, the Catalan Institute of technology in Barcelona and the Potsdam Leibniz Institute of astrophysics, he published an article on this observation in nature.
The instrument in this case is the erosita X-ray telescope, which is currently located 1.5 million kilometers away from the earth and has been investigating soft X-rays in the sky since 2019. On July 7, 2020, it measured strong X-ray radiation in an area of the sky, which was completely inconspicuous four hours ago. Four hours later, when the X-ray telescope measured the same position in the sky, the radiation had disappeared. It can be seen that the X-ray flash that was completely overexposed to the center of the detector must have lasted less than 8 hours.
Such an X-ray explosion was predicted by theoretical research more than 30 years ago, but it has not been directly observed until now. These X-ray fireballs occur on the surface of stars, which are about the size of the sun before running out of fuel composed mostly of hydrogen and later helium deep in their core. The bodies of these stars shrink until there are white dwarfs. They are similar in size to earth, but their mass may be similar to our sun. "One way to imagine these proportions is to think of the sun as an apple( https://apple.pvxt.net/c/1251234/435400/7639?u=https%3A%2F%2Fwww.apple.com%2Fcn%2Fmusic%2F ) The same size, which means that the earth will be the same size as a needle and run around the apple at a distance of 10 meters, "J ö RN Wilms explained.
Dr Victor doroshenko from the University of Tubingen added: "These so-called new stars do happen all the time, but it is really difficult to detect them at the initial moment of most X-ray emission. Not only is the flash short duration a challenge, but also the spectrum of the emitted X-ray is very soft. The energy of soft X-ray is small and easy to be absorbed by the interstellar medium, so we can't see far in this band, which limits the number of objects that can be observed - whether it's new stars or ordinary constant stars Stars. Telescopes are usually designed to be most effective with harder X-rays because the absorption there is less important, and that's why they miss such an event! " Victor doroshenko concluded.
On the other hand, if you want to reduce an apple to the size of a needle, this tiny particle will retain the relatively large weight of the apple. J ö RN Wilms continued: "a teaspoon of material from the interior of a white dwarf can easily have the same mass as a truck. Since these burned stars are mainly composed of oxygen and carbon, we can compare them to giant diamonds the same size as the earth floating in space. These precious gemstone objects are very hot and emit white light. However, this radiation is very weak and difficult to detect from the earth.
Unless the white dwarf is accompanied by a still burning star, that is, when the white dwarf's huge gravity attracts hydrogen from the accompanying star's shell. "Over time, this hydrogen can aggregate into a layer only a few meters thick on the surface of a white dwarf," said J ö RN Wilms, an astrophysicist at Fau This pressure is so great that it ignites the stars again. In a chain reaction, it will soon have a huge explosion, during which the hydrogen layer is blown off. The X-ray radiation of such an explosion was the reason why it hit the erosita detector on July 7, 2020, resulting in an overexposed image.
"We have a relatively good understanding of the physical sources of X-ray radiation from the atmosphere of white dwarfs. We can build their spectral models from the first principles and exquisite details. Comparing the models with the observations can understand the basic properties of these objects, such as weight, size or chemical composition," said Dr. Valery suleimanov from the University of Tubingen, "However, the problem in this particular case is that after 30 years without photons, we suddenly have too many photons, which distorts the spectral response of erosita, which is designed to detect millions of very weak objects rather than a very bright object," added Victor doroshenko.
J ö RN Wilms said: "using the model calculations we originally developed to support the development of X-ray instruments, we can analyze the overexposed images in more detail in a complex process, so as to obtain the behind the scenes view of a white dwarf or Nova explosion."
According to these results, the mass of the white dwarf is about the same as that of our sun, so it is relatively large. The explosion produced a fireball with a temperature of about 327000 degrees Celsius, which makes it 60 times hotter than the sun. "These parameters were obtained by combining the X-ray radiation model with the radiation model of very hot white dwarfs created by Valery suleimanov and Victor doroshenko in Tubingen, as well as a very in-depth analysis of instrument response under far beyond the specification system carried out by Fau and MPE. I think this well illustrates the importance of cooperation in modern Science - and the extensive expertise in the German erosita alliance." Professor Klaus Werner, PhD, from the University of Tubingen, added.
As these new stars quickly run out of fuel, they will cool rapidly, and the X-ray radiation will become weaker until it finally becomes visible light, which reaches the earth and is observed by optical telescopes half a day after being detected by erosita.
Ole K ö nig pointed out that a seemingly bright star appeared later, which was actually the visible light from the explosion and was so bright that it could be seen with the naked eye in the night sky. "Phenomena like this seemingly 'new stars' have been observed in the past. Because these new stars can only be seen after X-ray flash, it is difficult to predict this kind of explosion. When they hit X-ray detectors, they mainly rely on luck."