“It’s kind of a lucky coincidence, really,” points out Ole König from the FAU Astronomy Agency. “These X-ray flashes last only a few hours and are almost impossible to predict, but the observing instruments have to be direct at the exact time. Aim for the explosion.” He joined Prof. Dr. Jörn Wilms and a research team from the Max-Planck Institute for Extraterrestrial Physics, the University of Tübingen, the Polytechnic University of Catalunya in Barcelona and the Leibniz Institute for Astrophysics in Potsdam Together they published an article about the observation in Nature.
The instrument in this case is the eROSITA X-ray telescope, which is currently located 1.5 million kilometers from Earth and has been investigating soft X-rays in the sky since 2019. On July 7, 2020, it measured intense X-ray radiation in an area of the sky that was completely inconspicuous four hours earlier. Four hours later, the radiation was gone when X-ray telescopes measured the same location in the sky. It follows that the X-ray flash that had previously been completely overexposed in the center of the detector must have lasted less than 8 hours.
X-ray explosions like this were predicted by theoretical studies more than 30 years ago, but have not been directly observed until now. These X-ray fireballs occur on the surfaces of stars that are about the size of the Sun before they use up most of their fuel, consisting of hydrogen and later helium deep in their cores. These corpses of stars keep shrinking until there are white dwarfs left, which are about the size of Earth but probably about the same mass as our sun. “One way to imagine these proportions is to think of the sun asappleThe same size, which means that the Earth will be the size of a needle and orbit the apple at a distance of 10 meters,” explains Jörn Wilms.
Dr Victor Doroshenko, from the University of Tübingen, added: “These so-called novae do happen all the time, but it’s really hard to detect them in the very first moments of most X-ray emissions. Not only is the short duration of the flash a challenge, And the spectrum of the emitted X-rays is very soft. Soft X-rays are not very energetic and are easily absorbed by the interstellar medium, so we can’t see very far in this band, which limits the number of objects that can be observed — whether novae Or ordinary stars. Telescopes are usually designed to be most efficient for harder X-rays, where absorption is less important, and that’s why they miss such an event!” concludes Victor Doroshenko.
On the other hand, if an apple were to be shrunk down to the size of a pinhead, this tiny particle would retain the relatively large weight of the apple. Jörn Wilms continued: “A teaspoon of material from the interior of a white dwarf can easily have the same mass as a large truck. Since these burnt stars are mainly composed of oxygen and carbon, we can compare them to the Giant diamonds the size of Earth. Objects in the form of these precious gems are hot and glow white. However, this radiation is so weak that it is difficult to detect from Earth.
Unless the white dwarf is accompanied by a star that is still burning, that is, when the white dwarf’s massive gravitational pull attracts hydrogen gas from the accompanying star’s outer shell. “Over time, this hydrogen gas can accumulate on the surface of the white dwarf into a layer that is only a few meters thick,” says FAU astrophysicist Jörn Wilms. The pressure is so great that it causes the star to reignite. In a chain reaction, it quickly explodes with a huge explosion, during which the hydrogen layer is blown off. The X-ray radiation from an explosion like this is what hit the eROSITA detector on July 7, 2020, producing an overexposed image.
“The physical origin of X-ray radiation from white dwarf atmospheres is relatively well understood, and we can model their spectra from first principles and in fine detail. Comparing the models to observations can give insight into the fundamental properties of these objects, Such as weight, size or chemical composition,” says Dr Valery Suleimanov from the University of Tübingen, “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 faint objects instead of one but very bright one,” added Victor Doroshenko.
Jörn Wilms said: “Using the model calculations we originally developed in support of the development of the X-ray instrument, we can analyze the overexposed images in more detail in a complex process to gain a behind-the-scenes view of a white dwarf or nova explosion.”
Based on these results, the white dwarf is about as massive as our sun and therefore relatively large. The explosion created a fireball with a temperature of about 327,000 degrees Celsius, which makes it 60 times the temperature of the sun. “These parameters were obtained by combining X-ray radiation models with those emitted by very hot white dwarfs created by Valery Suleimanov and Victor Doroshenko in Tübingen, and by tuning the instruments under a well-out-of-spec regime at FAU and MPE. obtained from a very in-depth analysis of the responses. I think this is a good illustration of the importance of collaboration in modern science – and the wide range of expertise in the German eROSITA consortium,” added Prof. Dr. Klaus Werner from the University of Tübingen. .
As these novae quickly run out of fuel, they cool rapidly, and the X-ray radiation becomes weaker and eventually becomes visible light, which reaches Earth and is observed by optical telescopes half a day after eROSITA’s detection.
Ole König pointed out that a seemingly bright star followed, which was actually visible light from an explosion and so bright that it could be seen with the naked eye in the night sky, “appearing ‘novae’ like this in It has also been observed in the past. Since these novae can only be seen after an X-ray flash, it is difficult to predict such outbursts and it is mostly luck when they hit X-ray detectors.”
