For the first time, researchers observed an X-ray explosion on a white dwarf

(ORDO NEWS) — When stars like our Sun use up all their fuel, they shrink to form white dwarfs. Sometimes these dead stars come back to life in a superhot explosion and produce a fireball of X-rays.

A research team from several German institutions, including the University of Tübingen, led by the Friedrich-Alexander Erlangen-Nuremberg University (FAU), was able to observe such an X-ray burst for the first time.

“It was kind of a lucky coincidence,” explains Ole König of the FAA Astronomy Institute at the Dr. Carl Remeis Observatory in Bamberg, who published an article about the observation in the journal Nature along with Prof. Dr. Jorn Wilms and a research team from the Institute for Extraterrestrial Physics named after Max Planck.

“These X-ray flashes last only a few hours and are almost impossible to predict, but the observation instrument must be pointed directly at the explosion at a precisely defined time,” explains the astrophysicist.

In this case, such an instrument is the eROSITA X-ray telescope, which is currently located one and a half million kilometers from Earth and has been observing the sky for soft X-rays since 2019. On July 7, 2020, he recorded a strong X-ray emission in a region of the sky that was completely invisible four hours before.

When the X-ray telescope surveyed the same spot in the sky four hours later, the radiation disappeared. It follows from this that the X-ray flash, which previously completely overexposed the center of the detector, should have lasted less than eight hours.

Such X-ray explosions were predicted by theoretical studies more than 30 years ago, but have never been directly observed until now.

These X-ray fireballs originate on the surface of stars that were originally comparable in size to the Sun but then used up most of their hydrogen and then helium fuel deep inside their cores.

These stellar corpses shrink until they are left with white dwarfs that are similar in size to Earth but have a mass that could be the same as our Sun.

“One way to think of these proportions is to think that the Sun is the size of an apple and the Earth is the size of a pinhead orbiting the apple at a distance of 10 meters,” explains Jorn Wilms.

“These so-called new stars are constantly occurring, but detecting them in the very first moments, when most of the X-ray emission occurs, is very difficult,” adds Dr. Victor Doroshenko from the University of Tübingen.

“The difficulty is not only the short duration of the flare, but also the fact that the spectrum of emitted X-rays is very soft.

Soft X-rays are not very energetic and are easily absorbed by the interstellar medium, so we cannot see very far in this range, which limits the number of observed objects , whether it be a nova or an ordinary star.

Telescopes are usually designed to operate in harder X-rays where absorption is less important, and this is the reason they can miss such an event,” concludes Viktor Doroshenko.

Star corpses are like gems

On the other hand, if you reduced an apple to the size of a pinhead, this tiny particle would retain a relatively large mass of the apple.

“A teaspoon of matter from inside a white dwarf could easily have the same mass as a large loaf,” continues Jorn Wilms. Because these burned-out stars are mostly oxygen and carbon, we can compare them to giant diamonds that are the size of Earth and float in space.

These gem-shaped objects are so hot that they glow with white light. However, the radiation is so weak that it is difficult to detect from Earth.

This is true until the white dwarf is accompanied by a star that is still burning, and when the white dwarf’s huge gravitational pull pulls hydrogen from the shell of the accompanying star.

“Over time, this hydrogen can collect and form a layer just a few meters thick on the surface of a white dwarf,” explains FAA astrophysicist Jorn Wilms. reaction, a powerful explosion soon occurs, during which a layer of hydrogen is blown away.X-ray radiation from such an explosion hit the eROSITA detectors on July 7, 2020, creating an overexposed image.

“The physical origin of the X-ray emission from the atmospheres of white dwarfs is relatively well understood, and we can model their spectra from first principles and in great detail.

Comparing models with observations allows us to know the basic properties of these objects, such as mass, size or chemical composition “explains Dr. Valery Suleimanov from the University of Tübingen.

“However, in this particular case, the problem was that after 30 years without photons, we suddenly had too many of them, which distorted the spectral response of eROSITA, which was designed to detect millions of very faint objects, and not one, but very bright,” – adds Viktor Doroshenko.

“Using model calculations that we originally compiled with support from X-ray instrument development, we were able to analyze the overexposed image in more detail through a complex process to get a behind-the-scenes look at the explosion of a white dwarf, or nova,” explains Jörn Wilms.

According to the results obtained, the mass of a white dwarf is approximately equal to the mass of our Sun and is therefore relatively large. The explosion created a fireball with a temperature of about 327,000 Kelvin, making it about 60 times hotter than the Sun.

“These parameters were obtained by combining X-ray models with models of radiation emitted by very hot white dwarfs, created in Tübingen by Valery Suleimanov and Viktor Doroshenko, and very deep analysis of the response of instruments in a mode far beyond the specifications carried out in FAU and MPE.

I think this illustrates very well the importance of collaboration in modern science and the wide range of knowledge within the German consortium eROSITA,” adds Prof. Dr. Klaus Werner from the University of Tübingen.

As these new stars run out of fuel fairly quickly, they cool rapidly and the X-rays become weaker until they eventually turn into visible light, which reached Earth half a day after eROSITA was detected and was seen by optical telescopes.

“Then a seemingly bright star appeared, which was actually the visible light from the explosion, and so bright that it could be seen in the night sky with the naked eye,” explains Ole König.

Such seemingly “new stars” as this one have already been observed in the past and have been called “nova stella”, or “new star”, because of their unexpected appearance.

Since these new stars are only visible after an X-ray flare, such flares are very difficult to predict. and getting them into X-ray detectors depends mainly on chance. “We were very lucky,” says Ole Koenig.


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