University of Tübingen: Explosion observed directly on a white dwarf

When stars like our Sun run out of fuel, they shrink into white dwarfs. Sometimes such objects flare up again in a superheated explosion, producing a fireball of X-rays. A research team led by Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) has now been able to directly observe such a burst in X-ray light for the first time. Tübingen scientists were involved in the project.

“Chance also came to our aid here,” explains Ole König from the Astronomical Institute at FAU in the Dr. Karl Remeis Observatory Bamberg, who together with the FAU astrophysicist Prof. Dr. Jörn Wilms and the research team consisting of the Max Planck Institute for Extraterrestrial Physics in Garching, the University of Tübingen, the Universitat Politécnica de Catalunya in Barcelona and the Leibniz Institute for Astrophysics Potsdam reported on the observation in the renowned journal Nature. “Such X-ray flashes can hardly be predicted, they only last a few hours and the observation instrument has to aim at the burst during this time,” says the astrophysicist, describing the connections.

That instrument is the eROSITA X-ray telescope, which has been scanning the sky for soft X-rays since 2019, one and a half million kilometers from Earth. On July 7, 2020, strong X-ray radiation was measured in an area of ​​the sky that had been completely unremarkable four hours earlier. When the X-ray telescope again scanned the same spot in the sky four hours later, that radiation was gone. The X-ray flash, which had previously completely overexposed the center of the detector, lasted less than eight hours.

Such X-ray bursts had already been predicted by theoretical considerations more than 30 years ago. However, they had never been observed directly before. These fireballs of X-rays form on the surface of stars similar in size to our Sun before they had largely exhausted their fuel supplies of hydrogen and later helium deep within. These old stars shrink very sharply, leaving behind a “white dwarf” that is similar in size to Earth but contains a mass that may be similar in size to our Sun. “These conditions can be well imagined with an example,” explains Jörn Wilms: “If you imagine the sun the size of an apple, the earth would have the dimensions of a pinhead circling ten meters away from the apple.”

“Such so-called novae happen more frequently, but observing the very first moments of the eruption, when most of the X-rays are produced, is very difficult,” adds Dr. Victor Doroshenko from the University of Tübingen. “Not only the short duration of this X-ray flash is a challenge, but also the fact that the spectrum of the emitted radiation is very soft. Soft X-rays are not very energetic and are easily absorbed by interstellar matter, so we cannot see very deep into space in this wavelength band. This limits the number of objects that can be observed, whether a nova or a normal star. X-ray telescopes are therefore usually designed in such a way that they work particularly effectively in the hard X-ray range. And that’s exactly the reason

Stars in the shape of a gem
In turn, if you reduce an apple to the size of a pinhead, this tiny particle retains the comparatively huge weight of the apple. “A teaspoon of matter from the interior of a white dwarf therefore easily has the mass of a truck,” explains Jörn Wilms. Because these burned-out stars are composed mostly of oxygen and carbon, they resemble a giant Earth-sized diamond, also made of carbon, floating in space. While these gem-shaped objects are still hot, they glow white. However, this radiation is weak and can therefore hardly be detected from Earth’s perspective.

Unless the old star is accompanied by a star in which the solar fire is still burning and from which material can then transfer to it. “Over time, this hydrogen can collect in a layer just a few meters thick on the surface of the corpse,” explains FAU astrophysicist Jörn Wilms. In this layer, however, the enormous gravity creates a gigantic pressure that can become so great that the star fire ignites there again. A huge explosion quickly occurs in a chain reaction, in which the hydrogen layer is blasted off again. The X-rays from such an explosion then hit and overexposed the detectors of eROSITA on July 7, 2020.

“The physical origin of the X-ray emission from the white dwarf is quite well understood and we can model the spectrum very well. Comparing models with observation allows determination of the mass, size and chemical composition of these objects. Valery Suleimanov from the University of Tübingen. “The problem in this case was that after 30 unsuccessful years in finding such X-ray flashes, we suddenly observed such a bright event that overexposed the telescope’s detectors and made it difficult to interpret the data,” adds Victor Doroshenko.

“Using model calculations, with which we originally accompanied the development of the X-ray instrument, we were then able to analyze the overexposed image more precisely in a complex work and thus take a look behind the scenes of such an explosion on a white dwarf for the first time,” says Jörn Wilms the further research. According to these results, the white dwarf should have about the mass of our sun and thus be relatively large. The explosion created a 327,000 degree fireball, which was around sixty times warmer than our sun. “These results were obtained by combining models of X-rays and white dwarf emission models developed in Tübingen by Valery Suleimanov and Victor Doroshenko. This shows impressively

Because such novae lack energy replenishment, they cool down quickly and the X-rays become softer until they finally become visible light, which also reached Earth half a day after the eROSITA discovery and was observed with optical telescopes. “Then an apparently bright star appeared that was even visible to the naked eye,” explains Ole König. Such apparent “new stars” have also been observed before and have been named “Nova Stella,” meaning “new star,” because of their unexpected appearance. However, because this nova only becomes visible after the X-ray flash, it is very difficult to predict such outbursts, which therefore hit the X-ray detectors rather accidentally. “We were really lucky there,” says Ole König happily.

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