Explosion on a White Dwarf Star Observed for the Very First Time

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White Dwarf Exploding

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Artist impression of an exploding white dwarf. Credit: University of Tübingen

When stars like our Sun run out of gas, they contract to type white dwarfs. Such useless stars can typically flare again to life in a super-hot explosion and produce a fireball of X-ray radiation. A analysis staff from a number of German institutes together with Tübingen University and led by Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) has now noticed such an explosion of X-ray mild for the very first time.

“It was to some extent a fortunate coincidence, really,” explains Ole König from the Astronomical Institute at FAU within the Dr. Karl Remeis observatory in Bamberg, who has revealed an article about this statement within the respected journal Nature, along with Prof. Dr. Jörn Wilms and a analysis staff from the Max Planck Institute for Extraterrestrial Physics, the University of Tübingen, the Universitat Politécnica de Catalunya in Barcelona, and the Leibniz Institute for Astrophysics Potsdam. “These X-ray flashes last only a few hours and are almost impossible to predict, but the observational instrument must be pointed directly at the explosion at exactly the right time,” explains the astrophysicist.

“These so-called novae do happen all the time but detecting them during the very first moments when most of the X-ray emission is produced is really hard.” — Dr. Victor Doroshenko

The instrument on this case is the eROSITA X-ray telescope, which is presently positioned one and a half million kilometers from Earth and has been surveying the sky for delicate X-rays since 2019. On July 7, 2020, it measured sturdy X-ray radiation in an space of the sky that had been utterly inconspicuous 4 hours beforehand. When the X-ray telescope surveyed the identical place within the sky 4 hours later, the radiation had disappeared. It follows that the X-ray flash that had beforehand utterly overexposed the middle of the detector should have lasted lower than eight hours.

X-ray explosions similar to this have been predicted by theoretical analysis greater than 30 years in the past however have by no means been noticed straight till now. These fireballs of X-rays happen on the floor of stars that have been initially comparable in measurement to the Sun earlier than utilizing up most of their gas fabricated from hydrogen and later helium deep inside their cores. These stellar corpses shrink till “white dwarfs” stay, that are much like Earth in measurement however comprise a mass that may be much like that of our Sun. “One way to picture these proportions is to think of the Sun being the same size as an apple, which means Earth would be the same size as a pin head orbiting around the apple at a distance of 10 meters,” explains Jörn Wilms.

“These so-called novae do happen all the time but detecting them during the very first moments when most of the X-ray emission is produced is really hard,” provides Dr. Victor Doroshenko from Tübingen University. “Not only the short duration of a flash is a challenge, but also the fact that the spectrum of emitted X-rays is very soft. Soft X-rays are not very energetic and easily absorbed by interstellar medium, so we cannot see very far in this band, which limits the number of observable objects, be it a nova or ordinary star. Telescopes are normally designed to be most effective in harder X-rays where absorption is less important, and that’s exactly the reason why they would miss an event like this!” concludes Victor Doroshenko.

Stellar corpses resemble gem stones

On the opposite hand, in the event you have been to shrink an apple to the scale of a pin head, this tiny particle would retain the comparatively massive weight of the apple. “A teaspoon of matter from the within of a white dwarf easily has the same mass as a large truck,” Jörn Wilms continues. Since these burnt-out stars are mainly made up of oxygen and carbon, we can compare them to gigantic diamonds that are the same size as Earth floating around in space. These objects in the form of precious gems are so hot they glow white. However, the 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, and when the enormous gravitational pull of the white dwarf draws hydrogen from the shell of the accompanying star. “In time, this hydrogen can collect to form a layer only a few meters thick on the surface of the white dwarf,” explains FAU astrophysicist Jörn Wilms. In this layer, the huge gravitational pull generates enormous pressure that is so great that it causes the star to reignite. In a chain reaction, it soon comes to a huge explosion during which the layer of hydrogen is blown off. The X-ray radiation of an explosion like this is what hit the detectors of eROSITA on July 7, 2020, producing an overexposed image.

Since these burnt-out stars are mainly made up of oxygen and carbon, we can compare them to gigantic diamonds that are the same size as Earth floating around in space.

“The physical origin of X-ray emission coming white dwarf atmospheres is relatively well understood, and we can model their spectra from first principles and in exquisite detail. Comparison of models with observations allows then to learn basic properties of these objects such as weight, size, or chemical composition” explains Dr. Valery Suleimanov from Tübingen University. “The problem in this particular case was, however, that, after 30 years with no photons we suddenly had too many, which distorted the spectral response of eROSITA, which was designed to detect millions of very faint objects rather than one but very bright” adds Victor Doroshenko.

“Using the model calculations, we originally drew up while supporting the development of the X-ray instrument, we were able to analyze the overexposed image in more detail during a complex process to gain a behind-the-scenes view of an explosion of a white dwarf, or nova,” explains Jörn Wilms.

According to the results, the white dwarf has around the mass of our Sun and is therefore relatively large. The explosion generated a fireball with a temperature of around 327,000 degrees K (588,000 degrees F), making it around sixty times hotter than the Sun. “These parameters were obtained by combining models of X-ray radiation with the models of radiation emitted by very hot white dwarfs created in Tübingen by Valery Suleimanov and Victor Doroshenko, and very deep analysis of instrument response in a regime far outside specifications carried out at FAU and MPE. I think it illustrates very nicely the importance of collaboration in modern science, and wide range of expertise within the German eROSITA consortium” adds Prof. Dr. Klaus Werner from Tübingen University.

Since these novae run out of fuel quite quickly, they cool rapidly and the X-ray radiation becomes weaker until it eventually becomes visible light, which reached Earth half a day after the eROSITA detection and was observed by optical telescopes.

“A seemingly bright star then appeared, which was actually the visible light from the explosion, and so bright that it could be seen on the night sky by the bare eye,” explains Ole König. Seemingly “new stars” such as this one have been observed in the past and were named “nova stella,” or “new star” on account of their unexpected appearance. Since these novae are only visible after the X-ray flash, it is very difficult to predict such outbreaks and it is mainly down to chance when they hit the X-ray detectors.

“We were really lucky,” says Ole König.

Reference: “X-ray detection of a nova in the fireball phase” by Ole König, Jörn Wilms, Riccardo Arcodia, Thomas Dauser, Konrad Dennerl, Victor Doroshenko, Frank Haberl, Steven Hämmerich, Christian Kirsch, Ingo Kreykenbohm, Maximilian Lorenz, Adam Malyali, Andrea Merloni, Arne Rau, Thomas Rauch, Gloria Sala, Axel Schwope, Valery Suleimanov, Philipp Weber and Klaus Werner, 11 May 2022, Nature.
DOI: 10.1038/s41586-022-04635-y



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