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

White dwarfs are formed when stars use up all their fuel. Sometimes dead stars can produce a fireball of X-ray radiation when they explode in a super-hot explosion. An explosion of X-ray light has been observed by a research team from several German institutions.

It was to some extent a fortunate coincidence, says Ole König from the Astronomical Institute at FAU in the Dr. Karl Remeis observatory.

The eROSITA X-ray telescope is located one and a half million kilometers from Earth and has been surveying the sky for soft X-rays since 2019. On July 7, 2020, it measured strong X-ray radiation in an area of the sky that had been completely obscured four hours earlier. Four hours after the X-ray telescope surveyed the same position in the sky, the radiation had vanished. The X-ray flash that exposed the center of the detector must have lasted less than eight hours.

More than 30 years ago, theoretical research predicted X-ray explosions, but they have never been observed directly. These fireballs of X-rays occur on the surface of stars that were originally comparable in size to the sun before using up most of their fuel made of hydrogen and helium. White dwarfs are similar to Earth in size but have a mass that is similar to the sun.

The detection of novae during the very first moments when most of the X-ray emission is produced is difficult. We can't see very far in this band because soft X-rays aren't very energetic and can't be absorbed by the medium. Telescopes are normally designed to be most effective in harder X-rays where absorption is less important, and that is the reason why they would miss an event like this.

The corpses look like gemstones.

If you were to shrink an apple to the size of a pin head, this tiny particle would retain the large weight of the apple. We can compare burnt-out stars to diamonds that are the same size as Earth in space because they are mostly made up of oxygen and carbon. The objects in the form of gems are very hot. It is difficult to detect the weak radiation from Earth.

Unless the white dwarf is accompanied by a star that is still burning, and when the enormous pull of the white dwarf draws hydrogen from the shell of the accompanying star, it's not true. The star reignites because of the enormous pressure generated by the huge gravitational pull in this layer. In a chain reaction, a huge explosion takes place when the layer of hydrogen is blown off. The X-ray radiation of an explosion like this hit the detectors of eROSITA on July 7, 2020, producing an overexposed image.

The physical origin of X-ray emission coming from white dwarf atmospheres is well understood, and we can model their spectrum from first principles. The basic properties of objects such as weight, size, or chemical composition can be learned if we compare models with observations.

The problem in this particular case was that after 30 years without a single photon, we suddenly had too many, which distorted the response of eROSITA, which was designed to detect millions of very faint objects rather than one but very bright.

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.

The white dwarf is relatively large because it is around the mass of the sun. The fireball was around 60 times hotter than the sun and was created by the explosion. It shows the importance of collaboration in modern science and the wide range of expertise within the German eROSITA consortium.

Since these novae run out of fuel quickly, they cool rapidly and the X-ray radiation becomes weaker until it becomes visible light, which was observed by optical telescopes a day after the eROSITA detection. nova stella, or new star, is a name that has been given to new stars such as this one in the past. Since novae are only visible after the X-ray flash, it is very difficult to predict an outbreak and it is mainly down to chance.

More information: Ole König et al, X-ray detection of a nova in the fireball phase, Nature (2022). DOI: 10.1038/s41586-022-04635-y Journal information: Nature Citation: For the first time, researchers have observed an X-ray explosion on a white dwarf (2022, May 11) retrieved 11 May 2022 from https://phys.org/news/2022-05-x-ray-explosion-white-dwarf.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.