When Sun-like stars use up all their fuel, they shrink to form white dwarfs. Sometimes these dead stars revive in a very hot explosion and produce a ‘fireball’ of X-ray radiation. Now, a research team led by the University of Erlangen-Nuremberg (FAU, Germany) has managed to observe, for the first time, this type of explosion with the eROSITA X-ray Observatory. The results are presented this week in Nature.

In turn, the do not go are explosions caused by uncontrolled thermonuclear combustion in the hydrogen-rich envelopes of these accreting white dwarfs. These are unpredictable stellar explosions that apparently appear as ‘new’ star in the sky.

The German eROSITA X-ray observatory has detected this type of radiation from a ‘fireball’ produced by a white dwarf for the first time.

Its origin lies in the accumulation of material from a star similar in size to the sun (hydrogen from the outermost layers) towards a companion star white dwarfa type of highly compact star, which has a mass similar to that of the Sun, but concentrated in a celestial body with a radius equivalent to that of the Earth.

The extreme conditions on the white dwarf’s surface cause material accumulated on the surface to explode and be expelled into outer space in a massive thermonuclear explosion. The material expands rapidly and, within hours, causes an increase in the star’s visible magnitude. At that time, the phenomenon can be observed from Earth as a new star in the sky.

In this artist’s depiction, a white dwarf (left) steals gas from its companion star. The material spirals to the dwarf’s surface, where it is compressed and eventually ignited in a thermonuclear explosion. / IT / M. Kornmesser

The early stages of a new explosion “It had already been theoretically predicted: the high temperatures of the thermonuclear explosion would provoke an intense and brief emission of X-rays. This is what is known as the initial ‘fireball'”, explains one of the authors, Glory Roomprofessor at the Polytechnic University of Catalonia (CPU) and researcher at the Institute of Space Studies of Catalonia (IEEC) in the Astronomy and Astrophysics Group of that university.

The initial phases of a nova explosion had already been predicted: the high temperatures of the thermonuclear explosion would cause a brief, intense emission of X-rays, known as the initial ‘fireball’. It is very brief and occurs hours before the appearance of the star in the sky.

Glória Sala (UPC/IEEC)

During the days after the explosion, the expansion of the fireball causes a drop in temperature that causes it to evolve into a large sphere of cooler gas, which emits visible light and causes the new star to appear in the sky.

But, as Sala specifies, “this ‘fireball’ phase is very brief and occurs hours before the appearance of the star in the sky. So detecting X-rays before the source is discovered is tricky.”

Russian-German mission to scan the sky

Usually, the detection of stars with X-ray emission is carried out from satellites that receive the order to observe in the direction of the discovered source. However, there are some missions that aim to carry out a global mapping of the sky: this is the case of the German eROSITA X-ray telescopedeveloped in Max Planck Institute for Extraterrestrial Physics (MPE)traveling aboard the Russian-German mission Spectrum-X-Gammalaunched in 2019. The mission’s objective is to make a global X-ray map of the sky and, for that, it scans the entire celestial sphere every six months.

During the second sky survey, specifically on July 7, 2020, a new extremely bright X-ray source was detected that lasted less than eight hours. A week later, on July 15, the Nova explosion was discovered from Earth and in visible light. Reticles 2020 (YZ Ret), located at a distance of 2,500 parsecs (astronomical unit of length that corresponds, approximately, to three light years or 30 billion kilometers). This made it possible to identify for the first time that the intense X-ray flash detected by eROSITA corresponded to the initial fireball of the nova explosion.

Understanding the evolution of the galaxy

Studying nova explosions allows us to fit together some of the pieces of the galaxy’s chemical evolution and how it came to have the variety and distribution of chemical elements present in the galaxy. solar systemafter the Big Bang, starting from a primitive universe with a much simpler composition.

The detection of the initial ‘fireball’ predicted by the models is a key piece to verify and adjust the theories of explosions of stellar novae.

The observation of large terrestrial telescopes, together with the study of X-ray and gamma-ray emissions from satellites and theoretical modeling through numerical models, allow us to reconstruct the detailed processes that occur in these explosive phenomena and their contribution to the galaxy evolution.

Therefore, the detection of the initial ‘fireball’ predicted by the models is a key piece to verify and adjust the theories of explosions of stellar novae. For Sala, “the characteristics of the X-ray radiation that we detected with eROSITA coincide with what the theory predicts for this phase of the explosion and confirm, therefore, that it is the piece of the puzzle we were looking for”.

In addition to the FAU, the MPA, the UPC and the IEEC, the University of Tübingen and the Leibniz Institute for Astrophysics Potsdam in Germany participated in this study.

Exhibition of the seven eROSITA mirror modules before being launched into space. /P.Friedrich/MPE


Konig, O.. and others. “X-ray detection of a nova in the fireball phase”. Nature2022


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