The study, which was published in the Nature journal on 11 May, is led by the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and involves the Max Planck Institute for Extraterrestrial Physics (MPE), the University of Tübingen and the Leibniz-Institute for Astrophysics Potsdam. Gloria Sala, a researcher in the Astronomy and Astrophysics Group of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and the Institute of Space Studies of Catalonia (IEEC) and a professor at the UPC’s Barcelona East School of Engineering (EEBE), participates in the research.

Novas are unpredictable stellar explosions that seemingly appear as a “new” star in the sky. The origin of this phenomenon lies in the accumulation of material from a star of a similar size to the Sun (such as hydrogen from the outer layers) on a companion white dwarf, a highly dense type of star with a mass similar to that of the Sun concentrated in a radius equivalent to that of the Earth.

This week in Nature: nova fireball – short bursts of X-rays reveals early stage of an explosion on a white dwarf. Browse the full issue here: https://t.co/78COY06jOl pic.twitter.com/H6YCkN8zl9

— nature (@Nature) May 12, 2022

The extreme conditions on the surface of the white dwarf cause the material accumulated on the surface to explode and be ejected into outer space in a huge thermonuclear explosion. The material expands rapidly and, within a few hours, the visible magnitude of the star increases. The phenomenon can be observed from the Earth as a new star in the sky.

As researcher Glòria Sala explains, the initial stages of a nova explosion “had already been predicted theoretically: the high temperatures of a thermonuclear explosion would cause an intense and brief emission of X-rays. This is known as the initial fireball.” During the days following the explosion, the expansion of the fireball leads to 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 stage is very brief and occurs hours before the star appears in the sky. Therefore, detecting the X-rays before discovering the source is complicated.”

Scanning the sky
Usually, detecting stars with X-ray emissions is done from satellites that are given the order to observe in the direction of the discovered source. Some missions, however, aim to perform a global mapping of the sky: just like the German X-ray telescope eROSITA, developed at the MPE, which travels on board the Russian-German Spectrum-X-Gamma mission, launched from the Baikonur Cosmodrome (Russia) on 13 July 2019. The mission aims to make a global X-ray map of the sky and, to do so, it scans the entire celestial sphere every six months.

During its second mapping of the sky, on 7 July 2020, a new extremely bright X-ray source was detected, lasting less than eight hours. A week later, on 15 July, the Nova Reticuli 2020 (YZ Ret) explosion, located at a distance of 2.5 kpc from Earth (2,500 parsecs, an astronomical unit of length that is approximately equal to 3 light-years or 30 trillion kilometres), was discovered in visible light from Earth. This made it possible to identify, for the first time, that the intense X-ray flash detected by eROSITA corresponded to the initial fireball from the nova explosion.

Understanding the evolution of the Galaxy
The study of nova explosions allows us to fit together some of the pieces of the Galaxy’s chemical evolution and how we have come to have the variety and distribution of chemical elements present in the Solar System after the Big Bang, starting from an initial universe with a much simpler composition. Observation from large ground-based telescopes, together with the study of X-ray and gamma-ray emissions from satellites and theoretical modelling using numerical models, allows us to reconstruct the detailed processes that occur in these explosive phenomena and their contribution to the evolution of our galaxy.

For this reason, detecting the initial fireball predicted by the models is a key piece to test and adjust the theories of stellar explosions. According to Sala, “the characteristics of the X-ray radiation that we detected with eROSITA coincide with what the theory predicts for this stage of the explosion and, therefore, confirm that this is the piece of the puzzle that we were looking for.”

Further information:

• König et al. “X-ray detection of a nova in the fireball phase”, DOI: 10.1038/s41586-022-04635-y