The eROSITA X-ray Observatory detects for the first time the ‘fireball’ of a nova explosion

For the first time, a research team, led by the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), has been able to observe this type of explosion using the eROSITA X-ray Observatory.

Glòria Sala, a researcher in the Astronomy and Astrophysics Group at the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and a member of IEEC, participates in the study published today in the journal Nature.

When Sun-type stars finish all their fuel, they shrink to form white dwarfs. Sometimes these dead stars come back to life in a very hot explosion and produce a 'fireball' of X-ray radiation. Now, a research team led by the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) has been able to observe, for the first time, this type of X-ray light explosion with the eROSITA X-ray Observatory.

The study, published today, 11 May, in the prestigious journal Nature, involves the Max Planck Institute for Extraterrestrial Physics (MPE), the University of Tübingen, and the Leibniz Institute for Astrophysics Potsdam. Glòria Sala, researcher from the Institute of Space Studies of Catalonia (Institut d'Estudis Espacials de Catalunya — IEEC) at the Astronomy and Astrophysics Group of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and professor at the Barcelona East School of Engineering (EEBE) from the same university, participates in the research.

The novae are unpredictable stellar explosions that apparently appear as a "new" star in the sky. The origin of this phenomenon lies in the accumulation of material from a star similar in size to the Sun (such as hydrogen from the outermost layers) on a companion white dwarf star—a highly compact star—which has a mass similar to the Sun but concentrated in a celestial body with a radius equivalent to that of the Earth.

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 in a few hours causes an increase in the visible magnitude of the star. At this time, the phenomenon can be observed from Earth as a new star in the sky.

As IEEC researcher at UPC Glòria Sala explains, “the initial phases of the explosion of a nova had already been theoretically predicted: the high temperatures of the 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' causes a drop in temperature that causes it to evolve into a large sphere of cooler gas, which emits visible light and causes the appearance of the new star in the sky.

But, as Dr Sala specifies, “this 'fireball' phase is very brief and occurs hours before the appearance of the star in the sky. Therefore, detecting the X-rays before the discovery of the source is complicated”.

Mapping the sky

Normally, the detection of stars with X-ray emissions is performed from satellites that are commanded to observe in the direction of the discovered source. Some missions aim to perform a global mapping of the sky: this is the case of the German X-ray telescope eROSITA, developed at the MPE, which travels on board the Russian-German Spectrum-Roetgen-Gamma mission, launched on 13 July 2019. Its goal is 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 corresponding to approximately 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 explosion of the nova.

Understanding the evolution of the Galaxy

The study of novae explosions allows us to fit together some of the pieces of the Milky Way'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, as well as theoretical modelling using numerical models, allow us to reconstruct the detailed processes that occur in these explosive phenomena and their contribution to our galaxy.

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

Press release prepared in collaboration with the Communication Office of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC), together with the communication offices of the institutions participating in the study.

Main Image

Nova Reticuli 2020 (YZ Ret)
Artistic recreation of the ‘fireball’ that creates the Nova YZ Reticuli / Friedrich-Alexander-Universität Erlangen-Nürnberg.

Links

– IEEC
– UPC
– eROSITA X-ray Observatory

More information

This research is presented in a paper entitled “X-ray detection of a nova in the fireball phase”, by König, O. et al., that appears in the journal Nature on 11 May 2022 (DOI: 10.1038/s41586-022-04635-y).

The Institute of Space Studies of Catalonia (IEEC  — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of 25 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centers, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors. 

IEEC is a private non-profit foundation, governed by a Board of Trustees composed of Generalitat de Catalunya and four other institutions that each have a research unit, which together constitute the core of IEEC R&D activity: the Universitat de Barcelona (UB) with the research unit ICCUB — Institute of Cosmos Sciences; the Universitat Autònoma de Barcelona (UAB) with the research unit CERES — Center of Space Studies and Research; the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) with the research unit CTE — Research Group in Space Sciences and Technologies; the Spanish Research Council (CSIC) with the research unit ICE — Institute of Space Sciences. IEEC is a CERCA (Centres de Recerca de Catalunya) center.

Contacts

IEEC Communication Office
Barcelona, Spain
Ana Montaner and Rosa Rodríguez
E-mail: comunicacio@ieec.cat 

Lead Researcher at IEEC
Barcelona, Spain
Glòria Sala
Institute of Space Studies of Catalonia (IEEC)
Universitat Politècnica de Catalunya · BarcelonaTech (UPC)
E-mail: gsala@ieec.cat, gloria.sala@upc.edu