Novae are caused by runaway thermonuclear burning in the hydrogen-rich atmospheres of accreting white dwarfs, which leads to a rapid expansion of the atmosphere and the ejection of most of its mass. Theory predicts the existence of a ‘fireball’ phase following directly on from the runaway fusion, which should be observable as a short, bright and soft X-ray flash before the nova becomes visible in the optical wavelengths. Now, astronomers using the eROSITA X-ray telescope on board the Spektrum-Roentgen-Gamma (SRG) observatory have observed a X-ray flash associated with a classical nova called YZ Reticuli about 11 hours before its optical brightening.
“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,” said Dr. Glòria Sala, an astronomer in the Departament de Física EEBE at the Universitat Politécnica de Catalunya and the Institut d’Estudis Espacials de Catalunya.
“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 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.”
On July 7, 2020, the German X-ray telescope eROSITA detected an extremely bright X-ray source.
A week later, the Nova Reticuli 2020 (YZ Reticuli) explosion — located some 2,530 parsecs (8,252 light-years) away in the constellation of Reticulum — was discovered in visible light.
“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,” Dr. Sala said.
“It was to some extent a fortunate coincidence, really,” said Dr. Ole König, an astronomer with the Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics at the Friedrich-Alexander-Universität Erlangen-Nürnberg.
“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.”
“We were searching for strong flaring objects in the eROSITA data,” said Dr. Riccardo Arcodia, an astronomer at the Max Planck Institute for Extraterrestrial Physics.
“And this flare was so strong that at first we discussed whether it was even real. We immediately realized that we had stumbled across a unique event.”
“The study of nova explosions allows us to fit together some of the pieces of 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,” Dr. Sala said.
“Observation from large ground-based telescopes, together with the study of X-ray and gamma-ray emissions from satellites and theoretical modeling 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.”
“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.”
A paper on the findings appears in the journal Nature.
O. König et al. 2022. X-ray detection of a nova in the fireball phase. Nature 605, 248-250; doi: 10.1038/s41586-022-04635-y
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