On July 7, 2020, the eROSITA X-ray instrument captured an astronomical event that – until then – had only been theorized and never seen. He saw the detonation of a nova on a white dwarf star, which produced a so-called X-ray fireball explosion.
“It was to some extent a happy coincidence, really,” said Ole König of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), who led the team of scientists who published a new paper on the discovery. . “These X-ray flashes only last a few hours and are almost impossible to predict, but the observing instrument must be pointed directly at the explosion at exactly the right time.”
A white dwarf is a small, very dense star that forms when a low-mass star uses up all of its fuel. Typically, white dwarfs are the size of a planet. Sometimes these dead stars come back to life in a super hot explosion and produce a fireball of X-rays, then they gradually return to their original brightness. Unlike a supernova, which is an explosion of a star where most of the material is ejected from the star, a nova does not eject much mass from the star and can occur multiple times in a star’s lifetime. a white dwarf.
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“These so-called novae happen all the time, but detecting them in the very first moments when most of the X-ray emission is produced is really difficult,” said Dr Victor Doroshenko from the University of Tübingen, also a member of the research team. “Not only the short duration of a flash is a challenge, but also the fact that the spectrum of X-rays emitted is very soft. Soft X-rays are low in energy and easily absorbed by the interstellar medium, so we cannot see very far in this band, which limits the number of observable objects, whether it is a nova or an ordinary star. Telescopes are normally designed to be more effective in harder X-rays where absorption is less important, and that is exactly why they would miss an event like this!
eROSITA is the main instrument aboard the Russian-German Spectrum-Roentgen-Gamma (SRG) mission, which, like the James Webb Space Telescope, is in a halo orbit around the second Lagrange point. He has been in space since 2019 and is conducting the first all-sky survey in the medium-energy X-ray range (down to 10 keV).
The existence of a “fireball” phase immediately after the runaway fusion nova has been theorized for 30 years. Scientists predicted that it should be observable as a short, bright, soft X-ray flash before the nova becomes visible in the optical range. The trick to seeing it, as König said, is to have your telescope pointed in the right place at the right time.
In their paper, published in Nature, the team said no X-ray sources were detected 4 hours either side of when the visible flash was detected, limiting the length of time flash less than 8 hours.
The nova occurred on YZ Reticuli, also known as Nova Reticuli 2020, which was a naked-eye nova in the constellation Reticulum discovered on July 15, 2020. It was previously known as VY Sculptoris type object with the designation MGAB-V207. Nova Reticuli 2020 was spotted by veteran comet hunter Robert McNaught of Coonabarabran, Australia. It was visible in the southern hemisphere for several days, and barely visible to the naked eye.
“The physical origin of X-ray emission from white dwarf atmospheres is relatively well understood, and we can model their spectra from first principles and in exquisite detail,” said Dr Valery Suleimanov, also from the University of Tubingen. “Comparing the models with the observations then makes it possible to learn the basic properties of these objects such as weight, size or chemical composition.”
“The problem in this particular case,” Doroshenko added, “was, however, that after 30 years without photons, we suddenly had too many, which distorted the spectral response of eROSITA, which was designed to detect millions of objects. very faint rather than more than one but very bright.
Since these novae burn out very quickly, they quickly cool and the X-radiation weakens until it eventually becomes visible light. Since these novae are only visible after the X-ray flash, it is very difficult to predict such outbreaks and it is mostly chance when they hit the X-ray detectors. Visible light has reached Earth 12 hours after the detection of eROSITA, when it was first observed by McNaught.
“A seemingly bright star then appeared, which was actually visible light from the explosion, and so bright that it could be seen in the night sky with the naked eye,” König said. “We were really lucky.”
paper in nature
Press release from the University of Tübingen