One of the most powerful X-ray instruments in space has captured a brief eruption of a dead star.
The eROSITA telescope aboard the Spektr-RG space observatory caught for the first time what is known as the "fireball" phase of a classical telescope. The 1990 prediction about the physics of novae has been confirmed by the X-ray data.
On July 15, 2020, the nova YZ Reticuli was discovered at a distance of around 8,250 light-years from the southern constellation of Reticulum. A classical nova is an eruption from a white dwarf star.
Here is how it works. A white dwarf star is a star that was up to 8 times the mass of the Sun after it reached the end of its atomic fusion. Other objects of this kind include black holes and neutron stars.
White dwarfs are small and dense, between the size of Earth and the Moon, and up to as massive as 1.4 Suns. If a white dwarf exceeds the Chandrasekhar limit, it will blow up in a spectacular supernova.
White dwarfs can be in systems with less massive stars. The white dwarf can snatch material from the other.
The hydrogen accumulates on the white dwarf's surface, where it heats up. When the mass becomes so great that pressure and temperature at the bottom of the hydrogen layer are enough to ignite atomic fusion on the white dwarf's surface, this causes a thermonuclear explosion, violently expelling the excess material into space. Hello, nova.
During its second all-sky survey from June to December 2020, eROSITA swept the region of sky containing the white dwarf. Everything looked normal on its first 22 passes. On the 23rd pass, on July 7, 2020, a bright X-ray source appeared, only to disappear again at the next pass, meaning the entire flash couldn't be seen.
This was 11 hours before the source's optical brightening. Astronomers say this was consistent with theoretical modeling of the fireball phase of a nova. The nova fireball has been observed in optical wavelength, and the expanding ejection as the star erupts is a different stage of the nova entirely.
A very brief fireball phase should take place between the explosion and the star in optical wavelength, according to a prediction advanced in 1990. This phase should appear as a soft, short, and bright flash of X-radiation before the star lights up.
The theory says that the expanding material reaches the white dwarf's photoosphere. The white dwarf will heat up and shine with maximum luminosity for a brief period of time because of the outward acceleration of that material.
As the explosion continues to expand, it cools down, causing the light emitted to shift from the more energetic X-ray wavelength into the optical. That is usually when we see a nova.
The team has been able to make a few key measurements of the white dwarf. The timing of the thermonuclear reaction and the temperature evolution of the white dwarf are included. The mass of the white dwarf is thought to correspond to the duration of the fireball phase. The mass of the Sun was 0.98 times the mass of the team.
The team said that the observation was very lucky. The rate of novas in our galaxy is expected to cause eROSITA to detect just one or two fireballs.
The researchers wrote in their paper that the existence of X-ray flashes has been observationally confirmed after the successful detection of the flash of YZ Reticuli.
Our detection adds the missing piece to measure the total nova energetics and completes the picture of the photospheric evolution of the thermonuclear runaway.
The research has been published.
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