“Fireball” of a dead star confirmed a 30-year-old prediction

A short-lived but enormously powerful eruption of a dead star was captured by one of the most powerful X-ray instruments in space. The joint German-Russian eROSITA telescope on board the Spektr-RG Space Observatory at the L2 Lagrange point first caught the so-called “fireball” phase of a classical nova. These X-ray data finally confirmed the 1990 prediction about the physics of new stars.

 Nova, known as YZ Reticuli, erupted into a “fireball”

The nova in question, known as YZ Reticuli, was discovered on July 15, 2020 at a distance of about 8250 light-years near the southern constellation Reticulum. The analysis showed that this short-term increase in brightness was the result of what we call a classic nova – the eruption of a white dwarf.

Theory Confirmation

During its second observation of the entire sky from June to December 2020, eROSITA repeatedly scanned an area of the sky containing a white dwarf. On the first 22 passes, everything looked just fine. However, on the 23rd pass, starting on July 7, 2020, an extremely bright X-ray source appeared in the place that was later identified by the star YZ Reticuli. But there was no flash on the next pass, which means it couldn’t have lasted more than eight hours.

Image of eROSITA from May 7, 2020, which shows an X-ray fireball. Photo: Nature

The X-ray flash occurred 11 hours before the optical illumination of the source. This corresponded to the theoretical modeling of the phase of the “fireball” of the new star. According to a prediction made in 1990, a very short “fireball” phase should take place between uncontrolled synthesis and an increase in the brightness of a star in the optical range. This phase should manifest as a short and bright flash of X-ray radiation before the star shines brighter in the optical range.

What is Nova?

A white dwarf is a “dead” star, or rather its collapsed core, whose mass was about 8 times the mass of the Sun after it reached the end of its period of existence as a result of the cessation of nuclear fusion and dropped its outer material. Other objects of this type include neutron stars (from 8 to 30 solar masses) and black holes (more than 30 solar masses). White dwarfs are small and dense. In size, they are approximately between the size of the Earth and the Moon, but with a mass of 1.4 Suns. This mass limit is known as the Chandrasekhar limit: if a white dwarf exceeds this limit, it becomes so unstable that it explodes into a spectacular supernova.

White dwarfs are also sometimes found in binary systems with larger stars. If they are in a sufficiently close mutual orbit, the white dwarf can take material from its binary companion. This material, primarily hydrogen, accumulates on the surface of the white dwarf and heats up. Eventually, the mass becomes so large that the pressure and temperature at the bottom of the hydrogen layer is enough to ignite atomic fusion on the surface of the white dwarf – this causes a thermonuclear explosion, throwing excess material into space. This phenomenon is called a nova.

According to Nature

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