Astronomers witness the X-ray ‘fireball’ of a stellar nova for the first time

The brief but colossal flare of a dead star undergoing a nova explosion has been 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 in L2 Lagrange point (yes, Webb’s house), captured for the first time what is known as the “fireball” phase of a classical nova. These X-ray data finally confirmed by observation a 1990 prediction about the physics of novae.

The nova in question is known as YZ Reticuli, discovered on July 15, 2020, at a distance of about 8,250 light-years, near the southern constellation of Reticulum. Analysis revealed that this transient brightening was likely the result of what we call a classic nova – a flare of a white dwarf star.

This is how it works. A white dwarf star is what we consider a “dead” star – the collapsed core of a star that was up to about 8 times the mass of the Sun after reaching the end of its atomic fusion lifetime (sequence main), and ejected its outer material. Other such objects include neutron stars (between 8 and 30 solar masses) and black holes (anything bigger than that).

White dwarfs are small and dense: between the size of the Earth and the Moon, approximately, and up to 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 can also – frequently – be found in binary systems with larger (albeit less massive) stars. If they are in close enough mutual orbit, the white dwarf can siphon material from its binary companion.

This material is mostly hydrogen; it accumulates on the surface of the white dwarf, where it heats up. Eventually, the mass grows so large that the pressure and temperature at the bottom of the hydrogen layer is sufficient to initiate atomic fusion at the surface of the white dwarf; this triggers a thermonuclear explosion, violently expelling excess matter into space. Hello, new.

During its second all-sky survey from June to December 2020, eROSITA repeatedly scanned the region of the sky containing the white dwarf. On his first 22 passes, everything looked perfectly normal, beautiful as a dory could be. On the 23rd pass, however, from July 7, 2020, an extremely bright and soft X-ray source appeared on what was later identified as YZ Reticuli – only to disappear again on the next pass, meaning the entire flash couldn’t have lasted more than eight hours.

This was 11 hours before optical source illumination. This, the astronomers say, was entirely consistent with theoretical modeling of the “fireball” phase of a nova. (Previous observations of a nova fireball have been taken in optical wavelengths, and relate to the expanding ejecta when the star bursts – a completely different stage of nova.)

According to a prediction advanced in 1990, a very brief “fireball” phase should occur between the runaway fusion that triggers the explosion and the brightening of the star in the optical wavelengths. This phase should appear as a soft, short, bright flash of X-ray radiation before the star shines in optical wavelengths.

This, the theory goes, happens because the expanding material reaches the white dwarf’s photosphere, or “surface.” For a brief period of time, the outward acceleration of this material matches the inward acceleration due to the star’s gravity, causing the white dwarf to heat up and glow with maximum brightness, known as Eddington Brightness.

As the explosion continues to grow, it cools, causing emitted light to shift from the more energetic x-ray wavelengths toward the optics. It’s usually when you see a nova light up.

The results allowed the team to make some key measurements of the white dwarf in question. These include the precise timing of the thermonuclear reaction and the temperature trend of the white dwarf throughout the duration of the nova event. Theoretical work also suggests that the duration of the fireball phase corresponds to the mass of the white dwarf. Using this information, the team derived a mass 0.98 times the mass of the Sun.

The sighting, the team said, was very lucky. During its four-year mission, eROSITA should only detect one or two of these fireballs, given the rate of novas in our galaxy.

“With the successful detection of the YZ Reticuli flash by eROSITA, the existence of X-ray flashes has now been confirmed by observation”, the researchers write in their article.

“Our detection also adds the missing piece to measuring the total energetics of the nova and completes the full picture of the photospheric evolution of the thermonuclear runaway.”

The research has been published in Nature.

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