The short, but massive outburst of a dead star as it experienced a nova explosion was captured by one of the most powerful X-ray instruments in space.
The joint German-Russian eROSITA telescope, aboard the Spektr-RG Space Observatory at L2 Lagrange point (Yes, Webb’s house), was first captured by what’s known as the “fireball” phase of a new supernova. These X-ray data were finally confirmed by observation with a 1990 prediction about nova physics.
The nova in question is known as YZ Reticuli, which was discovered on July 15, 2020, at a distance of about 8,250 light-years, near the southern constellation of retina. Analysis revealed that this transient brightness was most likely the result of what we call a classic nova – an outburst from a white dwarf star.
Here’s how it works. A white dwarf star is what we think of as a “dead” star – the collapsed core of a star about 8 times the mass of the Sun after reaching the end of its atomic fusion (main sequence) period, expelling its outer matter. Other things of this kind include neutron stars (between 8 and 30 solar masses) and black holes (Anything greater than that).
White dwarfs are small and dense: between the size of the Earth and the Moon, roughly, and up to 1.4 suns in size. This mass limit is known as the Chandrasekhar limit: if a white dwarf exceeds this limit, it becomes so unstable that it explodes in a stunning supernova.
White dwarfs can also often be in binary systems with larger (albeit less massive) stars. If they are in a close enough mutual orbit, the white dwarf can pull material from its binary companion.
This substance is primarily hydrogen. It accumulates on the surface of the white dwarf as it heats up. Eventually, the mass becomes so large that the pressure and temperature at the bottom of the hydrogen layer are sufficient to ignite atomic fusion on the surface of the white dwarf; This leads to a thermonuclear explosion, which violently expels excess material into space. Hello, Nova.
During the second all-sky survey from June to December 2020, eROSITA repeatedly swept the region of the sky containing the white dwarf. On the first 22 passes, everything looked completely normal. However, in the 23rd pass, starting on July 7, 2020, an extremely bright source of X-rays appeared in what was later identified as the YZ Reticuli – only to disappear again on the next pass, meaning that the entire flash could not last more than eight hours .
This was 11 hours before the optical brightness of the source. The astronomers say this is in complete agreement with theoretical modeling of the “fireball” phase of the nova. (Previous observations of the nova fireball were taken at optical wavelengths, and Related to the expanding ejection when the star explodes – A completely different phase of the supernova.)
according to Forecasting progress in 1990, a very short “fireball” phase must occur between the runaway merger causing the explosion and the star’s brightness at optical wavelengths. This phase should appear as a soft, short, bright flash of X-rays before the star shines at optical wavelengths.
This happens, according to the theory, because the expanding matter reaches the white dwarf’s photosphere, or “surface”. For a brief period of time, the outward acceleration of that material matches the inward acceleration due to the star’s gravity, causing the white dwarf to heat up and shine with its maximum luminosity, known as Eddington gloss.
As the explosion continues to expand, it cools, causing the light emitted from the more energetic X-ray wavelengths to travel to light waves. This is usually when we see a bright nova.
The results allowed the team to make some key measurements of the white dwarf in question. This includes the exact timing of the thermonuclear reaction, and the evolution of the white dwarf’s temperature over the entire duration of the nova event. Theoretical work also indicates that the duration of the fireball phase corresponds to the mass of the white dwarf. Using this information, the team extracted a mass of 0.98 times the mass of the Sun.
The team said the note was very lucky. During its four-year mission, eROSITA is expected to detect only one or two of these fireballs, given the nova rate in our galaxy.
“With the successful detection of the YZ Reticuli flash by eROSITA, the presence of the X-ray flashes has now been confirmed by monitoring,” The researchers write in their paper.
“Our discovery also adds the missing piece to measuring the total energy of a supernova and completes the whole picture of thermonuclear escape photosphere evolution.”
The search was published in temper nature.