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The unexpected revival of supernova 1181 

Supernova 1181, produced by the dramatic collision of two white dwarf stars, was studied using a combination of historical records and modern astronomy. The remnant, now identified in the constellation Cassiopeia, reveals a complex structure with signs of recent re-ignition, providing a unique perspective on the behavior of supernova remnants and their ability to reactivate.

A supernova explosion. Illustration: depositphotos.com
A supernova explosion. Illustration: depositphotos.com

An elusive temporary star described in historical documents has been recreated using a new computer model, showing it may have recently started creating stellar winds. 

A supernova remnant from 1181, created by the collision of two white dwarf stars, has been analyzed using historical records and modern astronomy. The discovery in the constellation Cassiopeia reveals a complex structure with new stellar winds, which could indicate a re-ignition of the remnant star. This interdisciplinary approach provides deep insights into the dynamics of stellar phenomena.

 The mystery of the supernova is revealed

For the first time, an explanation has been given for the mysterious remnants of a rare type of supernova recorded in 1181. Two white dwarf stars collided, creating a temporary "guest star", now labeled as supernova (SN) 1181, documented in historical documents in Japan and elsewhere in Asia. However, after the star faded, its location and structure remained a mystery until the research team identified its location in 2021.

Now, using computer models and analysis of observations, researchers have reconstructed the structure of the white dwarf star, a rare phenomenon, and explained the formation of its double shock wave. They also discovered that fast interstellar winds may have been blowing off its surface only in the last twenty to thirty years. This finding improves our understanding of the diversity in supernova explosions, and highlights the benefits of interdisciplinary research that combines history with modern astronomy to enable new discoveries about our galaxy.

 Historical and observational context

The year is 1181 and in Japan the Genpei War (1180-1185) has recently started. The war would lead to a shift in political power from aristocratic families to a new military shogunate (from Shogun), which would be based in the coastal city of Kamakura near modern Tokyo. Records of this tumultuous period were recorded in a diary written by Bazuma Kagami. He recorded not only the lives of people and major events (with varying degrees of accuracy), but also other daily observations, including the appearance of a new star.

“There have been many records of this temporary guest star in historical documents from Japan, China and Korea. At its peak, the star's brightness was comparable to that of Saturn. It remained visible to the naked eye for about 180 days, until it gradually faded and disappeared from view. The explosion remnants of SN 1181 are now very old, and therefore dark and difficult to find," explained lead author Takatoshi Ko, a PhD student from the Department of Astronomy at the University of Tokyo.

This illustration depicts the evolution of the SN 1181 remnant, from its formation when a carbon-oxygen white star and an oxygen-neon white star merged, to the formation of its two shock regions. Credit: 2024 T. Ko
This illustration depicts the evolution of the SN 1181 remnant, from its formation when a carbon-oxygen white star and an oxygen-neon white star merged, to the formation of its two shock regions. Credit: 2024 T. Ko

 Discovery and analysis of the SN 1181 remnant

The remnant of this guest star, labeled Supernova Remnant (SNR) 1181, was found to have formed when two very dense, Earth-sized white dwarf stars collided. It produced a rare type of supernova, called an Iax type supernova, which left behind a bright, rapidly rotating white star. Using observations of its location recorded in a historical document, modern astrophysicists were able to identify its location in 2021 in a nebula toward the constellation Cassiopeia.

Because of its rare nature and location within our galaxy, SNR 1181 has been the subject of much observational research. This indicated that SNR 1181 consists of two shock regions, an outer region and an inner region. In this new study, the team studied the latest X-ray data to build a theoretical computer model that explains these observations, and that reconstructs the unexplained structure of the supernova remnant.

 Challenges in understanding supernova properties

The main challenge was that, according to conventional understanding, when two white dwarf stars collide like this, they should explode and disappear. However, this merger left behind a white star. The rotating white star was expected to produce a stellar wind (a fast stream of particles) immediately after its formation. However, what the researchers found was different.

"If the wind had started blowing immediately after the formation of SNR 1181, we would not have been able to reproduce the size of the observed inner shock region," Ko said. “However, by treating the wind onset time as a variable, we were able to explain all the observed properties of SNR 1181 accurately and resolve the mysterious properties of this fast wind. We were also able to follow the time evolution of each shock zone at the same time, using numerical calculations."

 Future research and interdisciplinary contributions

The team was very surprised to find that according to their calculations, the wind may have started blowing only recently, in the last twenty-thirty years. They suggest that this may indicate that the white star has begun to burn again, perhaps due to material ejected from the explosion observed in 1181 falling back onto its surface, increasing its density and temperature above the threshold necessary for re-ignition.

To validate their computer model, the team is now preparing for additional observations of SNR 1181 using the Very Large Array (VLA) radio telescope in central New Mexico and the 8.2-meter Subaru Telescope in Hawaii.

"The ability to determine the age of supernova remnants or their luminosity at the time of their explosion through archaeological perspectives is a rare and invaluable asset to modern astronomy," Ko said. "Such interdisciplinary research is exciting and highlights the enormous potential for combining diverse fields to reveal new dimensions of astronomical phenomena."

for the scientific article

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