Elusive temporary star described in historical documents recreated using new computer model

A mysterious remnant from a rare type of supernova recorded in 1181 has been explained for the first time. Two white dwarf stars collided, creating a temporary “guest star,” now called supernova (SN) 1181, which was recorded in historical records in Japan and elsewhere in Asia. However, after the star dimmed, its location and structure remained a mystery until a team discovered its location in 2021.

Now, through computer modeling and observational analysis, researchers have recreated the structure of the white dwarf remnant, a rare occurrence, explaining its double shock formation. They also found that high-velocity stellar winds may have started blowing from its surface within just the last 20-30 years. The work was published in The Astrophysical Journal.

This discovery improves our understanding of the variety of supernova explosions and highlights the benefits of interdisciplinary research, combining history with modern astronomy to enable new discoveries about our galaxy.

The year is 1181, and the Genpei War (1180–85) has recently begun in Japan. This would lead to a shift of political power from aristocratic families to the new military-based shogunate, which would be based in the coastal city of Kamakura near present-day Tokyo.

A record of this turbulent period was compiled in a diary format in Azuma Kagami. It describes not only people’s lives and major events (with varying accuracy), but other everyday observations, including the appearance of a new star.

“There are many accounts of this temporary guest star in historical records 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 from the explosion remnant SN 1181 is now very old, so it is dark and difficult to find,” explained lead author Takatoshi Ko, a doctoral student from the Department of Astronomy at the University of Tokyo.

The remnant of this guest star, named supernova remnant (SNR) 1181, was found to have been created when two extremely dense, Earth-sized stars called white dwarfs collided. This created a rare type of supernova, called an Iax-type supernova, which left behind a single, bright, fast-spinning white dwarf. Aided by observations of its position noted in the historical document, modern astrophysicists finally pinpointed its location in 2021 in a nebula toward the constellation Cassiopeia.

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

The main challenge was that, according to the conventional understanding, when two white dwarfs collide in this way, they should explode and disappear. However, this merger left behind a white dwarf. The rotating white dwarf was expected to create a stellar wind (a fast-flowing stream of particles) shortly after its formation. However, what the researchers found was something else.

“If the wind had started blowing immediately after the formation of SNR 1181, we could not reproduce the observed size of the inner shock region,” Ko said.

“However, by treating the wind onset time as variable, we were able to accurately explain all the observed features of SNR 1181 and reveal the mysterious properties of this high-speed wind. We were also able to simultaneously track the time evolution of each shock. region, using numerical calculations.”

The team was very surprised to find that, according to their calculations, the wind may have only started blowing very recently, within the last 20-30 years. They suggest that this may indicate that the white dwarf is starting to burn again, perhaps due to some of the material ejected from the explosion witnessed in 1181 falling back to its surface, increasing its density and temperature. above a threshold to restart combustion.

To validate their computer model, the team is now preparing to further observe SNR 1181 using the Very Large Array (VLA) radio telescope based in the central US state of New Mexico, and the 8.2-meter Subaru-class telescope in the US state of Hawaii. .

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