Elusive temporary star described in historical documents recreated with new computer model

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white dwarf stars collided to form a temporary “guest star,” now designated supernova (SN) 1181, which was recorded in historical documents in Japan and elsewhere in Asia.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

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

This finding improves our understanding of the diversity of supernova explosions and highlights the benefits of interdisciplinary research that combines history with modern astronomy to enable new discoveries about our galaxy.

The year is 1181 and the Genpei War (1180-1185) has recently begun in Japan. This will lead to a shift in political power from aristocratic families to a new military shogunate that will establish itself in the coastal city of Kamakura near present-day Tokyo.

A record of this tumultuous period was compiled in diary format in Azuma Kagami. He recorded not only people’s lives and key events (with varying degrees of accuracy), but also other daily observations, including the appearance of a new star.

“In historical records from Japan, China, and Korea, there are many reports of this temporary guest star. 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. The remnant of the SN 1181 explosion is now very old , so it is dark and hard to find,” explained lead author Takatoshi Ko, a doctoral student in the Department of Astronomy at the University of Tokyo.

The remnant of this guest star, designated supernova remnant (SNR) 1181, was found to have formed when two extremely dense Earth-sized stars, called white dwarfs, collided. This resulted in a rare type of supernova called a Type Iax supernova, which left behind a single, bright, rapidly rotating white dwarf. With the help of observations of its position recorded in a historical document, modern astrophysicists finally pinpointed its position in the nebula in the constellation Cassiopeia in 2021.

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

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

“If the wind started blowing immediately after SNR 1181 formed, we would not be able to reproduce the observed size of the inner shock region,” Ko said.

“However, by treating the onset time of the wind 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 track the time evolution of each shock simultaneously.” region using numerical calculations.”

The team was very surprised to find that, according to their calculations, the wind could have started blowing only very recently, within the last 20-30 years. They suggest that this may indicate that the white dwarf has started burning again, possibly because some of the matter ejected by the explosion observed in 1181 has fallen back onto its surface, raising its density and temperature past the threshold for it to start burning again.

To validate their computer model, the team is now preparing to make further observations of 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 Telescope in the US state of Hawaii. .

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