Elusive temporary star described in historical documents recreated using new computer model

A mysterious remnant of a rare type of supernova recorded in 1181 has been explained for the first time. Two white dwarfs collided, creating a temporary “guest star,” now called supernova (SN) 1181, which was recorded in historical documents in Japan and elsewhere in Asia.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 a team pinpointed its location in 2021.

Using computer modeling and observational analysis, the researchers have now recreated the structure of the remnant white dwarf, a rare phenomenon that explains its formation by double shock. They also discovered that high-speed stellar winds may have started blowing from its surface in just the last 20 to 30 years. The work was published in The Journal of Astrophysics.

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

The year is 1181 and the Genpei War (1180-1185) has just begun in Japan. It will lead to a transfer of political power from aristocratic families to the new army-based shogunate, which will be established in the coastal city of Kamakura, near present-day Tokyo.

The Azuma Kagami diary chronicles this tumultuous period. It not only records people’s lives and significant events (with varying accuracy), but also other daily 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 view. The remnant of the SN 1181 explosion is now very old, so it is dim and difficult 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, called supernova remnant (SNR) 1181, is thought to have been created by the collision of two extremely dense, Earth-sized stars called white dwarfs. This created a rare type of supernova, called a type Iax supernova, which left behind a single, bright, rapidly rotating white dwarf. Aided by observations of its position noted in the landmark paper, modern astrophysicists finally pinpointed its location in 2021 in a nebula near the constellation Cassiopeia.

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

The main challenge was that, according to classical theory, 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 generate a stellar wind (a fast flow of particles) immediately after its formation. But the researchers discovered something else.

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

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

The team was surprised to find that, according to their calculations, the wind had only started blowing very recently, in the last 20 to 30 years. They suggest that this could indicate that the white dwarf has started burning again, perhaps because some of the material released by the explosion observed in 1181 fell back to its surface, increasing its density and temperature beyond a threshold that would allow burning to restart.

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 telescope in the US state of Hawaii.

“The ability to determine the age of supernova remnants or their brightness at the time of their explosion through archaeological insights is a rare and invaluable asset to modern astronomy,” Ko said. “Such interdisciplinary research is both exciting and underscores the immense potential of combining diverse fields to uncover new dimensions of astronomical phenomena.”