Ancient People Saw a Kilonova Light up the Sky

What happens when aging white dwarf stars come together? Observers in feudal Japan in the year 1181 had a front-row view of the superpowerful kilonova created by such a merger. Their records show that a rare “guest star” flared up and then faded. It took until 2021 for astronomers to find the place in the sky where it occurred.

“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 Saturn’s. It remained visible to the naked eye for about 180 days, until it gradually dimmed out of sight. The remnant of the SN 1181 explosion is now very old, so it is dark and difficult to find,” said Takatoshi Ko, a doctoral student from the Department of Astronomy at the University of Tokyo. Ko led a team that analyzed observations and did computer modeling to relocate this ancient stellar disaster.

This explosion site of the kilonova is still active some 1,800 years later. Astronomers now see a white dwarf embedded in a nebula in Cassiopeia. The star appears to have just started blowing high-speed winds from its surface in the past few decades.

Anatomy of a White Dwarf Kilonova

The original “guest star” is called SN 1181, surrounded by a remnant (SNR 1181) of the explosion. It formed when two very dense, Earth-sized white dwarfs collided. The result was a very rare type of supernova explosion, labeled Type 1ax. The explosion blew away rings of material from both stars. At the center of the merger remained a very bright, very hot, fast-rotating white dwarf called WD J00531. It’s surrounded by an infrared nebula called IRAS 00500+6713.

When a merger of white dwarfs takes place, astronomers expect that both should explode and disappear. Instead, this one created a new white dwarf. It’s pinning rapidly and blowing off a strong stellar wind at a velocity of 15,000 km/sec. It’s also experiencing a high mass-loss rate via that wind.

Usually, kilonova explosions occur when two neutron stars or a neutron star and a black hole collide. So, for one to occur between white dwarfs says a lot about the progenitors. Given those characteristics, astronomers think this one is a “super-” or “near-Chandrasekahr limit” white dwarf. To get that kind of weirdo stellar corpse, the progenitors had to be double-degenerate white dwarfs. In other words, they are at or above the Chandrasekhar limit. That’s the mass above which electron degeneracy pressure in the star’s core isn’t enough to balance its self-gravity. In this case, when these two oddball white dwarfs combined, they made a newer, weirder version.

Rings around the White Dwarf

SN 1191 lies about 10,100 light-years from Earth—so not close enough to affect us. Nonetheless, kilonovae can be pretty catastrophic. Experts estimate that if you were within a dozen or so light-years away from one, it could affect life as the gamma rays and other radiation slam into a planet.

The resulting remnant of the kilonova is itself somewhat weird. It contains two shock regions in addition to that superfast wind. The outer region is bright in X-rays and is the interface between material ejected from the merger and material in interstellar space. The inner one is a more recent creation. It appears to have begun blowing around 1990 and is dust-rich. “If the wind had started blowing immediately after SNR 1181’s formation, we couldn’t reproduce the observed size of the inner shock region,” said Ko.

“However, by treating the wind’s onset time as variable, we succeeded in explaining all of the observed features of SNR 1181 accurately and unraveling 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.”

What’s Happening Now?

The team thinks the resulting white dwarf has started to burn again. That’s possibly due to matter thrown out by the kilonova explosion witnessed in 1181 falling back to its surface. When that happens, the density of the surface area and the temperature both increase enough to restart burning.

The team deduced this from computer models based on X-ray observations by the Chandra X-ray Observatory, XMM-Newton, and IRAS in the infrared. Now they’ll focus on further observations of SN 1181 using the Very Large Array radio telescope and the Subaru Telescope in Hawai’i. This should allow scientists to probe the history of this event more deeply.

“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 asset to modern astronomy,” said Ko. “Such interdisciplinary research is both exciting and highlights the immense potential for combining diverse fields to uncover new dimensions of astronomical phenomena.”

For More Information

Fresh Wind Blows from Historical Supernova
A Dynamical Model for IRAS 00500+6713: The Remnant of a Type Iax Supernova SN 1181 Hosting a Double Degenerate Merger Product WD J005311