What happens when aging white dwarf stars merge? Observers in feudal Japan in 1181 had their first glimpse of the superpowerful kilonova created by such a merger. Their data shows 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 happened.
“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 SN 1181 explosion remnant is now very old, so it’s dark and hard to find,” said Takatoshi Ko, a Ph.D. from the Department of Astronomy at the University of Tokyo. Ko led a team that analyzed the observations and made computer modeling to relocate this ancient stellar disaster.
This kilonova explosion site 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 only begun to blow high-speed winds from its surface in recent decades.
Anatomy of a Kilonova white dwarf
The original “guest star” is called SN 1181, surrounded by a remnant (SNR 1181) of the explosion. It was formed when two very dense, Earth-sized white dwarfs collided. The result was a very rare type of supernova explosion, named Type 1ax. The explosion blasted rings of material from both stars. At the center of the merger remained a very bright, very hot, rapidly rotating white dwarf called WD J00531. It is surrounded by an infrared nebula called IRAS 00500+6713.
When a white dwarf merger occurs, astronomers expect both to explode and disappear. Instead, it created a new white dwarf. It is rapidly accelerating and is blowing a strong stellar wind at a speed of 15,000 km/sec. It is also experiencing a high rate of mass loss through that wind.
Typically, kilonova explosions occur when two neutron stars or a neutron star and a black hole collide. So for one to happen among white dwarfs says a lot about the ancestors. Given these characteristics, astronomers think this is a “super” or “near Chandrasekahr limit” white dwarf. To get that kind of strange stellar corpse, the progenitors had to be binary degenerate white dwarfs. In other words, they are at or above the Chandrasekhar limit. This is the mass above which the electron degeneracy pressure in the star’s core is not sufficient to balance its self-gravity. In this case, when these two weird white dwarfs got together, they made a newer, weirder version.
Ring around the White Dwarf
SN 1191 is about 10,100 light-years from Earth—so not close enough to affect us. However, kilonovae can be quite catastrophic. Experts estimate that if you were a dozen or more light years away from one, it could affect life as gamma rays and other radiation collide with a planet.
The resulting kilonova remnant is itself somewhat strange. It features two shocking regions in addition to that super fast 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 interior is a newer creation. It appears to have started blowing around 1990 and is rich in dust. “If the wind had started blowing right after the formation of SNR 1181, we could not 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 simultaneously track the temporal evolution of each shock region, using numerical calculations.
What is happening now?
The team thinks the resulting white dwarf has begun to burn again. This is probably due to matter ejected from the kilonova explosion, witnessed in 1181, falling back to its surface. When this happens, the surface density and temperature increase enough to restart combustion.
The team deduced this from computer models based on X-ray observations from the Chandra X-ray Observatory, XMM-Newton and IRAS infrared. They will now 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 deeper into the history of this event.
“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.”
For more information
Fresh wind blows from the Historic Supernova
A dynamical model for IRAS 00500+6713: The remnant of a type Iax supernova SN 1181 hosting a degenerate double merger product WD J005311