What happens when aging white dwarfs merge? Observers in feudal Japan in 1181 had a front-row view of the superpowerful kilonova produced by such a fusion. Their records show that a rare “guest star” flared up and then went out. It took astronomers until 2021 to find the spot in the sky where it happened.
“There are many reports of this temporary guest star in the historical records of 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 disappeared from view. The explosion remnant of SN 1181 is now very old, so it is dark and hard to find,” said Takatoshi Ko, a doctoral student at the University of Tokyo’s Department of Astronomy. Ko led a team that analyzed the observations and performed computer modeling to reposition this ancient stellar disaster.
This kilonova explosion site is still active some 1,800 years later. Astronomers now see a white dwarf embedded in the Cassiopeia Nebula. The star appears to have just started blowing high-velocity winds from its surface in the last few decades.
Anatomy of the Kilonov white dwarf
The original “guest star” is called SN 1181, surrounded by a remnant (SNR 1181) after the explosion. It was formed when two very dense Earth-sized white dwarfs collided. The result was a very rare type of supernova explosion, designated Type 1ax. The explosion blew away rings of material from both stars. At the center of the merger was left a very bright, very hot, rapidly rotating white dwarf called WD J00531. It is surrounded by an infrared nebula called IRAS 00500+6713.
When the white dwarfs merge, astronomers expect both to explode and disappear. Instead, this one created a new white dwarf. A strong stellar wind of 15,000 km/s is rapidly building up. It also experiences a high rate of mass loss through this wind.
Kilonova explosions usually occur when two neutron stars or a neutron star and a black hole collide. So the fact that it appears among the white dwarfs says a lot about the ancestors. Given these characteristics, astronomers think it is a “super-” or “near Chandrasekahr boundary” white dwarf. To obtain such a strange stellar corpse, the ancestors must have been doubly degenerate white dwarfs. In other words, they are at or above the Chandrasekhar limit. This is the mass above which the pressure of electron degeneracy in the star’s core is insufficient to balance its own gravity. In this case, when these two weird white dwarfs merged, they created a newer, weirder version.
Rings around the White Dwarf
SN 1191 lies about 10,100 light-years from Earth – so it’s not close enough to affect us. However, kilonovae can be pretty catastrophic. Experts estimate that if you were a dozen light-years away, it could affect life as gamma rays and other radiation hit the planet.
The resulting kilonova remnant is rather strange in itself. In addition to the super-fast wind, it contains two impact areas. 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 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 simultaneously track the time evolution of each shock region using numerical calculations.
What’s happening?
The team thinks the resulting white dwarf started burning again. This is possibly due to the mass ejected by the kilonova explosion witnessed in 1181 falling back onto its surface. When this happens, the surface density and temperature increase enough to start burning again.
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. They will now focus on further observations of SN 1181 using the Very Large Array radio telescope and the Subaru Telescope in Hawaii. This should allow scientists to further explore the history of this event.
“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 enormous potential for combining different disciplines to reveal new dimensions of astronomical phenomena.”
For more information
A fresh wind blows from the historic supernova
Dynamical Model for IRAS 00500+6713: Remnant of Type Iax Supernova SN 1181 Hosted by Double Degenerate Merger Product WD J005311