What happens when aging white dwarf stars merge? Observers in feudal Japan in the year 1181 had a front-row view of the superpowerful kilonova created by such a merger. Their accounts show that a rare “guest star” flared up and then disappeared. It wasn’t until 2021 that astronomers finally found the spot in the sky where it happened.
“There are many records of this temporary guest star in historical documents from Japan, China and Korea.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 explosion remnant of SN 1181 is now very old, so it is dark and difficult to find,” said Takatoshi Ko, a doctoral student in the Department of Astronomy at the University of Tokyo. Ko led a team that analyzed observations and created computer models to locate 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 seems to have started blowing high-speed winds from its surface only 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 was created 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 rings of material away 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 a white dwarf merger occurs, astronomers expect both to explode and disappear. Instead, it created a new white dwarf. It is pinning rapidly, blowing out a strong stellar wind at 15,000 km/sec. It is also experiencing a high rate of mass loss via that wind.
Normally, kilonova explosions happen when two neutron stars or a neutron star and a black hole collide. So when one happens between white dwarfs, it tells us a lot about the progenitors. Given those characteristics, astronomers think this is a “super” or “near-Chandrasekhar limit” white dwarf. To get that kind of weird stellar corpse, the progenitors would have to be doubly degenerate white dwarfs. In other words, they’re at or above the Chandrasekhar limit. That’s the mass above which the electron degeneracy pressure in the core of the star isn’t enough 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 is about 10,100 light-years away from Earth, so not close enough to affect us. Still, kilonovae can be quite catastrophic. Experts estimate that if you were within a dozen or so light-years of a kilonovae, it could affect life, as the gamma rays and other radiation would slam into a planet.
The resulting kilonova remnant is itself a bit odd. 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 region is a more recent creation. It appears to have started blowing around 1990 and is rich in dust. “If the wind had started blowing right after SNR 1181 formed, we would not have been able to reproduce the observed size of the inner shock region,” Ko said.
“However, by treating the wind onset time as a variable, we were able 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 track the time evolution of each shock region, using numerical calculations.”
What happens now?
The team thinks the resulting white dwarf has started burning again. This is possibly because matter thrown out by the kilonova explosion of 1181 falls back to the surface. When this happens, the surface density and temperature both increase enough to start the burning again.
The team derived 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 turn to further observations of SN 1181 using the Very Large Array radio telescope and the Subaru Telescope in Hawaii. This should allow scientists to probe the history of this event in more depth.
“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 for modern astronomy,” Ko said. “Such interdisciplinary research is both exciting and highlights the immense potential for combining diverse fields to discover new dimensions of astronomical phenomena.”
For more information
Fresh wind blows through historic supernova
A dynamical model for IRAS 00500+6713: The remnant of a Type Iax Supernova SN 1181 with a doubly degenerate merger product WD J005311