Elusive new supernova discovery sheds light on ancient mystery

29 Jun 2021

Colour composite of the electron-capture supernova 2018zd (the large white dot on the right) and the host starburst galaxy NGC 2146. Image: NASA/STScI/J. DePasquale; Las Cumbres Observatory

After eluding scientists for decades, observations from a nearby galaxy are providing evidence for a previously theoretical supernova model.

Evidence of a new electron-capture supernova supports a 40-year-old astronomical theory and could also shed light on 1,000-year-old records from Japan and China.

While there are two primary forms of supernovas observed to date, a recent observation from the ‘nearby’ (31m light-years away) galaxy NGC 2146 fit into neither of those typical forms, leading to a new study and exciting evidence for a decades-old theory.

The new research, published in Nature Astronomy, is led by Daichi Hiramatsu, a graduate student at the University of California, Santa Barbara, and Las Cumbres Observatory.

Hiramatsu is part of the Global Supernova Project, a worldwide team of scientists that found the supernova SN 2018zd had many unusual characteristics, some of which were seen for the first time in a supernova.

“We started by asking, ‘What’s this weirdo?’’ Hiramatsu said. “Then we examined every aspect of SN 2018zd and realised that all of them can be explained in the electron-capture scenario.”

Theorised in 1980 by Ken’ichi Nomoto of the University of Tokyo and colleagues, the theoretical underpinnings for electron-capture supernovas have been set out for years.

This type of supernova should emerge from the explosions of the super-asymptotic giant branch (SAGB) stars.

The stars should have a lot of mass and lose much of it before exploding. This mass near the dying star should have an unusual chemical composition. Then, the electron-capture supernova should be weak, with little radioactive fall-out and with neutron-rich elements at its core.

This is what the supernova should look like, but Nomoto had no physical observations to back up his theory. As SAGB stars are elusive, there was little chance of spotting one’s dying moments.

The only potential written evidence for an electron-capture supernova was in Chinese and Japanese records from 1054 CE, which described a celestial sight that was so bright it was visible for 23 consecutive days and remained in the night sky for two years.

The remnant of this, the Crab Nebula, has been studied in detail. Researchers hypothesised this was an electron-capture supernova – a suspicion that has been supported by observations in how bright this new supernova appears.

The research team said that the luminosity of the historic SN 1054 supernova was probably increased by its supernova ejecta colliding with material from its progenitor star, as was seen with SN 2018zd. So, it would make sense that ancient observers would be so taken with the celestial phenomenon.

Nomoto was excited that his theory had been confirmed. “I am very pleased that the electron-capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago,” he said.

“I very much appreciate the great efforts involved in obtaining these observations. This is a wonderful case of the combination of observations and theory.”

Sam Cox was a journalist at Silicon Republic covering sci-tech news