Clue To A 1,000-Year-Old Astrophysical Mystery

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

Researchers from Tel Aviv University and the University of Tokyo have identified a rare “electron-capture” supernova, and in the process may have shed light on an astrophysical mystery dating back nearly 1,000 years.

An electron-capture supernova occurs when a star eight to nine times the mass of the Earth’s sun explodes. A thermonuclear supernova occurs after the explosion of a white dwarf star that has gained matter in a binary star system. A core-collapse supernova occurs when a massive star runs out of nuclear fuel and its core collapses, creating a black hole or a neutron star.

Iair Arcavi, from TAU’s faculty of Exact Sciences, participated in a study that uncovered a third type of stellar explosion. The newly identified electron-capture supernova falls somewhere between the two more well-known types.

The study, published in the scientific journal Nature Astronomy, focuses on a supernova named SN2018zd, discovered in 2018 by a Japanese amateur astronomer.

SN2018zd has unique properties that fit the electron-capture category hypothesized 40 years ago by a Japanese astronomer — specifically, a strong pre-supernova mass loss, an unusual chemical composition, a weak explosion, little radioactivity, and neutron-rich material.

“We started by asking ‘What’s this weirdo?’” said Daichi Hiramatsu of the University of California Santa Barbara and Las Cumbres Observatory, who led the study. “Then we examined every aspect of SN2018zd and realized that all of them can be explained in the electron-capture scenario.”

From left: Japanese amateur astronomer Koichi Itagaki, who discovered the supernova, Tel Aviv University researcher Iair Arcavi, who participated in the study, and University of California graduate student Daichi Hiramatsu, lead author of the study, at one of Itagaki’s telescopes in Japan. (Courtesy of Iair Arcavi)

SN2018zd is relatively nearby — only 31 million light-years away. That’s close enough that it could be identified in pre-explosion archival images taken by the Hubble Space Telescope.

And it may shed light on a supernova observed in A.D. 1054 right in our own Milky Way galaxy.

According to Chinese and Japanese records, this supernova was so bright that it could be seen in the daytime and cast shadows at night. The resulting remnant is known today as the Crab Nebula. While researchers can’t know for sure that the Crab Nebula developed because of an electron-capture supernova, the latest results increase their confidence that this was, in fact, the case.

“It’s amazing that we can shed light on historical events in the universe with modern instruments,” said TAU’s Arcavi. “Today, with robotic telescopes that scan the sky in unprecedented efficiency, we can discover more and more rare events which are critical for understanding the laws of nature, without having to wait 1,000 years between one event and the next.”

Arcavi is a member of the Global Supernova Project and makes use of the Las Cumbres telescope network to study rare transient phenomena like supernovae, neutron star mergers, and stars torn apart by black holes.

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