Astronomers have spotted a rare and spectacular sight in the distant universe: a supernova whose light was split into four images by the gravity of a galaxy in front of it. The supernova, named SN Zwicky, was a type Ia explosion, which occurs when a white dwarf star is destroyed by its companion star. Type Ia supernovas are important for measuring cosmic distances and studying the mysterious dark energy that is accelerating the expansion of the universe.
SN Zwicky was first detected by the Zwicky Transient Facility (ZTF) at Palomar Observatory in California on August 21, 2022. The ZTF is a survey telescope that scans the sky for transient objects, such as supernovas, asteroids and black hole mergers. The supernova appeared unusually bright, which prompted further observations by other telescopes, including the W.M. Keck Observatory, the Very Large Telescope, the Hubble Space Telescope and the Nordic Optical Telescope.
The follow-up observations revealed that SN Zwicky was actually four images of the same supernova, arranged in a cross-like pattern around a faint galaxy. This galaxy acted as a gravitational lens, bending and magnifying the light from the supernova as it passed by. This phenomenon was predicted by Albert Einstein more than a century ago, based on his theory of general relativity. According to this theory, massive objects can warp the fabric of space-time around them, creating a lens-like effect for distant light sources.
Gravitational lensing is a rare and powerful tool for astronomers, as it can reveal objects that would otherwise be too faint or too small to be seen. For example, gravitational lensing can help detect planets around other stars, reveal hidden galaxies and clusters of galaxies, and probe the distribution of dark matter in the universe.
However, finding a gravitationally lensed supernova is even rarer, as supernovas are short-lived events that last only a few weeks or months. Only a handful of such cases have been discovered so far, and SN Zwicky is the first one to be found by ZTF.
SN Zwicky is also special because it is a type Ia supernova, which has a consistent brightness that can be used as a standard candle to measure distances in the universe. By comparing the apparent brightness of SN Zwicky with its true brightness, astronomers can estimate how far away it is. In this case, SN Zwicky exploded more than 4 billion light-years away from us, while the lensing galaxy is about 2.5 billion light-years away.
Type Ia supernovas were also used to discover the accelerated expansion of the universe in 1998, which implies that there is an unknown force called dark energy that is pushing the universe apart faster and faster. By studying more type Ia supernovas, especially those that are gravitationally lensed, astronomers hope to learn more about the nature and origin of dark energy.
“Strongly lensed type Ia supernovas allow us to see further back in time because they are magnified. Observing more of them will give us an unprecedented chance to explore the nature of dark energy,” said Joel Johansson, co-author of a study on SN Zwicky published in Nature Astronomy on June 12, 2023.
The study was led by Ariel Goobar of Stockholm University in Sweden, who said that SN Zwicky is “the smallest resolved gravitational lens system found with optical telescopes.” The team also found that two of the four images of SN Zwicky were brighter than expected, which could be due to smaller microlensing events within the lensing galaxy that further amplified the supernova’s light.
The discovery of SN Zwicky is an example of how modern astronomical instruments can reveal new and exciting phenomena in the cosmos. “I was observing that night and was absolutely stunned when I saw the lensed image of SN Zwicky,” said Christoffer Fremling of Caltech, who was part of the ZTF team that detected the supernova. “The discovery showcases the remarkable capabilities of ZTF and its ability to find rare transients.”
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