Maunakea, Hawai’i – Astronomers have captured a bizarre image of a supernova, the powerful explosion of a star, whose light was so warped by the gravity of a galaxy that it appears as multiple images in the sky. This effect, known as gravitational lensing, occurs when the gravity of a dense object distorts and brightens the light of an object behind it.
A team led by Ariel Goobar from the Oskar Klein Centre at Stockholm University discovered that the unusual Type Ia supernova dubbed “SN Zwicky” was quadruply lensed, meaning that four images of the same supernova could be seen from Earth.
The results, which include observations from W. M. Keck Observatory on Maunakea, Hawai’i Island, are published in the publiction Nature Astronomy.
Within weeks of detecting the supernova at the Zwicky Transient Facility (ZTF) at Palomar Observatory, Goobar and his team used Keck Observatory’s Near-Infrared Camera 2 (NIRC2) paired with its adaptive optics system and successfully resolved SN Zwicky, revealing that the lensing of the supernova was strong enough to have created multiple images of the same object.
“I was observing that night and was absolutely stunned when I saw the lensed image of SN Zwicky,” says Christoffer Fremling, a staff astronomer at the Caltech Optical Observatory who leads the ZTF supernova survey, called the Bright Transient Survey.“We catch and classify thousands of transients with the Bright Transient Survey, and that gives us a unique ability to find very rare phenomena such as SN Zwicky.”
“With ZTF, we have the unique ability to catch and classify supernovae in near real time. We noticed that SN Zwicky was brighter than it should have been given its distance to us and quickly realized that we were seeing a very rare phenomenon called strong gravitational lensing,” says Goobar, lead author of the study and the director of the Oskar Klein Centre at Stockholm University. “Such lensed objects can help us to uniquely probe the amount and distribution of matter at the inner core of galaxies.”
The Very Large Telescope, NASA’s Hubble Space Telescope, Hobby-Eberly Telescope, Liverpool Telescope, and the Nordic Optical Telescope were also used inthis study.
As predicted by Albert Einstein more than a century ago, light from one cosmic object,that encounters a dense object on its way to us can undergo gravitational lensing. The dense object acts like a lens that can bend and focus the light. Depending on how dense the lens is and the distance between the lens and us, this warping effect can vary in strength. With strong lensing, the light from the cosmic object is so distorted that it is magnified and split into several copies of the same image.
Astronomers have been observing the gravitational bending of light since 1919, just a few years after Einstein developed the theory, but the transient nature of supernovae makes events such as SN Zwicky, also known as SN 2022qmx, very hard to spot. In fact, while scientists have spotted lensed duplicated images of distant objects called quasars many times before, only a handful of supernovae lensed into duplicated images have been found. One classic example, called iPTF16geu, was discovered by the intermediate Palomar Transient Factory (iPTF), a predecessor to ZTF.
“SN Zwicky is the smallest resolved gravitational lens system found with optical telescopes. iPTF16geu was a wider system but had larger magnification,”says Goobar. SN Zwicky is what is known as a Type Ia supernova. These are dying stars that end their lives with a light show that is always the same in brightness from event to event. This unique property was used to reveal the accelerated expansion of our universe back in 1998 due to an as-yet unknown phenomenon called dark energy.
“Strongly lensed Type Ia supernovae 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,” says Joel Johansson, a postdoctoral fellow at Stockholm University and a co-author of the study.
“What are missing components needed to model the expansion history of the universe? What is the dark matter that makes up the vast majority of the mass in galaxies? As we discover more ‘SN Zwickys’ with ZTF and the upcoming Vera Rubin Observatory, we will have another tool to chip away at the mysteries of the universe and find answers,”says Goobar.