Science: 'Superluminous' Supernova Brightened by Galactic Lens

An exceptionally bright supernova reported in 2013 gets its intense glow from the lensing effect of a nearby galaxy, according to a study in the 24 April issue of Science.

The galaxy's gravity may bend light from the supernova, called PS1-10afx, in the same way that a glass lens focuses light. The bend directs that light toward Earth and makes the supernova appear super-luminous.

The discovery of the lens settles an important controversy in the field of astronomy, said senior author Robert Quimby of the University of Tokyo's Kavli Institute for Physics and Mathematics of the Universe.

Science authors Robert Quimby (left) and Marcus Werner demonstrate strong gravitational lensing. The water glass serves as a lens, producing the stripe of brighter light against the wall. | AAAS/Science
Schematic illustration of the magnification of PS1-10afx. A massive object between Earth and the supernova bends light rays much as a glass lens can focus light. As more light rays are directed toward the observer than would be without the lens, the supernova appears magnified. | Kavli IPMU

When a team of scientists first observed PS1-10afx in 2013, its exceptional glow puzzled many, leading some to conclude it was a new type of extra-bright supernova. Other researchers suggested it was a normal Type Ia supernova (SN1a) magnified by a lens in the form of a massive object, such as a nearby supermassive black hole.

"The team that discovered it proposed that it was a new type of supernova, one that no theory predicted," said Quimby. There are a few, rare supernovae that have been found with comparable luminosities, he noted, "but PS1-10afx was different in just about every way. It evolved too fast, its host galaxy is too big, and it was way, way too red."

"We proposed that [its exceptional glow] could be explained as a gravitationally lensed SN1a, but we had no direct evidence for the lens," Quimby said. "Thus each explanation [to date] required a bit of magic-new physics or an unseen magnifier-and scientists don't generally buy into magic."

Quimby and colleagues thought that if there was a gravitational lens there to magnify the supernova, it would still be there after the supernova faded away. The team compared spectroscopic data from PS1-10afx's peak brightness period to data from the period after it had faded.

The scientists uncovered two sets of gas emission lines in the data, suggesting there was an additional galaxy coincident with PS1-10afx during its bright period. Their data indicate that the galaxy was in front of PS1-10afx, at just the right angle and distance to amplify the supernova's light and serve as a sort of magnifying glass.

Quimby explained why the lens for PS1-10afx was missed before. "It turns out that PS1-10afx's host galaxy appears brighter than the lens galaxy...so the light of the lens was simply lost in the host's glare."

The lens identified here is the first to strongly magnify a Type 1a supernova. The strong gravitational lensing created "multiple images of the supernova" to give its extra-bright appearance, according to Quimby.

"Strong gravitational lensing is actually quite rare," said Marcus Werner, co-author and Project Researcher at Kavli IPMU. "When it can be observed, it's very important in astronomy. Rather like a telescope, it lets us see objects we couldn't see otherwise." 

The glow of PS1-10afx faded away before researchers could detect each of those images, but in principle, the strong lensing effect offers a way to measure cosmic expansion. "Each image will arrive at a different time, with the exact delay dependent on how the universe is expanding," said Quimby.

Quimby, Werner and several of their coauthors presented the findings at a press conference in Tokyo, organized by Science/AAAS in cooperation with The Kavli IPMU at The University of Tokyo. Reporters from Japan's major media outlets, including The Asahi Shimbun, Yomiuri Shimbun, and NHK, the Japan Broadcasting Corporation attended the event, held in a large, bright room overlooking a pond at the University of Tokyo.

One reporter in the audience, curious for a closer glimpse of the gravitational lens identified in this study, asked Quimby where citizens might look in the sky to see it. "It is in the constellation of Aquarius," Quimby said. "But you'll need a really big telescope to catch a glimpse," he joked.