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An exceptionally bright supernova reported in 2013 is so luminous, a new study reports, because a lens in the sky amplified its light. The discovery of the lens settles an important controversy in the field of astronomy.
In 2010, a team of scientists observed a supernova, PS1-10afx, shining brighter than any other in its class.
"PS1-10afx is like nothing we have seen before," said senior author Robert Quimby of the University of Tokyo's Kavli Institute for the Physics and Mathematics of the Universe.
Its exceptional glow puzzled many, leading to some to conclude it was a new type of extra-bright supernova, while others suggested it was a normal Type Ia supernovae (SNIa) magnified by a lens in the form of a massive object, such as supermassive black hole, nearby.
"The team that discovered it," Quimby said, "proposed that it was a new type of supernova, one that no theory predicted."
There are a few, rare supernovae that have been found with comparable luminosities, according to Quimby "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."
His team had another idea. "We proposed that [its exceptional glow] could be explained as a gravitationally lensed SNIa, but we had no direct evidence for the lens. 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's hypothesis, however, came with a testable prediction. If there was a gravitational lens there to magnify the supernova, it would still be there after the supernova faded away. "Thus we could go back and get new and better data to check for a signature of the lens," he said.
His team compared spectroscopic data from PS1-10afx's peak brightness period to data from the period after it had faded. If there were an additional galaxy coincident with PS1-10afx during the bright period, serving as the lens, the scientists would expect to see two sets of gas emission lines -- and that is indeed what they saw.
In this way, the researchers inferred the presence of another galaxy right in front of PS1-10afx - one that, at just the right angle and distance to amplify the supernova's light, served 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."
Critically, the lens identified here is the first to strongly magnify a Type Ia supernova. "We have lots of examples of gravitational lensing," Quimby explained, "but most of these are so-called 'weak' gravitational lensing."
Strong gravitational lensing is different.
"It means that there were multiple images of the supernova formed," Quimby explained. That's what created the extra-bright appearance.
The multiple images created through strong lensing offer a means to test cosmic expansion, the increase in distance between two parts of the universe over time. "Each image will arrive at a different time with the exact delay dependent on how fast the universe is expanding. In principle, measuring this delay provides a direct way [more so than existing methods] to measure cosmic expansion."
The scientists couldn't do this with PS1-10afx because it faded away before they identified its importance.
"But now we know what to look for," Quimby said. And this may enable scientists to use future lensed supernova events to measure expansion in the universe.
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