Mystery of Superluminous Supernova Solved

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In 2010, a team of scientists reported an exceptionally luminous stellar explosion that took place nine billion years ago. Named PS1-10afx, the supernova was shining 30 times brighter than any other in its class. Some astronomers thought it was a new type of extra-bright supernova. 
New research shows how it was all an illusion created with a cosmic magnifying glass. The superluminous supernova was just a normal Type Ia supernova (SNIa) amplified by a hidden lens. In this case, a galaxy filled with dim, old stars. 
PS1-10afx was discovered using the Panoramic Survey Telescope & Rapid Response System 1 in Hawaii. It looked a lot like an SNIa, but it was just too bright. There are a few, rare supernovae that have been found with comparable luminosities, “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,” study author Robert Quimby from the University of Tokyo explains in an AAAS news release. Those rare supernovae usually have higher temperatures, bluer colors, and larger sizes. “New physics would thus be required to explain PS1-10afx as an intrinsically luminous supernova,” he adds
Or, the explanation could simply lie in well-demonstrated physics. Gravitational lensing is a technique astronomers use to measure distances. If there’s nothing between us and a faraway object of known brightness, we just see one image of it. But if something massive drifts by and gets in the way, the gravitational field around this closer, intervening object bends the light — acting like a lens to redirect the light rays. We end up seeing multiple images of the farther object. 
If a closer, massive galaxy is perfectly aligned with PS1-10afx, it would form magnified images of that background SNIa. “Although this warping of space time probably created four separate images of the supernova when viewed from Earth, we find that these likely appeared as a single source after atmospheric blurring,” Quimby tells BBC. Check out this very cool gravitational lensing gif from the researchers.  
However, the gravitational lens would still be there after the supernova faded away. To go back and check for the lens signature, Quimby and an international team used the Low-Resolution Imaging Spectrograph on the 10 meter Keck-I telescope in Hawaii to observe PS1-10afx’s host galaxy and neighboring objects. They spent seven hours collecting light at the location of the supernova, which had by then faded away itself (pictured right). Then they compared data from PS1-10afx’s peak brightness period to data from the period after it had faded. 
If there was an additional galaxy coincident with PS1-10afx during the bright period — serving as the lens — they would see two sets of gas emission lines. “Looking at the spectra we could check to see if there was light coming from two sources at two separate distances,” Quimby tells BBC. And that’s what they found: The presence of a galaxy right in front of PS1-10afx. At just the right angle and distance, it amplified the supernova’s light thirtyfold.
“Buried in the glare of the relatively bright host galaxy, we found a second, foreground galaxy,” ​University of Tokyo’s Anupreeta More says. “This second galaxy was faint enough to have previously gone unnoticed.” It’s the first time a galaxy is found strongly magnifying an SNIa.
The work was published in Science this week. 
Images: Aya Tsuboi/Kavli IPMU (top), Kavli IPMU/Canada-France-Hawaii-Telescope (middle)
Gif: Anupreeta More/Kavli IPMU

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