PS1-10afx, an unusually bright supernova that was discovered in 2010 using the Panoramic Survey Telescope & Rapid Response System, is so luminous because a gravitational lens amplified its light.
This image, taken with the Canada-France-Hawaii-Telescope, shows the field before the supernova PS1-10afx. Image credit: Kavli Institute for the Physics and Mathematics of the Universe / CFHT. The right inset shows PS1-10afx. Image credit: R. Chornock et al.
PS1-10afx demonstrated the same color and the change in brightness over time as a Type Ia supernova, but its peak brightness was 30 times greater than expected.
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 too red.
These anomalies led some astronomers to conclude that PS1-10afx was a completely new type of supernova.
“Generally, the rare supernovae that have been found to shine brighter than Type Ia usually have higher temperatures (bluer colors) and larger physical sizes (and thus slower light curves). New physics would thus be required to explain PS1-10afx as an intrinsically luminous supernova,” explained Dr Robert Quimby from the University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe, the lead author of the study published in the journal Science.
Other astronomers suggested PS1-10afx was a normal Type Ia supernova magnified by a gravitational lens in the form of a massive object.
“We proposed that its exceptional glow could be explained as a gravitationally lensed SNIa (Type Ia supernova), 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,” Dr Quimby said.
“We found a second explanation and it required only well demonstrated physics: gravitational lensing. If there was a massive galaxy in front of PS1-10afx, it could warp space-time to form magnified images of the supernova,” said study co-author Dr Marcus Werner, also from the University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe.
Dr Quimby added: “the 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.”
Magnification of the supernova PS1-10afx by a gravitational lensing. Image credit: Aya Tsuboi / Kavli Institute for the Physics and Mathematics of the Universe.
In 2013, Dr Quimby, Dr Werner and their colleagues aimed to find the hidden lens. Using the Low-Resolution Imaging Spectrograph on the 10-m Keck-I telescope located in Hawai’i, they collected light at the location of PS1-10afx.
They then 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, they would expect to see two sets of gas emission lines – and that is indeed what they saw.
“After carefully extracting the signal from the data, we had confirmation. In the glare of the relatively bright host galaxy, we found a second, foreground galaxy,” said co-author Dr Anupreeta More, also from the University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe.
“This second galaxy was faint enough to have previously gone unnoticed. But our analysis showed that it was still the right size to explain the gravitational lensing of PS1-10afx.”
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Robert M. Quimby et al. 2014. Detection of the Gravitational Lens Magnifying a Type Ia Supernova. Science, vol. 344, no. 6182, pp. 396-399; doi: 10.1126/science.1250903
