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“Standard candle” supernova extraordinarily magnified by gravitational lensing

Researchers have identified what may prove to be the first gravitationally lensed type Ia supernova, which could be used to place constraints on the cosmology of the universe.
SNIa_type-Ia-supernova illustration
Artist's concept of type Ia supernova. // Kavli IPMU
A team of researchers at the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) led by Robert Quimby has identified what may prove to be the first ever type Ia supernova (SNIa) magnified by a strong gravitational lens. In this work, the “standard candle” property of type Ia supernovae is used to directly measure the magnification due to gravitational lensing. This provides the first glimpse of the science that will soon come out of dark matter and dark energy studies derived from deep, wide-field imaging surveys.

Supernova PS1-10afx was discovered by the Panoramic Survey Telescope & Rapid Response System 1 (Pan-STARRS1). PS1-10afx exploded over 9 billion years ago, which places it farther than typical Pan-STARRS1 discoveries. Based on this distance and its relatively bright appearance, the Pan-STARRS1 team concluded that PS1-10afx was intrinsically very luminous. The inferred luminosity, about 100 billion times greater than our Sun, is comparable to members of a new, rare variety of superluminous supernovae (SLSNe), but that is where the similarities end.

SLSNe typically have blue colors, and their brightness changes relatively slowly with time. PS1-10afx, on the other hand, was rather red even after correcting for its redshift, and its brightness changed as fast as normal supernovae. There is no known physical model that can explain how a supernova could simultaneously be so luminous, so red, and so fast.

Soon after the findings were announced, Robert Quimby from Kavli IPMU independently analyzed the data. Quimby is an expert in SLSNe and has played a key role in their discovery. He quickly confirmed part, but not all, of the conclusions. PS1-10afx was, indeed, rather distinct from all known SLSNe, but the data struck Quimby as oddly familiar. He compared the features seen in the spectra of PS1-10afx to known supernovae, and, surprisingly, found an excellent match. The spectra of PS1-10afx are almost identical to normal SNIa.

SNIa have a useful property that has enabled cosmologists to chart the expansion of our universe over the last several billion years: SNIa have strikingly similar peak luminosities that can be rendered even more standard by correcting for how quickly they brighten and fade (their "light curves"). This property allows astronomers to use SNIa as standard candles to measure distances, as was key to the discovery of the accelerating expansion of the universe.

How does the light curve of PS1-10afx compare to SNIa? After correcting for time dilation — another consequence of our expanding universe — the light curve of PS1-10afx is perfectly consistent with a SNIa, but the observed brightness of PS1-10afx is far too high for such a distant SNIa.

To understand this mysterious discovery, Quimby tapped into cosmologists and mathematicians at Kavli IPMU, including Marcus Werner, who specializes in mathematical theory of gravitational lensing, and found an explanation: The anomalously high brightness could indicate that PS1-10afx was gravitationally lensed by an object between us and the supernova. While light travels through space in "straight" lines, massive objects warp space and thus cause rays of light to "bend" around them. Thus, if there is a sufficiently massive object aligned between us and PS1-10afx, light rays that would have gone off to other parts of the cosmos will be focused on us, making PS1-10afx appear brighter. This does not change the colors or spectra of the lensed object, nor does it change how fast the supernova evolves. The supernova simply appears brighter than it would otherwise, just as was observed for PS1-10afx. In this case, the lensing object may be detectable even after the supernova has faded away; future observations may thus provide final confirmation of this scenario.

The Kavli IPMU team's identification of the first strongly lensed SNIa is unprecedented but not entirely unexpected. Masamune Oguri, part of Quimby's team, led a paper a few years ago predicting that Pan-STARRS1 was capable of discovering strongly lensed SNIa. He also has shown that such objects may be exploited to place precise constraints on the cosmology of the universe. Now that Quimby's team has shown how to identify them, next-generation surveys with the Hyper Suprime-Cam on Subaru and the planned LSST can be tuned to discover even more strongly lensed SNIa. These discoveries can be used to study the nature of dark matter, test theories of gravity, and help reveal what our universe is made of.
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