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Sixteen ways of looking at a supernova

Thanks to fast thinking, luck, and gravitational lensing, four telescopes managed to observe a quadruple image of a single supernova. Andrew Masterson reports.

The light from the supernova iPTF16geu and of its host galaxy is warped and amplified by the curvature of space by the mass of a foreground galaxy.
ALMA (ESO/NRAO/NAOJ), L. Calçada (ESO), Y. Hezaveh et al., edited and modified by Joel Johansson

In September 2016, when astronomer Ariel Goobar and his colleagues at the Intermediate Palomar Transient Factory in California saw the image recorded by the facility’s field camera, they knew they had to move fast.

They were looking at something that was simultaneously massive, spectacular, new, short-lived, and a triumphant demonstration of Einstein’s theory of general relativity.

As reported in the journal Science, Goobar, from Stockholm University in Sweden, and his team had discovered a brand new Type 1a supernova, which they later dubbed iPTF16geu.

Any freshly discovered supernova is a significant astronomical find, but in this case its importance was magnified – quite literally – by circumstance.

Einstein’s theory of general relativity predicts that matter curves the spacetime surrounding it. The region of curved spacetime around a particularly massive object – a galaxy, say – can, if the alignment is correct, bend the paths of light travelling through it in such a way as to act as a lens, enlarging the appearance of objects in the distance behind it.

The effect is known as “gravitational lensing” and is well known to astronomers.

From left: an image from the SDSS survey; a zoomed view showing the foreground lensing galaxy; two versions of the four resolved images of the supernova, resolved by the Hubble Space Telescope and the Keck/NIRC2 instrument.
Joel Johansson

Goobar’s team quickly realised that its view of iPTF16geu was an extreme example of the phenomenon. A galaxy situated between Earth and the supernova was magnifying the phenomenon by 50 times, providing an unparalleled view of the stellar explosion. They were also able to see four separate images of the supernova, each formed by light taking a different path around the galaxy.

The light burst from a Type 1 supernova starts to fade precipitously after only a couple of minutes, and disappears pretty much completely after a year.

Realising that the window of opportunity was limited and closing fast, the team hit the phones and did some rapid talking. In a very short period, three other big facilities homed in on iPTF16geu.

As well as the initial Palomar shot, the astronomers captured images from the Hubble Telescope, the Very Large Telescope in Chile, and the Keck Observatory in Hawaii.

The results – multiple observations of multiple images of the supernova event – provide data that will offer insights not only into the supernova itself, but also into the structure of the intervening galaxy and the physics of gravitational lensing.

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Andrew Masterson is news editor of Cosmos.
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