There are about three supernovae per galaxy per century. That means if you’re an astronomer hunting for them, you need to make sure to look at the right object at the right time. And sometimes, by chance, you can stumble upon an incredible find like iPTF16geu.
The supernova is a quadruple treat. The galaxy that hosts iPTF16geu emitted its light more than 4 billion years ago, and that light is gravitationally lensed by a dense galaxy right in front of it. This well-aligned galaxy creates a peculiar optical effect, whereby the background galaxy light is distorted so that the lens splits it into four images of the supernova, each with a magnification of about 50 times.
The discovery, which is reported in Science, was possible thanks to the Intermediate Palomar Transient Factory – an automated survey that scans the sky for lights that shouldn’t be there. It spotted iPTF16geu in September 2016, and the team, led by Professor Ariel Goobar at Stockholm University, was able to follow up with the object using several other instruments.
The high-resolution data showed that the supernova was a type Ia (one-a), a special kind of supernova. These types happen only in binary systems made of a regular star and a white dwarf. The white dwarf often ends up stealing material from the companion, and once it has a mass of 1.44 times our Sun, it turns into a supernova.
Composite image of the supernova, foreground galaxy, and where they are in the sky. ESA/Hubble, NASA, Sloan Digital Sky Survey, Palomar Observatory/California Institute of Technology
The precise limit leads to a predictable and consistent explosion. Astronomers know what to expect from type Ia supernovae. For this reason, these kinds of cosmic explosions are used to work out crucial information, like the acceleration of the universe. They are considered standard candles. We can measure how bright they appear from Earth, and since we can work out how bright they should be up close, we can work out how far they and their host galaxies are.
“Resolving, for the first time, multiple images of a strongly lensed standard candle supernova is a major breakthrough," Professor Goobar said in a statement. "We can measure the light-focusing power of gravity more accurately than ever before, and probe physical scales that may have seemed out of reach until now."
If our worry is the important constants of the cosmos, then the discovery of a lensed type Ia supernova has a lot more importance than just another distance measurement. When massive objects – like galaxies or black holes – bend space-time, they can magnify and distort light, but they don’t act like a common magnifying glass.
The foreground lensing galaxy is not perfectly symmetric, so the path that each photon takes will be slightly different. Some may be longer, some may be shorter. This means that the four images are different snapshots of the supernova at slightly different moments. By studying the differences in the four images, astronomers can learn a lot about the possible changes in the cosmological parameters.
Graphic of how the four images came to form. ALMA (ESO/NRAO/NAOJ), L. Calçada (ESO), Y. Hezaveh et al., edited and modified by Joel Johansson