Gravity-lensed supernova magnifies understanding of universe

Using an automated supernova-hunting "pipeline," scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have identified a Type Ia supernova that exploded about four billion years ago. Called iPTF16geu, it's sitting behind a galaxy two billion light years away that is acting like a gigantic gravitational magnifying glass to split the star into four bright images in four different parts of the sky. This is the first such supernova to be seen by gravitational lensing and astronomers believe it could provide us with a better understanding of how the universe is expanding.

If you're looking for spectacular, look for supernovae. These aren't so much stars as stellar detonations that explode suddenly and generate enough energy to briefly outshine an entire galaxy. They are also very rare, which is lucky for us, because you don't want to be anywhere in the vicinity of one when it goes off.

However, astronomers are interested in such supernovae for a number of reasons and, in a universe made up of 100 billion galaxies, they still have quite a few to look at on a regular basis. One class of supernova of particular interest is Type Ia, which make up one in every 50,000 supernovae discovered.

Type Ia supernovae are caused when a white dwarf star orbits another star. The dwarf draws in gases from the other star, which eventually build up to the point that the dwarf suddenly and catastrophically collapses in on itself. This implosion detonates the hydrogen and helium in its core, destroying both the dwarf and its companion.

As gripping a light show as this is, scientists are particularly interested in these because if they can get a good look at one, it could help them figure out the rate of the Universe's expansion with much more accuracy.

According to Berkeley Lab, supernova iPTF16geu was found using a technique called gravitational lensing, derived from Einstein's Theory of General Relativity which states that mass bends light. The more mass, the more bending. This means that light traveling past an object will be refracted in a similar way that it does when passing through a lens.

In everyday life, this effect is much too small to be seen, but when a truly massive object, like a black hole or a galaxy is involved, the lens effect is very apparent. If this galaxy is between the Earth and something interesting, that something will look brighter and even as if it is in several different places at the same time. The latter is because, while a galaxy can act as a lens, it isn't a very good lens.

The hard part is putting this effect to practical use. Not only does it require something interesting sitting in just the right spot, but scientists have to be able to sight it and recognize it. This is where the "pipeline" comes in. Situated at the Palomar Observatory in Southern California is the Intermediate Palomar Transient Factory (iPTF). This uses a wide-field camera mounted on the robotic Samuel Oschin Telescope to collect data, which is transmitted to the Department of Energy's (DOE's) National Energy Research Scientific Computing Center (NERSC) at Berkeley Lab.

There, a supercomputer uses machine learning algorithms to process the information in real time and identify candidates for further study. Berkeley Lab says that so far, this has been so successful that the discovery rate has gone from two per month a generation ago to one a day – some within hours of the supernova detonating.

Supernova iPTF16geu was identified by the pipeline on September 5, 2016 and was originally thought to be about one billion light years away, but spectral analysis showed that it was not only a Type Ia supernova, but four time farther than first estimated. In addition, there was another galaxy two billion light years from Earth that was acting as a gravitational lens that was magnifying and splitting the image into four distinct versions in four places.

This last fact is important because it helps measure the expansion rate. For iPTF16geu, each image takes a slightly different path around the lens before reaching Earth. These paths are of different lengths, so they take different amounts of time to arrive. The difference between these times can help to more accurately measure the rate of the expansion of the universe.

This is because Type Ia supernovae have the same luminosity, which makes them what are called "standard candles." In other words, they have properties that allow astronomers to calculate how bright they are and they can use this to figure out how far away they are.

For Type Ia supernovae, they only explode when the white dwarf reaches a critical mass, so the resulting explosion is always the same brightness. Since a supernova can be seen clear across the Universe, this makes them very useful for measuring distances. And if you know the distance and can compare the light-travel times from lensing, you've got a pretty good yardstick for calculating how fast the Universe is expanding.

According to the Berkeley Lab team, the hope is that with the new pipeline, new instruments like the Large Synoptic Survey Telescope should detect 500 strongly lensed Type Ia supernovae over the next decade. If successful, each find could give scientists a four percent or better estimate of the Universe's expansion rate.

"We are just now getting to the point where our transient surveys are big enough, our pipelines are efficient enough, and our external data sets are rich enough that we can weave through the data and get at these rare events," says Danny Goldstein, a UC Berkeley astronomy graduate student. "It's an exciting time to be working in this field."

The research was published in Science.

The animation below shows how the gravity lens works.