Cosmic Lens Provides Unique View of Supernova

Astronomers have discovered a gravitationally lensed Type Ia supernova that will soon give them a new measure of the universe’s expansion.

“I was baffled.”

That’s how Mansi Kasliwal (Caltech) describes his first impression when he looked at initial images of the supernova dubbed iPTF 16geu. It was clearly a Type Ia, the kind of supernova made famous in its role as standard candle. In 2011 a team of astronomers used just this kind of supernovae to measure luminosity independently of distance — and in the process, discover the existence of dark energy. (They won the Nobel Prize for their effort.)

But this supernova didn’t seem to follow the rule that governed the rest of its type. “It was much brighter than it should have been given its distance from us,” Kasliwal says.

Follow-up observations from the Hubble Space Telescope, the Keck telescope in Hawai‘i, and the European Southern Observatory’s Very Large Telescope in Chile revealed three more supernovae near the first one. They were just like it — identical, in fact. Turns out a foreground galaxy’s gravity had bent the light of supernova iPTF 16geu during its 4.3 billion-year journey toward Earth, dialing up its brightness by a factor of 52 and splitting its light into four images just 0.3 arcseconds apart. The result is a classic Einstein Cross, the first ever for a Type Ia supernova.

Zoom-in on gravitationally lensed supernova

This composite image shows the gravitationally lensed Type Ia supernova iPTF16geu, as seen with different telescopes. The background image shows a wide-field view of the night sky as seen with the Palomar Observatory. The leftmost image shows a Sloan Digital Sky Survey (SDSS) observation. Hubble captured the central image, which shows the foreground galaxy responsible for bending the supernova’s light (SDSS J210415.89-062024.7). The rightmost image was also taken with Hubble and depicts the four images of the supernova explosion around the lensing galaxy.
ESA / Hubble / NASA / SDSS / Palomar Observ. / Caltech

Astronomers have discovered gravitational lenses by the dozen — massive foreground galaxies or galaxy clusters magnify (and distort) the light from background galaxies, giving astronomers a view of the early universe that would otherwise lie out of reach.

But supernovae are rarely captured in such lenses. Their flare of light is too brief — on the order of months or years depending on the type of supernova and its distance. Only one supernova with multiple images has been caught before: Supernova Refsdal, a core-collapse supernova.

Supernova Einstein Cross

Gravitationally lensed supernova iPTF 16geu appears in the shape of an Einstein Cross.
ESA / Hubble / NASA

This discovery was made possible by the Intermediate Palomar Transient Factory, a fully automated 1.2-meter telescope that scans the sky with a large field camera. It’s designed to catch fast-changing celestial events, such as supernovae, in near-real-time. The researchers in Global Relay of Observatories Watching Transients Happen (GROWTH) led the observations that followed up on the supernova’s discovery.

As cool as it is to catch a supernova in a cosmic lens, it’s even cooler that it’s a Type Ia supernova — knowing how bright it really is (and not just how bright it appears to be thanks to the lens’s magnification) tells astronomers how far away it really is and makes tricky lensing calculations a lot more straightforward, with fewer assumptions involved.

Since the light from Supernova iPTF 16geu was split into four images, each of those images took a slightly different path to Earth. Now, the international team of astronomers is calculating the length of each of those paths. Soon, Goobar and colleagues will have a measure of the Hubble constant, which tells us how quickly the universe is accelerating. That’s a valuable piece of data, as astronomers have been arguing about the Hubble constant for decades, and — even though they’re narrowing in on an answer — debate has only intensified in recent years.

“When people measure the expansion rate of the universe now locally, using supernovae or Cepheid stars, they get a different number from those looking at early universe observations and the cosmic microwave background,” says Ariel Goobar (Stockholm University, Sweden). “There is tension out there and it would be neat if we could contribute to resolving that quest.”

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