Finding brighter ‘Standard Candles’

Print edition : May 12, 2017

Image of iPTF16geu Type Ia supernova. Photo: W.M. Keck Observatory

AN international team of astronomers has, for the first time, seen a cosmic magnification of the light from a class of supernova called Type Ia. Type Ia supernovas, often referred to as “standard candles” because of their well-known intrinsic brightness, are frequently used by astronomers as yardstick to accurately measure cosmic distances and deduce the expansion rate of our universe at different epochs. Finding a magnified, or “gravitationally lensed”, Type Ia supernova is like discovering a brighter candle with which to view the universe.

The researchers say that this discovery is the first of many to come, and that having a whole collection of similarly lensed Type Ia supernovae will lead to more precise measurements which could answer some of the most intriguing questions about the universe.

Gravitational lensing occurs when the gravity of a cosmic object, such as a galaxy, bends and magnifies the light of a more distant object. The effect can produce multiple images of the same object. While this phenomenon of gravitational lensing has been observed many times since Albert Einstein predicted it, imaging a lensed Type Ia supernova has proven formidably difficult, until now.

In the new study, published in the latest issue of Science, the researchers imaged the Type Ia supernova called iPTF16geu and found its four different images.

“Resolving, for the first time, multiple images of a strongly lensed ‘standard candle’ supernova is a major breakthrough,” said Ariel Goobar of the University of Stockholm, Sweden, the lead author of the study. “Normally, when we view a lensed object, we don't know the intrinsic brightness of that object, but with a Type Ia supernova, we do. This will allow us to better quantify and understand the phenomenon of gravitational lensing.”

Goobar and his group are partners in two Caltech-led international scientific collaborations, the intermediate Palomar Transient Factory (iPTF) and the Global Relay of Observatories Watching Transients Happen (GROWTH) project. The iPTF takes advantage of the Palomar Obser<FZ,1,1,11>vatory and its unique capabilities to scan the skies and discover, in near real-time, fast-changing cosmic events such as supernovas. GROWTH manages a global network of researchers and telescopes that can swiftly perform follow-up observations to study these transient events in detail.

“I was baffled when I saw the initial data for iPTF16geu from the Palomar Observatory. It looked like a normal Type Ia supernova but it was much brighter than it should have been, given its distance from us. The rapid follow-up with more powerful facilities confirmed that we had stumbled upon an extremely interesting and rare event,” said co-author Mansi Kasliwal of Caltech, the principal investigator of GROWTH. Within two months of detection, the team observed the iPTF16geu supernova with the NASA/ESA Hubble Space Telescope; the adaptive-optics instruments on the W.M. Keck Observatory atop Mauna Kea, Hawaii; and the VLT telescopes in Chile. Apart from producing a striking visual effect, capturing the image of a strongly lensed Type Ia supernova such as iPTF16geu is extremely useful scientifically. Astronomers can measure very accurately how much time it takes for the light from each of the multiple images of the supernova to reach us.