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Supernova hunting with supercomputers

As the yardstick against which outer space is measured, type Ia supernovae are famous for consistency, yet new observations suggest their origins may not be so uniform. Using theoretical calculations and National Energy Research Scientific Computing Center (NERSC) supercomputer simulations, astronomers have for the first time observed a flash of light signaling a supernova collision. This discovery points them to the supernova's home star system and implies there could be two distinct types of Ia supernova.

Type Ia supernovae are significant due to the metric they provide. Their luminosity functions as a constant brightness against which to measure other lights, much like viewing a 100-Watt light bulb on your porch and then from a neighbor's house across the lake. This "standardizable candle" helps scientists calculate cosmological distances.

Simulation of the expanding debris from a supernova explosion (shown in red) running over and shredding a nearby star (shown in blue). Courtesy Daniel Kasen, Berkeley Lab/ UC Berkeley.

"By calibrating the relative brightness of Type Ia supernovae, astronomers use them to discover the acceleration of the universe. But if we want to push further and constrain the detailed properties of the dark energy driving acceleration, we need more accurate measurements," says Daniel Kasen, associate professor of astronomy and physics at the UC Berkeley Astronomy department. "If we don't know where Type Ia supernovae come from, we can't be totally confident that our cosmological measurements are correct."

There are two theories of Ia supernova origin: the single-degenerate model and the double-degenerate model. In both theories, the white dwarf star that eventually becomes a Type Ia supernova is one of a pair of stars orbiting around a common center of mass.

In the double-degenerate model, the supernova ignites when both white dwarfs merge. In the single-degenerate model, a white dwarf star orbits with a sun-like star or a red giant star, pulling material from its neighbor. The white dwarf expands and its core temperature and pressure increase, igniting a runaway nuclear reaction that ends in a dramatic explosion.

In 2010, with the help of US Department of Energy (DOE) Scientific Discovery through Advanced Computing (SciDAC) resources, Kasen made a prediction: In the single-degenerate model, the material ejected from the explosion would slam into its companion star, generate a shockwave that heats the surrounding material, and leave telltale ultraviolet signals in the immediate hours and days after explosion. Recent findings by California Institute of Technology (Caltech) graduate student Yi Cao, lead author of an article published in the 20 May issue of Nature, indicate Kasen was right.

Daniel Kasen. Courtesy UC Berkeley.

Thanks to the intermediate Palomar Transient Factory (iPTF), which uses NERSC supercomputer machine learning algorithms, astronomers were able to find a recent Ia explosion and quickly train NASA's Swift Space Telescope to record the ultraviolet signals Kasen had predicted.

"Kasen's paper was very important to our work. Without it, we wouldn't have known what to look for," says Yi Cao, a graduate student at Caltech and lead author of the Nature article. "With the help of NERSC's Edison supercomputer, the iPTF pipeline can turn up supernova candidates 10-15 minutes after their initial detection. This is crucial to our work searching for the ephemeral signals predicted by Kasen."

Kasen's prediction supported the single-degenerate model of supernova origins, but evidence from the iPTF also supports the double-degenerate model. "The news is that it seems that both sets of theoretical models are right, and there are two very different kinds of Type Ia supernovae," says Sterl Phinney, Caltech professor of theoretical astrophysics.

"It's really exciting to learn that something that once only existed in your imagination is actually out there," says Kasen. "Automated surveys like iPTF have revolutionized the field by catching these events earlier and earlier. It opens up a new avenue for studying the life and death of stars."

Animation showing a binary star system in which a white dwarf accretes matter from a normal companion star. Matter streaming from the red star accumulates on the white dwarf until the dwarf explodes. With its partner destroyed, the normal star careens into space. This scenario results in what astronomers refer to as a Type Ia supernova. Courtesy Walt Feimer and NASA's Goddard Space Flight Center.

Read more aboutsupernova hunting with supercomputers here.

--Linda Vu, Berkeley Lab Computing Sciences communications specialist

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