radioactivity in a supernova remnant
February 20, 2014

NuSTAR Takes A Peek Inside A Supernova

[ Watch the Video: Sloshing Star Goes Supernova ]

April Flowers for - Your Universe Online

For the first time, astronomers have peered into the heart of an exploding star during the final minutes of its life.

This groundbreaking achievement is one of the primary goals of NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) mission, which launched in June 2012. NuSTAR is tasked to measure high energy X-ray emissions from exploding stars, also known as supernovae, and black holes, including the massive black hole at the center of our own Milky Way Galaxy.

The NuSTAR findings, published in Nature, describe the first map of titanium thrown out from the core of a star that exploded in 1671, which produced a beautiful supernova remnant known as Cassiopeia A (Cas A).

Cas A is a well-known supernova remnant that has been photographed by many optical, infrared and X-ray telescopes. These photographs, however, only revealed how the star's debris collided in a shock wave with the surrounding gas and dust, heating it up. NuSTAR's map of high-energy X-ray emissions — created from material produced in the core of the exploding star — is the first ever produced. The core material is the radioactive isotope titanium-44, which is produced in the star's core as it collapsed to a neutron star or black hole. The star's outer layers were blown off by the energy released in the core collapse supernova, and the debris from this explosion has expanded outward ever since about the approximate rate of 3100 miles per second.

"This has been a holy grail observation for high energy astrophysics for decades," said NuSTAR investigator Steven Boggs, UC Berkeley professor and chair of physics. "For the first time we are able to image the radioactive emission in a supernova remnant, which lets us probe the fundamental physics of the nuclear explosion at the heart of the supernova like we have never been able to do before."

"Supernovae produce and eject into the cosmos most of the elements are important to life as we know it," said UC Berkeley professor of astronomy Alex Filippenko, who was not part of the NuSTAR team. "These results are exciting because for the first time we are getting information about the innards of these explosions, where the elements are actually produced."

Among those elements ejected into the cosmos are the gold in jewelry, the calcium in bones and the iron in blood. Small stars like our own sun die less violent deaths, but stars at least eight times as massive blow up in violent supernova explosions, creating high temperatures and particles in the blast that fuse together light elements to create heavier ones.

The data from NuSTAR will help astronomers build 3D computer models of exploding stars, and might eventually lead to understanding some of the mysterious characteristics of supernovae — including the jets of material ejected by some. Prior imaging of Cas A by the Chandra X-ray Observatory has shown jets of silicon emerging from the star.

"Stars are spherical balls of gas, and so you might think that when they end their lives and explode, that explosion would look like a uniform ball expanding out with great power," said Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology. "Our new results show how the explosion's heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating."

The most studied nearby supernova remnant, Cas A is about 11,000 light years from Earth. Since the star exploded 343 years ago, the debris has expanded to about 10 light years across. This has magnified the pattern of the explosion, allowing it to be seen from Earth.

Based on prior observations of the shock-heated iron in the debris cloud, some astronomers thought the explosion was symmetric, or equally powerful in all directions. According to Boggs, the origins of the iron are so unclear, however, that its distribution may not reflect the explosion pattern from the core.

"We don't know whether the iron was produced in the supernova explosion, whether it was part of the star when it originally formed, if it is just in the surrounding material, or even if the iron we see represents the actual distribution of iron itself, because we wouldn't see it if it were not heated in the shock," he said.

The newly created map of titanium-44 does not match the distribution of iron in the remnant. It shows the titanium concentrated at the remnant's center. This contrast strongly suggests that there is cold iron in the interior that Chandra is unable to see. UC Berkeley research physicist Andreas Zoglauer said that iron and titanium are produced in the same place in a star, so they should be similarly distributed in the explosive debris.

"The surprising thing, which we suspected all along, is that the iron does not match titanium at all, so the iron we see is not mapping the distribution of elements produced in the core of the explosion," Boggs said.

The titanium distribution might also point to a possible answer to the mystery of how the star met its end. Using computers to simulate the supernova, researchers have noticed that the main shock wave created as the massive star dies and collapses often stalls out and the star fails to shatter. The NuSTAR data strongly suggests that the star that formed Cas A literally sloshed around, re-energizing the stalled shock wave and allowing the star to finally blast off its outer layers.

"With NuSTAR we have a new forensic tool to investigate the explosion," said Brian Grefenstette of Caltech. "Previously, it was hard to interpret what was going on in Cas A because the material that we could see only glows in X-rays when it's heated up. Now that we can see the radioactive material, which glows in X-rays no matter what, we are getting a more complete picture of what was going on at the core of the explosion."

In addition to using NuSTAR data, Boggs and his colleagues also employ balloon-borne high-energy X-ray and gamma-ray detectors to record the radioactive decay of other elements, including iron, in supernovae to learn more about the nuclear reactions that take place during these brief, catastrophic explosions.

"The radioactive nuclei act as a probe of supernova explosions and allow us to see directly into densities and temperatures where nuclear processes are going that we don't have access to in terrestrial laboratories," Boggs said.

In addition to Cas A, NuSTAR maintains observations of radioactive titanium-44 emissions from a handful of other supernova remnants to determine if the pattern holds for other supernovae as well. To be useful in this determination, the supernova remnants must be close enough to Earth for the debris structure to be seen, yet young enough for radioactive elements like titanium – which has a 60-day half-life – to still be emitting high-energy X-rays.

The data from the NuSTAR map also casts doubt on other supernova explosion models in which the star is rapidly rotating just before it dies, launching narrow streams of gas that drive the explosion. Imprints of jets have been observed before surrounding Cas A, it has never been known if they were triggering the explosion. In the narrow regions matching the jets, NuSTAR does not see titanium, essentially the radioactive ash from the explosion. This means the jets were not the explosive trigger.

"This is why we built NuSTAR," said Paul Hertz, director of NASA's astrophysics division in Washington. "To discover things we never knew – and did not expect – about the high-energy universe."

Image 2 (below): These illustrations show the progression of a supernova blast. A massive star (left), which has created elements as heavy as iron in its interior, blows up in a tremendous explosion (middle), scattering its outer layers in a structure called a supernova remnant (right). Credit: NASA/CXC/SAO/JPL-Caltech