NuSTAR Proves 1987A Supernova Is Asymmetrical by Detecting Titanium-44 Leftovers

May 8, 2015

A new study using NASA’s X-ray space telescope NuSTAR has found evidence proving that the star that left behind the supernova remnant 1987A exploded in an asymmetrical fashion.

NuSTAR data on the right indicates the titanium-44 is moving away from us. On the left is the diagram showing the neutron star moving in the opposite direction. Credits: NASA/JPL-Caltech/UC Berkeley

Stars are spherical objects that end their life in spectacular explosions called supernovae. Being spherical, one would expect that they explode by expanding in all directions in a similar matter. However, the latest study of the supernova remnant 1987A shows that the bulk of the material is moving away from us while the neutron star left behind is most likely moving toward us.

The star, that left the supernova remnant that was analysed by this study, exploded in 1987, or rather the light of its demise reached us then, since the star was 166,000 light-years away. It was a Type II supernova meaning its core collapsed under the immense gravity near the end of star’s life. In the star’s last moments, there must have been some mechanism that caused the star to explode asymmetrically.

"Stars are spherical objects, but apparently the process by which they die causes their cores to be turbulent, boiling and sloshing around in the seconds before their demise," said Steve Boggs, from the University of California, Berkeley, USA. He is the lead author of the study. "We are learning that this sloshing leads to asymmetrical explosions."

The key to this finding was titanium-44, the radioisotope created at the core of the star, moments before the supernova. It has the mean-lifetime of 85 years, so it is still visible today by X-ray telescopes and it provides us with a direct insight into the mechanism of Type II supernovae.

"Titanium is produced in the very heart of the explosion, so it traces the shape of the engine driving the disassembly of the star," said Fiona Harrison. She is the principal investigator of NuSTAR at the California Institute of Technology, Pasadena, USA. "By looking at the shift of the energy of the X-rays coming from titanium, the NuSTAR data revealed that, surprisingly, most of the material is moving away from us."

Previous observations hinted at asymmetrical nature of Type II supernovae. Chandra X-ray Observatory detected iron heated by the 1987A supernova, but it was not clear whether it originated in the star or it was in the star’s surroundings and was heated after the blast. NuSTAR detected titanium-44 in another supernova remnant, Cassiopeia A, also hinting at asymmetrical explosion, but the evidence was not as solid as in the case of the 1987A.

"Radioactive titanium-44 glows in the X-rays no matter what and is only produced in the explosion," said Brian Grefenstette, a co-author of the study. "This means that we don’t have to worry about how the environment influenced the observations. We are able to directly observe the material ejected in the explosion."

The titanium-44 radioisotope is moving away from us at the speed of 2.6 million kilometres per hour. Although it’s dark and hard to observe, the neutron star that was left behind after the 1987A supernova is travelling in the opposite direction from the titanium-44.

"These explosions are driven by the formation of a compact object, the remaining core of the star, and this seems to be connected to the core blasting in one direction, and the ejected material in the other," added Boggs.

The study appeared in the May 8 issue of Science magazine. NuSTAR stands for Nuclear Spectroscopic Telescope Array and was launched in June 2012.

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