Splotchy supernova shakes assumptions.

Splotchy supernova shakes assumptions

The expanding shell of gas surrounding supernova 1987 A is more likely to be lumpy or disk-like than a uniform sphere, says Scott Barthelmy of the NASA Goddard Space Flight Center in Greenbelt, Md. Until now, astronomers have always assumed in their theoretical models that the gas cloud from a supernova explosion has a spherical symmetry as it expands.

Barthelmy and his collaborators measured the gamma-rays given off by the radioactive decay of cobalt-56, which was synthesized in the supernova explosion. The standard model of a supernova explosion predicts that Earth-based observers should see only the gamma-rays coming from the Earth-facing side of the explosion. This forward motion would shift the emitted gamma-rays to a slightly shorter wavelength. Furthermore, because the gamma-rays should be coming from a narrow shell of gas, the range of wavelengths would be small, producing a sharp line in the gamma-ray spectrum. However, the researchers saw a wide spectral line and detected no shift to a shorter wavelength.

Two possible scenarios explain these results. In one, the expanding envelope of gas has broken up into fragments, allowing observers to detect gamma-ray emissions from both the front and the back of the gas cloud. "There may be tunnels and holes so that we can see all the way to the back side," Barthelmy says. Alternatively, the gaseous material may be flattened into a disk that's tilted with respect to the line of sight to the Earth. This would allow observers to detect gamma-rays emitted from material traveling toward or away from the Earth.

"By studying the shape of gamma-ray lines, we're learning a lot about the explosion itself," Barthelmy says. "Until these results, nobody had ever speculated that asymmetries [in the expanding gases] could show up at such an early age."