Swift satellite throws light on origins of supernovae

Posted by Kate Taylor

X-ray and ultraviolet observations from NASA's Swift satellite have given new infoprmation about the origins of Type Ia supernovae.

Because such supernovae release large and consistent amounts of energy at visible wavelengths, it's easy to determine how far away they are, making them extremely valuable tools for measuring distance.

For decades, it's been known that Type Ia supernovae take place when a white dwarf reaches a critical mass and detonates. What exactly trigggers this, though, has been harder to establish.

"For all their importance, it's a bit embarrassing for astronomers that we don't know fundamental facts about the environs of these supernovae," says Stefan Immler, an astrophysicist at NASA's Goddard Space Flight Center.

"Now, thanks to unprecedented X-ray and ultraviolet data from Swift, we have a clearer picture of what's required to blow up these stars."

According to the most popular theory, a white dwarf orbits a normal star and pulls a stream of matter from it. This gas flows onto the white dwarf, which gains mass until it reaches a critical threshold and undergoes a catastrophic explosion. It's unclear, though, what sort of stars reside in these systems - whether it's a mix of stars like the sun or much more massive red and blue supergiants.

A competing model suggests that a supernova arises when two white dwarfs in a binary system eventually spiral inward and collide.

Observations suggest both scenarios occur in nature, but it's unknonw which is more common.

Swift's X-ray Telescope (XRT) has studied more than 200 supernovae to date, of which about 30 percent are Type Ia.

And X-ray data for 53 of the nearest known Type Ia supernovae failed to find an X-ray point source, indicating that supergiant stars, and even sun-like stars in a later red giant phase, likely aren't present in the host binaries.

Meanwhile, a companion study looked at 12 Type Ia events observed by Swift's Ultraviolet/Optical Telescope (UVOT) less than 10 days after the explosion. A supernova shock wave should produce enhanced ultraviolet light as it interacts with its companion, with larger stars producing brighter, longer enhancements. Swift's UVOT detected nothing of the kind, leading the researchers to exclude large, red giant stars from Type Ia binaries.

Taken together, the studies suggest the companion to the white dwarf is either a smaller, younger star similar to our sun or another white dwarf.