Studies using X-ray and ultraviolet observations from NASA’s Swift satellite provide new
insights into the elusive origins of an important class of exploding star called Type Ia supernova. These explosions, which can outshine their galaxy for weeks, release large and consistent amounts of energy at visible wavelengths. These qualities make them among the most valuable tools for measuring distance in the universe. Because astronomers know the intrinsic brightness of Type Is supernova, how bright they appear directly reveals how far away they are. Astronomers have known for decades that Type Is supernova originate with a remnant star called a white dwarf, which detonates when pushed to a critical mass. The environment that sets the stage for the explosion, however, has been harder to pin down. According to the most popular scenario, 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. In a competing model, the supernova arises when two white dwarfs in a binary system eventually spiral inward and collide. Observations suggest both scenarios occur in nature, but no one knows which version happens more often. Swift’s primary mission is to locate gamma-ray bursts, which are more distant and energetic explosions associated with the birth of black holes. Between these bursts, astronomers can use Swift’s unique capabilities to study other objects, including newly discovered supernovae. The satellite’s X-ray Telescope (XRT) has studied more than 200 supernova to date, with about 30 percent being Type Ia. Stars shed gas and dust throughout their lives. When a supernova shock wave plows into this material, it becomes heated and emits X-rays. The lack of X-rays from the combined supernova shows that supergiant stars, and even sun-like stars in a later red giant phase, likely aren’t present in the host binaries. In a companion study, a team led by Peter Brown at the University of Utah in Salt Lake City 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 no such emission, 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.