TYPE 1A SUPERNOVAE ORIGINS

The following excerpt is from the NASA website:

WASHINGTON -- 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 supernovae. 

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 Ia supernovae, how bright they appear directly 
reveals how far away they are. 

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

Astronomers have known for decades that Type Ia supernovae 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. 

"A missing detail is what types of stars reside in these systems. They 
may be a mix of stars like the sun or much more massive red- and 
blue-supergiant stars," said Brock Russell, a physics graduate 
student at the University of Maryland, College Park, and lead author 
of the X-ray study. 

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 supernovae to date, with about 30 percent being Type Ia. 

Russell and Immler combined X-ray data for 53 of the nearest known 
Type Ia supernovae but could not detect an X-ray point source. 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 supernovae 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. The X-ray findings will appear in the April 1 issue of The 
Astrophysical Journal Letters; the ultraviolet results appear in the 
April 10 edition of The Astrophysical Journal. 

The ultraviolet studies rely on early, sensitive observations. As 
Brown's study was being written, nature provided a great case study 
in SN 2011fe, the closest Type Ia supernova since 1986. Early Swift 
UVOT observations show no ultraviolet enhancement. According to the 
findings in an unpublished study led also by Brown, this means any 
companion must be smaller than the sun. 

Swift data on SN 2011fe also figure prominently in unpublished studies 
led by Alicia Soderberg at the Harvard-Smithsonian Center for 
Astrophysics in Cambridge, Mass. Preliminary results suggest that the 
explosion was caused by merging white dwarfs. 

Swift launched in November 2004 and is managed by Goddard. It is 
operated in collaboration with Pennsylvania State University and 
other national and international partners.