So-called Type Ia supernovas serve as standard candles to measure the cosmic expansion because they can be seen to large distances and follow a reliable pattern of brightness. However, until now, scientists have been unsure what actually causes the explosions. "These are such critical objects in understanding the Universe. It was a major embarrassment that we did not know how they worked," said Marat Gilfanov of the Max Plank Institute for Astrophysics (MPA) in Germany and lead author of the study that appears in this week's journal of Nature. "Now we are beginning to understand what lights the fuse of these explosions."

Most scientists agree that a Type Ia supernova occurs when a white dwarf star - a collapsed remnant of an elderly star - exceeds its weight limit, becomes unstable and explodes. There are two main possibilities for pushing the white dwarf over the edge: accretion, in which the white dwarf pulls material from a Sun-like companion star until it exceeds its weight limit, or two white dwarfs merging.

"Our results suggest that the supernovas in the galaxies we studied almost all come from two white dwarfs merging," said co-author Akos Bogdan also of the MPA. "This is probably not what many astronomers would expect."

The difference between these two scenarios may have implications for how these supernovas can be used to track vast cosmic distances. Because white dwarfs can come in a range of masses, it means that the merger of two of them could result in explosions that vary somewhat in brightness.

Because these two different scenarios would generate different amounts of X- ray emission, Gilfanov and Bogdan used NASA's Chandra observatory to observe five nearby elliptical galaxies and the central region of the Andromeda galaxy (a.k.a. M31). A Type Ia supernova caused by accreting material produces significant X-ray emission prior to the explosion. A supernova from a merger of two white dwarfs, on the other hand, would create significantly less.

The scientists found that the observed X-ray emission was a factor of 30 to 50 times smaller than expected from the accretion scenario, effectively ruling out this mechanism. This implies that the white dwarf merger trigger dominates in these galaxies. An open question remains whether these white dwarf mergers are the primary catalyst for Type Ia supernovas in spiral galaxies. This latest result does not directly address this issue, so further studies are required to know if they too are caused by mergers or a mixture of the two processes.

Another intriguing consequence of this result is that a pair of white dwarfs is relatively hard to spot even with the best telescopes. "To many astrophysicists the merger scenario seemed to be less likely because too few double white dwarf systems appeared to exist", said Gilfanov. "Now this path to supernovas will have to be investigated in more detail."

In addition to the X-rays observed with Chandra, other data that were critical for this result came from NASA's Spitzer Space Telescope and the 2MASS survey. The infrared brightness of the galaxies allowed the team to estimate how many supernovas should occur.

Original publication

Marat Gilfanov & Akos Bogdan, "An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate", Nature, 18 February 2010

More information, including images and other multimedia, can be found at http://chandra.harvard.edu, http://chandra.nasa.gov and Computer simulations of merging white dwarfs

Contact:
Dr. Hannelore Hämmerle
Press Officer
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone: +49 89 30000-3980
E-Mail: hhaemmerlempa-garching.mpg.de

Dr. Marat Gilfanov
Max Planck Institute for Astrophysics
Phone: +49 89 30000-2227
E-mail: mgilfanovmpa-garching.mpg.de