NASA's Chandra Reveals Origin of Key Cosmic Explosions
WASHINGTON -- New findings from NASA's Chandra X-ray Observatory have
provided a major advance in understanding a type of supernova
critical for studying the dark energy that astronomers think pervades
the universe. The results show mergers of two dense stellar remnants
are the likely cause of many of the supernovae that have been used to
measure the accelerated expansion of the universe.
These supernovae, called Type 1a, serve as cosmic mile markers to
measure expansion of the universe because they can be seen at large
distances, and they 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," said
Marat Gilfanov of the Max Planck Institute for Astrophysics in
Germany and lead author of the study that appears in the Feb. 18
edition of the journal Nature. "It was a major embarrassment that we
did not know how they worked. Now we are beginning to understand what
lights the fuse of these explosions."
Most scientists agree a Type 1a supernova occurs when a white dwarf
star -- a collapsed remnant of an elderly star -- exceeds its weight
limit, becomes unstable and explodes. Scientists have identified two
main possibilities for pushing the white dwarf over the edge: two
white dwarfs merging or accretion, a process in which the white dwarf
pulls material from a sun-like companion star until it exceeds its
weight limit.
"Our results suggest the supernovae in the galaxies we studied almost
all come from two white dwarfs merging," said co-author Akos Bogdan,
also of Max Planck. "This is probably not what many astronomers would
expect."
The difference between these two scenarios may have implications for
how these supernovae can be used as "standard candles" -- objects of
a known brightness -- to track vast cosmic distances. Because white
dwarfs can come in a range of masses, the merger of two could result
in explosions that vary somewhat in brightness.
Because these two scenarios would generate different amounts of X-ray
emission, Gilfanov and Bogdan used Chandra to observe five nearby
elliptical galaxies and the central region of the Andromeda galaxy. A
Type 1a 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
X-ray emission than the accretion scenario.
The scientists found the observed X-ray emission was a factor of 30 to
50 times smaller than expected from the accretion scenario,
effectively ruling it out. This implies that white dwarf mergers
dominate in these galaxies.
An open question remains whether these white dwarf mergers are the
primary catalyst for Type 1a supernovae in spiral galaxies. Further
studies are required to know if supernovae in spiral galaxies 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 supernovae will have to be investigated
in more detail."
In addition to the X-rays observed with Chandra, other data critical
for this result came from NASA's Spitzer Space Telescope and the
ground-based, infrared Two Micron All Sky Survey. The infrared
brightness of the galaxies allowed the team to estimate how many
supernovae should occur.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science
and flight operations from Cambridge, Mass.
More information, including images and other multimedia, can be found
at:
http://chandra.nasa.gov
and
http://chandra.harvard.edu
Source: NASA
