Dec.
18, 2013 — Astronomers affiliated with the Supernova Legacy Survey (SNLS) have
discovered two of the brightest and most distant supernovae ever recorded, 10
billion light-years away and a hundred times more luminous than a normal
supernova.
These
newly discovered supernovae are especially puzzling because the mechanism that
powers most of them -- the collapse of a giant star to a black hole or normal
neutron star -- cannot explain their extreme luminosity. Discovered in 2006 and
2007, the supernovae were so unusual that astronomers initially could not
figure out what they were or even determine their distances from Earth.
"At
first, we had no idea what these things were, even whether they were supernovae
or whether they were in our galaxy or a distant one," said lead author D.
Andrew Howell, a staff scientist at Las Cumbres Observatory Global Telescope
Network (LCOGT) and adjunct faculty at UC Santa Barbara. "I showed the
observations at a conference, and everyone was baffled. Nobody guessed they
were distant supernovae because it would have made the energies mind-bogglingly
large. We thought it was impossible."
One
of the newly discovered supernovae, named SNLS-06D4eu, is the most distant and
possibly the most luminous member of an emerging class of explosions called
superluminous supernovae. These new discoveries belong to a special subclass of
superluminous supernovae that have no hydrogen.
The
new study finds that the supernovae are likely powered by the creation of a
magnetar, an extraordinarily magnetized neutron star spinning hundreds of times
per second. Magnetars have the mass of the sun packed into a star the size of a
city and have magnetic fields a hundred trillion times that of Earth. While a
handful of these superluminous supernovae have been seen since they were first
announced in 2009, and the creation of a magnetar had been postulated as a
possible energy source, the work of Howell and his colleagues is the first to
match detailed observations to models of what such an explosion might look
like.
Co-author
Daniel Kasen from UC Berkeley and Lawrence Berkeley National Lab created models
of the supernova that explained the data as the explosion of a star only a few
times the size of the sun and rich in carbon and oxygen. The star likely was
initially much bigger but apparently shed its outer layers long before
exploding, leaving only a smallish, naked core.
"What
may have made this star special was an extremely rapid rotation," Kasen
said. "When it ultimately died, the collapsing core could have spun up a
magnetar like a giant top. That enormous spin energy would then be unleashed in
a magnetic fury."
Discovered
as part of the SNLS -- a five-year program based on observations at the
Canada-France-Hawaii Telescope, the Very Large Telescope (VLT) and the Gemini
and Keck telescopes to study thousands of supernovae -- the two supernovae
could not initially be properly identified nor could their exact locations be
determined. It took subsequent observations of the faint host galaxy with the
VLT in Chile for astronomers to determine the distance and energy of the
explosions. Years of subsequent theoretical work were required to figure out
how such an astounding energy could be produced.
The
supernovae are so far away that the ultraviolet (UV) light emitted in the
explosion was stretched out by the expansion of the universe until it was
redshifted (increased in wavelength) into the part of the spectrum our eyes and
telescopes on Earth can see. This explains why the astronomers were initially
baffled by the observations; they had never seen a supernova so far into the UV
before. This gave them a rare glimpse into the inner workings of these
supernovae. Superluminous supernovae are so hot that the peak of their light
output is in the UV part of the spectrum. But because UV light is blocked by
Earth's atmosphere, it had never been fully observed before.
The
supernovae exploded when the universe was only 4 billion years old. "This
happened before the sun even existed," Howell explained. "There was
another star here that died and whose gas cloud formed the sun and Earth. Life
evolved, the dinosaurs evolved and humans evolved and invented telescopes,
which we were lucky to be pointing in the right place when the photons hit
Earth after their 10-billion-year journey."
Such
superluminous supernovae are rare, occurring perhaps once for every 10,000
normal supernovae. They seem to explode preferentially in more primitive
galaxies -- those with smaller quantities of elements heavier than hydrogen or
helium -- which were more common in the early universe.
"These
are the dinosaurs of supernovae," Howell said. "They are all but
extinct today, but they were more common in the early universe. Luckily we can use
our telescopes to look back in time and study their fossil light. We hope to
find many more of these kinds of supernovae with ongoing and future
surveys."
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