Earlier this
year, astronomers witnessed for the first time the final death throes of an
aged and collapsing star as it spewed high-energy light beams into space before
exploding as a supernova.
The first
analyses of that remarkable event, conducted by four different research teams
and detailed in the Aug. 30 issue of the journal Nature, provides the strongest evidence yet that supernovas
and the brilliant but mysterious energy flare-ups known as gamma-ray
bursts, or GRBs, might be linked.
Astronomers
have traditionally thought that GRBs form when matter
from a swirling disk falls onto a black hole.
"Some of
the rotation and magnetic field of the black hole somehow gets transformed into
[gamma-ray] jets," explained Timothy Young, an astronomer at the
University of North Dakota who was not involved in the
studies.
But the
detection of GRB
060218, a gamma ray burst observed by NASA's Swift
Telescope in February, strongly suggest that the energetic flare-ups might
also be associated with certain types of supernovas.
Possible
hybrid
Analyses of
GRB 060218 revealed it showed traits common to both GRBs
and supernovas. It had the twin jet-like streams of material shooting out in
opposite directions characteristic of GRBs but also a
not-quite spherical shockwave typical of supernovas.
Also like
classical supernovas, GRB 060218 dimmed before brightening again. Scientists
think the extra luminescence occurs when radioactive nickel-56 created in the
initial supernova explosion decays to cobalt-56—this reheats the
material, causing it to glow at optical frequencies.
GRB 060218
differs from other known GRBs, however, in that the
burst lasted much longer than normal. GRBs typically
last from a few milliseconds to tens of seconds, but this one lasted about 33
minutes. It was also about 100 times less energetic than typical GRBs, releasing a comparatively modest ten million billion
times the output of the Sun throughout
its entire duration. Other GRBs have been known to
outshine whole galaxies, releasing in a few seconds more energy than the Sun
during its entire lifetime.
Because it was
less bright and did not produce as many gamma-rays as other GRBs,
scientists refer to GRB 060218 as a mild type of GRB known as an X-ray flash.
Astronomers
also think the star that created GRB 060218 was on the lower end of the mass
spectrum compared to most stars that
go supernova. Astronomers think it was a so-called Type 1C supernova, which
form when ancient stars that are devoid of helium and hydrogen explode.
Astronomers estimate that before it exploded, the star had a mass of about 20
times that of the Sun.
Uncertain
fate
The star's low
mass means astronomers are unsure about what its ultimate fate was after it
exploded. Twenty solar masses is generally believed to
be the minimum requirement needed for a star to become a black hole after exploding as a
supernova. A research team led by Elena Pian at the
INAF Astronomical Observatory of Trieste, Italy thinks GRB 060218 was right at
this lower limit, and thus may have produced a highly
magnetic version of a dense, city-sized neutron
star instead.
"It's an
interesting result whether it's a neutron star or a black hole," said University of California, Berkeley researcher Alex Filippenko,
who was involved in the analyses. "Because massive stars are hard to form,
there may be many more supernovae like this producing X-ray flashes than
explosions of very massive stars to produce gamma-ray bursts."
GRB 060218 occurred in a star-forming galaxy about 440 million light-years away
towards the constellation Aries, making it the second-closest GRB ever
detected. The closest was GRB 980425, which
went off 140 million light-years away.
The closest supernova ever detected was SN 1987A,
which exploded about 165,000 light-years away in the Large
Magellanic Cloud, a nearby dwarf
galaxy.
SN 1987A is
one of the best studied supernovas to date, but even so, astronomers missed the
very initial stages of the explosion.
"Someone
saw [SN 1987A] in the sky and they turned their telescopes towards it,"
Young said. "So the light from the supernova had already arrived."
But with GRB
060218, astronomers got their first glimpse of the full evolution of a star's
death, something that could prove invaluable for deciphering the mechanism
behind both gamma-ray bursts and supernovas.
"This
object showed both the gamma-ray signature and the supernova signature. This is
going to spawn a whole bunch of papers on what the connection between the two
is. It's really getting close to the actual mechanism of how these stars
explode" Young said. "This is what astronomers
dream of having—data like this."
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