While
peering at her computer screen four months ago, astronomer Alicia Soderberg
expected to see the small glowing smudge of a month-old supernova. But what she
and her colleague saw instead was a strange, extremely bright, five-minute
burst of X-rays.
With that
observation, they became the first astronomers to catch a star in the act
of exploding.
"For
years we have dreamed of seeing a star just as it was exploding, but actually
finding one is a once-in-a-lifetime, event," said Soderberg, a Hubble and
Carnegie Princeton Fellow at Princeton University.
The
discovery, detailed in the May 22 issue of the journal Nature, will shed
light on the early stages of this violent
stellar death, acting as a deciphering key or "Rosetta Stone" for
supernova studies, as Soderberg puts it.
And
analysis of the energy emitted by the new supernova, dubbed SN 2008D, could
help astronomers better understand this explosive process and the properties of
the stars that lead to it.
X-ray
'breakout'
A typical supernova occurs when the core of a
massive star runs out of nuclear fuel and collapses under its own gravity to
form an ultradense object known as a neutron star. But only so much material
can compress into the neutron star, so some of the original star's collapsing
gaseous outer layers can't fit; instead, they simply bounce off the neutron
star, Soderberg explained, triggering a shock wave that plows back through the outer
layers and blows the star to smithereens.
Astronomers
had predicted for decades that this "breakout" phase would produce an
X-ray blast lasting several minutes, but until Soderberg and Princeton
postdoctoral researcher Edo Berger's discovery, no one had ever observed the
signal. Supernovas were only found as they brightened days or weeks after their
initial explosion.
"Using
the most powerful radio, optical and X-ray telescopes on the ground and in
space, we were eventually able to observe the evolution of the explosion right
from the start," Berger said. "This eventually confirmed that the big
X-ray blast marked the birth of a supernova."
The
discovery was a case of serendipity, Soderberg said, as the team had NASA's
Swift satellite pointed at NGC 2770 to observe supernova SN 2007uy (located 90
million light years from Earth in the constellation Lynx) and happened to catch
the X-ray outburst.
"We
were in the right place, at the right time, with the right telescope on January
9th and witnessed history," Soderberg said.
World-wide
monitoring
After
observing the X-ray outburst, Soderberg mounted an international observing
campaign, with telescopes all over the world joining in to monitor the baby
supernova, including the Hubble Space Telescope, the Gemini South Telescope in Chile, Lick Observatory and the Keck I telescope in Hawaii, among others.
The
combined observations helped to pin down the energy of the initial X-ray burst
and showed that it was a typical Type Ibc supernova, which occurs when a massive,
compact star explodes.
The
observations will also provide insight into the early stages of supernovas.
"This
first instance of catching the X-ray signature of stellar death is going to
help us fill in a lot of gaps about the properties of massive stars, the birth
of neutron stars and black holes, and the impact of supernovae on their
environments," said Neil Gehrels, principal investigator of the Swift
satellite.
Studying
this initial X-ray outburst will also give astronomers a signature to help them
spy other newborn supernovas and set their time of explosion to within a few
seconds, instead of a few days like previous timing estimates.
"We
also now know what X-ray pattern to look for," Gehrels said.
"Hopefully we will be able to find many more supernovae at this critical
moment."
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