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.
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}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."
