December 8, 2012
Supernovae Birth Could Be Caught Sooner By Looking For X-ray Signatures
April Flowers for redOrbit.com - Your Universe Online
An excess of X-ray radiation has been observed in the first few minutes of the collapse of massive stars. This excess may be the signature of the supernovae shock wave as it first escapes from the star.
Dr. Rhaana Starling of the University of Leicester Department of Physics and Astronomy said, "The most massive stars can be tens to a hundred times larger than the Sun. When one of these giants runs out of hydrogen gas it collapses catastrophically and explodes as a supernova, blowing off its outer layers, which enrich the Universe. But this is no ordinary supernova; in the explosion narrowly confined streams of material are forced out of the poles of the star at almost the speed of light. These so-called relativistic jets give rise to brief flashes of energetic gamma-radiation called gamma-ray bursts, which are picked up by monitoring instruments in space, that in turn alert astronomers."
Because coincident supernovae are seen with ground-based optical telescopes approximately ten to twenty days after the high-energy flash, astronomers know that gamma-ray bursts arise in stellar deaths. However, the true moment of birth for a supernova — called the supernova shock breakout - when the star's surface reacts to the core collapse, is missed. Gamma-ray bursts are associated with only the most energetic supernovae.
For this sub-class of supernovae, however, it may be possible to identify X-ray emission signatures in the infancy of the supernova. If an X-ray early warning system could detect the supernova's birth earlier, astronomers could monitor the event as it happens, allowing them to pinpoint the drivers behind one of the most violent events in our Universe.
Partly built at the University of Leicester, the X-ray detectors the team are using for this study are on the X-Ray Telescope aboard NASA's Swift. Swift is able to nimbly turn around to catch a gamma-ray burst in action. The observatory has returned data on a number of gamma-ray bursts with visible supernovae showing an excess in X-rays received compared with expectations. This excess is thermal emission, also known as blackbody radiation.
Dr Starling added, "We were surprised to find thermal X-rays coming from a gamma-ray burst, and even more surprising is that all confirmed cases so far are those with secure supernova identification from optical data. This phenomenon is only seen during the first thousand seconds of an event, and it is challenging to distinguish it from X-ray emission solely from the gamma-ray burst jet. That is why astronomers have not routinely observed this before, and only a small subset of the 700+ bursts we detect with Swift show it.
"It all hangs on the positive identification of the extra X-ray radiation as directly emerging from the supernova shock front, rather than from the relativistic jets or central black hole. If this radiation turns out to be from the central black-hole-powered engine of the gamma-ray burst instead, it will still be a very illuminating result for gamma-ray burst physics, but the strong association with supernovae is tantalizing."
Comprised of scientists from the UK, Ireland, USA and Denmark, the team plans to extend their searches to make more quantitative comparisons with theoretical models both for stellar collapse and the dynamics of fast jet-flows.
For most supernovae, astronomers will continue to observe their visible-light peak at approximately ten days after the explosion. However, for the most energetic, it may become possible to routinely witness the very moment they are born, through X-ray eyes.