Astronomers watch supernova unfold in real time
For the first time ever, scientists have watched a star go supernova and linked it to the bright X-ray flash which preceded it. Astronomers around the world watched the event unfold, and what they saw has major implications for our understanding of supernovae. The observations and analyses of the event of four international teams of astronomers are described in the latest issue of the journal Nature. Supernovae occur when a mature star runs out of energy and is no longer able to support its own gravity, causing the stellar layers to collapse inwards. 'This creates a shockwave that rebounds outwards, fuelled by energy gained from internal magnetic fields and rotation,' explains Timothy R Young of the University of North Dakota in an accompanying article. 'At 'shock breakout', when the shockwave emerges from the surface of the collapsing star, its energy is unleashed. It is sent out into space as radiation of all frequencies over a period of days to months - the classic signal of a supernova.' It has long been suspected that gamma ray bursts (GRBs) are some kind of early warning signal for a supernova, but until now hard evidence of this has been lacking, and in fact no-one had ever observed a supernova from the moment of shock breakout. Furthermore, it was not clear why some supernovas appear to be linked to GRBs and others are not linked. It all started on 18 February 2006, when NASA's Swift satellite detected an unusual gamma ray burst in a galaxy 440 million light years away in the constellation Aries. Gamma ray bursts are the most powerful events in the universe, releasing more energy in just a few seconds than the Sun will in its entire lifetime. This GRB (named GRB060218 after its date of discovery) was 25 times closer and 100 times longer than most GRBs. It was also observed to be a relatively mild kind of GRB known as an X-ray flash. Its 40 minute duration gave astronomers the time to point a range of instruments at the event, and so capture images of it across a range of wavelengths and observe its behaviour. Meanwhile, scientists at the European Southern Observatory's Very Large Telescope (VLT) watched the afterglow of the gamma ray burst grow brighter in optical light, which suggested the beginnings of a supernova. A few days later, the classical supernova was clearly visible. For the first time, astronomers had established a clear link between the X-ray flash and a supernova. A team led by Paolo Mazzali from the Max Planck Institute for Astrophysics in Garching, Germany, used data from the VLT to analyse the type of star that had exploded so dramatically. He calculated that it was a relatively small star (20 times the mass of the sun); in contrast stars which cause supernovae linked to strong gamma ray bursts are usually twice this size. Larger stars which generate strong GRBs when they go supernova are thought to become black holes. However, in this case the star was too small to become a black hole, and Dr Mazzali's team believes the event probably lead to the creation of a highly magnetic type of neutron star called a magnetar. The astronomers believe supernovae associated with X-ray flashes may be much more common than those associated with strong gamma ray bursts. 'The properties of GRB060218 suggest the existence of a population of events less luminous than 'classical' GRBs, but possibly much more numerous,' commented Dr Mazzali. 'Indeed, these events may be the most abundant form of X- or gamma-ray bursts in the Universe, but instrumental limits allow us to detect them only locally.' Professor Keith Mason, UK lead investigator for the UVOT telescope on Swift, explained that astronomers were so excited about this supernova. 'Usually these events are not detected until after the supernova has brightened substantially in the optical wavelength, many days after the initial explosion,' he said, 'but on this occasion we were able to study the remarkable event in all its glory from the very beginning.'