UK astronomers are part of an international team that has used the Swift satellite to observe the first moments of a supernova explosion as it happens. The results are presented in a paper in the journal Nature today (22 May 2008) led by Dr. Alicia Soderberg of Princeton University.

This extraordinarily fortunate discovery led to multiple observations with other telescopes and allowed the totality of a supernova explosion to be observed for the first time.

Dr. Kim Page of the University of Leicester, who led the X-ray analysis, said that "this observation is by far the best example of what happens when a star dies and a neutron star is born". 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 ultra-dense object known as a neutron star.

The newborn neutron star compresses and then rebounds, triggering a shock wave that ploughs through the star's gaseous outer layers and blows the star to smithereens. Theoretical models have long predicted that this shock "break-out" will produce bright X-ray emission lasting a few minutes but until now the X-ray signal has never been detected.

Dr. Paul O'Brien, also from the University of Leicester, said "the X-rays from the shock break out are the earliest electromagnetic radiation that has ever been detected from a supernova. Understanding supernovae is important as these nuclear furnaces make the heavy elements from which planets like ours form."

Until this discovery, astronomers have only found supernovae days or weeks later, when the expanding shell of debris is energized by the decay of radioactive elements forged in the explosion.

Dr. Patricia Schady of the Mullard Space Science Laboratory, University College London said "Within 2 hours of the burst of X-rays we could see the expanding ultraviolet fireball with Swift. Telescopes in space and around the world were alerted well in time to see the radioactive debris as it brightened over the next few days."

The discovery of the first shock breakout can be attributed to luck and Swift's unique design. On January 9, 2008, Swift was observing a supernova known as SN 2007uy in the spiral galaxy NGC 2770, located 90 million light-years from Earth in the constellation Lynx.

During this observation an extremely bright 5-minute X-ray outburst occurred in another part of NGC 2770 which was recognized as another supernova occurring while Swift looked.

In a paper to appear in the May 22 issue of Nature, Soderberg and colleagues show that the energy and pattern of the X-ray outburst are consistent with a shock wave bursting through the surface of the progenitor star. This marks the birth of the supernova now known as SN 2008D.

Although astronomers were lucky that Swift was observing NGC 2770 just at the moment when SN 2008D's shock wave was blowing up the star, Dr. Mat Page also of the Mullard Space Science Laboratory, said "Swift is well equipped to study such an event because of its multiple instruments observing in gamma rays, X-rays, and ultraviolet light. " The X-ray camera and the Ultraviolet-Optical Telescope on Swift were built in the UK.

Professor Keith Mason, CEO of the Science and Technology Facilities Council which funds UK involvement with Swift, said "Supernova explosions are taking place all over the Universe right now, but astronomers can usually only detect them as they grow brighter after the star has started its violent death.

This is a once in a lifetime moment, where Swift happened to be looking in the right area of the sky and is built to respond extremely rapidly, so could study supernova SN2008D right from the first moment it started."

Due to the significance of the X-ray outburst, Soderberg and colleagues immediately mounted an international observing campaign to study SN 2008D. Observations were made with major telescopes such as the Hubble Space Telescope, the Chandra X-ray Observatory, the Very Large Array in New Mexico, the Gemini North telescope in Hawaii, the Keck I telescope in Hawaii, the 200-inch and 60-inch telescopes at the Palomar Observatory in California, and the 3.5-meter telescope at the Apache Point Observatory in New Mexico.

The combined observations helped pin down the energy of the initial X-ray outburst, which will help theorists better understand supernovae.

The observations also show that SN 2008D is an ordinary Type Ibc supernova, which occurs when a massive, compact star explodes. Significantly, radio and X-ray observations found no evidence that a jet played a role in the explosion, ruling out a rare type of stellar explosion known as a gamma-ray burst.

Swift spends most of its time observing GRBs, but due to its agility can also observe many other objects each day. This unique capability makes Swift a particularly capable multi-wavelength observatory.

Swift is a NASA mission in collaboration with the Science Technology and Facilities Council (STFC) in the UK and the Italian Space Agency (ASI) in Italy.