Rare observation of cosmic explosion

"That we have observed a gamma ray burst and a supernova simultaneously is unusual and it gives us a long awaited confirmation of the theory that gamma ray bursts are indeed associated with exploding giant stars", explains Johan Fynbo, astrophysicist at the Dark Cosmology Center, Niels Bohr Institute at the University of Copenhagen.

An international team of researchers made the discovery using NASA's Swift satellite, which was first used to find the exploding star in the universe.

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Seen here is a little section of the X-shooter spectrum. The figure illustrates how astronomers calculate the distance from the spectrum. By measuring how much certain features in the spectrum are shifted towards the red wavelengths, the so-called redshift can be determined. Thus, the distance is given based on the redshift, if you know the correct Big Bang model, and they believe they do.

Star explosion in the near universe

By analysing the light spectra and measuring the so-called redshift the researchers could calculate the distance to the supernova, which was located in a galaxy 820 million light years from the Earth. It sounds extremely far away, but for astronomers it actually a relatively near galaxy and that is a true delight.

"Only about one out of 10,000 supernovae produce gamma ray bursts. So gamma ray bursts are rare and gamma ray bursts in the relatively near universe are even more rare, so it gives us a unique opportunity to study the phenomena in detail. When you have one that is so near, it is like a clock telling you that the star is dying just NOW, and you can follow the early phases and the entire development of the supernova explosion that follows", explains Johan Fynbo.

It is only the very large, massive stars of more than 30-40 solar masses that produce gamma ray bursts. What happens is that when the star dies, it collapses and forms a black hole. If the black hole spins very quickly, a jet might appear - a gamma ray burst, which is powerful, high-energy radiation, which shoots out at nearly the speed of light. It happens very quickly and only lasts a few seconds.

The actual star explosion, the supernova, happens instantaneously, but it evolves and becomes brighter and brighter over the next 10 - 14 days, after which it gradually becomes weaker again over the course of several months.

The spectra show that it was a metal-poor star that exploded. It only had around half the amount of heavy elements as the Sun - our star. This means that it was an early development of stars. The galaxy the star was located in was irregular and it is typically in such galaxies, that you find metal-poor massive stars in the local universe.

"It is fantastic that we can now study exploding giant stars and the related gamma ray bursts in detail - we have only been able to do this 3-4 times before", explains Johan Fynbo, who is in full swing with the detailed studies of the supernova.

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Provided by University of Copenhagen