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Unique gamma-ray burst gives clues to formation
By Jim Allen
Posted: 26 July 2010

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A team of astronomers from Kyoto University, the Tokyo Institute of Technology and the National Astronomical Observatory of Japan have identified a dark gamma-ray burst (GRB) which could explain the origin of these exotic objects.

On 25 March 2008 the team identified a GRB in the constellation of Lyra. Using the Subaru telescope, Mauna Kea, they were able to observe the object in the near infrared part of the electromagnetic spectrum. Using the Multi-Object Infra-Red Camera and Spectrograph (MOIRCS) the team were able to make the first detection of a GRB host galaxy and its afterglow in the near infrared just nine hours after the initial burst.

The afterglow of the dark GRB taken by MOIRCS a) 9 hours after and b) 34 hours after the initial burst. c) The subtraction of image b) from a) leaving only the image of the GRB afterglow. The green circle in a) shows the uncertainty in the position of the X-ray afterglow.Image: Subaru Telescope.

GRBs are bright, short lived blasts of gamma radiation found at distances up to billions of light years away from Earth, and are some of the most radiant objects in the sky. They last for less than a minute (and sometimes only a few seconds), but despite this short lifetime they produce an afterglow that can last from anywhere between hours and days. It is this afterglow that the team were able to observe. Some GRBs, such as this one, have very faint afterglows and so are named dark GRBs. Despite making up half of all GRBs they are rarely investigated due to the difficulty of their detection.

This GRB was dimmer than expected, which led the scientists to believe that the radiation was being blocked by gas and dust from inside the source galaxy. In order to investigate this, one year after the initial measurement the team used the Subaru Prime-Focus camera (Suprime-Cam) and found that the host galaxy was a similar size to that of our own. This is one of the more massive galaxies found to have produced a GRB, and has the highest metallicity than any previously measured (hydrogen and helium are described as low metallicity, whilst any heavier elements are described as high metallicity).

A diagram showing the possible formation mechanism of a dark GRB involving the merger of a binary star system. Image: Subaru Telescope.

It is thought that GRBs are created in a low metallicity environment during a supernova – a large explosion experienced by a star at the end of its life. Previous investigations of GRBs have found that the galaxies in which they are formed usually have a low metallicity content. Relativistic jets, high speed jets of gas created during a supernova event and formed in low metallicity conditions, have also been found to be a cause of GRBs.

The discovery of a GRB in this high metallicity galaxy suggests that a new mechanism is needed to describe the formation of dark GRBs. One possibility is that instead of a supernova from a single star, the dark GRBs are caused by a pair of stars orbiting one another, in which the larger of the two stars expands and engulfs the other, to form one large star. As this happens the resulting star spins at such a rate to produce a GRB.

This discovery could also have implications for the history of life on Earth. The theory that a mass extinction 435 million years ago (during the Ordovician period) was caused by a GRB inside the Milky Way has previously been rejected due to the belief that a GRB cannot occur in a high metallicity environment such as our own. Finding a GRB in a high-metallicity galaxy suggests that they could also occur in our own.