Over The Horizon

The Light of 10 Million Billion Suns

Reacting quickly to an international alert, New Zealand astronomer Dr Karen Pollard has imaged the fading glow of a gamma ray burst, the most powerful type of explosion known in the universe.

On May 10, Pollard, from Canterbury University, and Dr John Menzies, from the South African Astronomical Observatory (SAAO), were about to begin the night's observing at the 1-metre telescope at SAAO Sutherland, South Africa, looking for evidence of planets around other stars as part of a worldwide collaboration called PLANET.

They were called by the Amsterdam/Huntsville gamma ray burst followup team with the news that the BeppoSAX Italian-Dutch satellite had detected tell-tale gamma rays and X-rays from an explosion in a distant galaxy near the South Celestial Pole. Phone calls were exchanged through the night as the PLANET observers obtained images of the region where the X-rays had been detected, then shipped them off electronically to Amsterdam.

One of the dim star-like objects in the region proved to be fading fast and, with a precise position now available, the giant 8-metre VLT telescope in Chile was able to obtain a spectrum so that the power and distant of the outburst could be measured. The distance to the "afterglow" of the gamma-ray burst was determined to be a staggering 10 billion light years.

Gamma ray bursts are common; observing them optically so that their distances can be measured is not. In the 27 months since the detection of the first optical counterpart, only 11 more have been found. The extreme distance and extreme brightness of these mysterious explosions is telling us something we don't understand yet. Every clue is vital, and early measurements crucial.

The Burst and Transient Source Experiment (BATSE) on the orbiting Compton Gamma Ray Observatory picks up about one of these brief flashes of high-energy radiation per day. Thousands have been detected since they were discovered in the late 1960s, but nobody knew what they were. BATSE did establish that they were so evenly spread over the sky that they must be very distant (and hence very powerful) outbursts of some kind. The breakthrough came two years ago when, for the first time, the fading "afterglow" of one of these explosions was detected in visible light, thanks to the more accurate positions available from the BeppoSAX satellite.

Three months later, another optical afterglow was detected, this time in a faint galaxy whose distance could be measured. Last year, a Nature article announced the optical detection of a gamma ray burst in a galaxy more than 12 billion light years away. Even the most exotic ideas proposed for these explosions -- supergiant stars collapsing to black holes, black holes merging with each other -- have trouble accounting for explosions with the power of 10 million billion suns.

The PLANET collaboration, which involves Pollard and Dr Michael Albrow at the University of Canterbury and others at institutes around the world, looks for planets using a technique called "gravitational microlensing". This method can pick up planets as small as the Earth, and will give important statistical results about how common planets really are.