Skip Navigation | ANU Home | Search ANU | Directories

RSAA News of the Month: April 2003

Two of the Best: Catching Gamma-ray Bursts in Action

 

RSAA astronomers have made some of the most detailed observations yet of gamma-ray bursts. Gamma-ray bursts (GRBs) are some of the most powerful events in the universe, and their cause is unknown. They were first discovered by satellite observatories in 1967, and occur at the rate of about one per day. At present, GRBs are thought to be caused by exploding stars (hypernovae) or by the collision of either black holes or neutron stars.

The afterglows of bursts are visible in ordinary light as well as gamma-rays, but they fade quickly. The afterglow is caused by matter ejected by the explosion smashing into material in surrounding space at near the speed of light.

Monitoring the fading afterglow of a burst is the principal method we have to investigate these events, so quick action by ground-based telescopes is essential. RSAA's 40" and 2.3m telescopes at Siding Spring are part of the international Gamma-ray Burst Coordinates Network (GCN) and respond whenever a burst visible from Australia is detected.

Paul Price, a final-year student at Mt Stromlo, is the coordinator for the Australian GRB network. He says, "Gamma-ray bursts are far more powerful than a supernova, or exploding star. They let off more energy than 1000 supernovas going off in the same time at the same place. They've been described as the biggest bang since the Big Bang."

On October 4th last year, the HETE spacecraft recorded a GRB and triggered the network. Shortly after, a Japanese telescope spotted the afterglow of the gamma-ray burst. The network of telescopes, including those at Siding Spring, were able to observe the event (now called GRB021004) almost continuously as it faded over the next 10 days.

Three images of the region near GRB021004
The GRB is arrowed in the first image, from the Near Earth Asteroid Tracking telescope in Hawaii.
Note that it is not visible on the second, earlier, image from the Digitised Palomar Sky Survey.
The third (colour) image is from the 40" at Siding Spring; note that the afterglow has faded compared with the first, earlier, image.

One vital observation was made with the Siding Spring 2.3m, a spectrum which allowed the minimum distance of the GRB to be estimated. Combining the 2.3m data with that from the Keck 10m telescope in Hawaii, the distance to the GRB was estimated to be around 11 billion light-years. Images from the 40" helped pin down the brightness of the fading afterglow.

The spectrum of GRB021004 obtained with the 2.3m at Siding Spring
The positions of the absorption lines of Magnesium (Mg) and Iron (Fe) show that the distance to the GRB is at least 10 billion light-years.

The data collected makes GRB021004 the most closely examined gamma-ray burst to date. They show that the explosion faded slowly and far from smoothly. This is quite different from the predictions of standard models of GRBs and analysis of the data will help in eventually explaining the nature of the explosion.

Results have been published as "Early optical emission from the gamma-ray burst of 4 October 2002" in the 20 March 2003 issue of the journal "Nature". The article can be read online here.

On March 29th another GRB occurred, again discovered by HETE. This one, GRB030329, is one of the brightest ever seen. It is a mere two billion light-years away, the second closest GRB known. Dr Bruce Peterson and Paul Price were the first to identify the afterglow and data is currently (April 1) being collected by observatories and amateur astronomers worldwide.

The discovery image for GRB030329, taken on the Siding Spring 40" telescope

Read the ANU press release about GRB030329 here.

The GCN Circulars can be read here.

Paul's network is ready for instant action whenever a GRB is detected. They are helping solve one of the great mysteries of modern astronomy.

P.S. Read June 2003 New York Times article about astronomers linking Gamma-ray bursts to supernovae

---

RSAA News of the Month Archive