Yet another record: Ulysses detects most distant gamma-ray burst
19 October 2000Ulysses has helped to set another record. On 31 January this year, the intrepid spacecraft detected the most distant gamma-ray burst ever recorded.Other spacecraft also picked up the burst, enabling astronomers to estimate its position in the sky using triangulation methods. A message was sent to ground-based telescopes and shortly afterwards the European Southern Observatory's Very Large Telescope (VLT) in Chile identified the optical counterpart - a rapidly-fading source of visible light in the southern constellation of Carina.
"The burst didn't look that interesting. It looked average: it wasn't weak," says Kevin Hurley from the University of California at Berkeley and Principal Investigator for the gamma-ray burst experiment (GRB) on Ulysses. However, analysis of the optical counterpart revealed that the burst originated in a galaxy 11 billion light-years away, making it the most distant gamma-ray burst ever recorded.
The Ulysses GRB experiment measures only the intensity of bursts. To measure the distance of the source of a burst, astronomers must first identify its optical counterpart and then measure its redshift.
Just as the pitch of a siren lowers as it recedes, so light becomes redder as its source recedes. By measuring the extent to which light has been shifted towards the red end of the spectrum, astronomers can estimate how fast its source is travelling away. Ever since the Big Bang, objects have been receding faster the further away they are. So by knowing the red shift, astronomers can estimate the distance of an object.
The optical source in Carina turned out to have a redshift of 4.5, which translates into a distance of about 11 billion light years (one light year is the distance light travels in a year). The previous record for a gamma ray burst was a red shift of 3.4, or about 9 billion light years, for a source in Ursa Major.
Gamma ray bursts originate in the most powerful explosions in the universe. In just a few seconds, they emit more energy than the Sun in its entire lifetime. No-one knows what causes them, but their observation at such great distances opens up a new tool to study the universe during its infancy.
A light year is a measure not just of distance, but also of time. Light from a source 11 billion light years away has taken 11 billion years to reach us, which means that the explosion causing the gamma ray burst took place 11 billion years ago. As the universe is thought to be 12-14 billion years old, the gamma ray burst occurred during the first 10-20% of its lifetime. Only quasars have been detected at further distances and earlier times.
"It's possible we could detect gamma ray bursts even further away than the furthest quasars, opening up new possibilities for studying the early universe. The further away the sources are, the more interesting they get," says Hurley.
"This exciting new result from Ulysses shows once again what an amazingly wide-ranging mission this is," Dr Richard Marsden, ESA Ulysses Project Scientists says. "Not only is Ulysses giving us a new picture of the Sun's environment, it is also teaching us about comets, our local interstellar neighbourhood, and now cosmology!"
Kevin Hurley gave a talk on these results yesterday at a conference on "Gamma-Ray Bursts in the Afterglow Era" in Rome, Italy. A paper will appear in the 1 December issue of Astronomy and Astrophysics.