By Dr David Whitehouse
Science Editor, BBC News website
Astronomers are poring over images of a distant galaxy for what may be evidence of the birth of a black hole.
Target region: Swift is designed to move quickly to observe the gamma-ray bursts
On Monday, the US space agency's (Nasa) Swift satellite detected a brief burst of gamma-rays - high energy radiation - originating from deep space.
Within a minute, Swift was homing in on the burst to be followed by dozens of the world's most powerful telescopes.
It could be due to two neutron stars merging or a collision between a neutron star and black hole.
"It's incredibly exciting. It's what we've been waiting for for years," Professor Josh Bloom, of the University of California, US, told the BBC News website.
Swift, launched in November 2004, is designed to monitor the sky for fleeting bursts of gamma-rays. When one is picked-up, it has the ability to quickly turn its main telescopes towards it.
This happened on Monday when it detected a burst designated GRB 050509b. Within 53 seconds of the burst, Swift was registering its decaying radiation. The faint X-ray afterglow had faded completely after 200 seconds.
An e-mail alert was dispatched to observers worldwide.
Swift alerts telescopes on Earth to join the observation
The giant twin 10m Keck telescopes in Hawaii had their scheduled observations interrupted and, within an hour, one was recording an image of the burst region while the other was obtaining spectra of the optical sources in the region.
They were followed by other optical telescopes in Asia, Europe and the US. Radio telescopes in Europe and the Very Large Array in New Mexico were also employed, though as yet no radio waves have been reported.
To the surprise of the astronomers, the brief burst came from the outskirts of an old elliptical galaxy.
"This was remarkable," said Professor Bloom. "It seems to be coming from a fairly old galaxy, a galaxy with no new stars being formed."
"We have never seen a gamma-ray burst (GRB) coming from an old galaxy like this before."
Short duration burst
Astronomers divide GRBs into two types. The long duration type seems to come from the collapse of young massive stars into black holes.
The short duration type - like GRB 050509b - appears to come from the collision of two neutron stars (which also result in black holes) or a collision between a neutron star and a black hole.
Currently, astronomers are scrutinising the outskirts of the elliptical galaxy to see if they can further refine its location. They have found four, possibly five, faint smudges of light in the region but remain unconvinced that they are anything other than chance alignments.
Some scientists believe that the event could have been accompanied by a burst of neutrinos and the team examining the details of GRB 050509b is asking colleagues monitoring neutrinos from the Sun and deep space to check their detectors' records for the time of the event.