Two peculiar gamma-ray bursts have been spotted that do not fit into the orderly classification system that astronomers had previously developed. The discoveries suggest that black holes may be colliding with stellar corpses called neutron stars much more often than thought, implying that gravitational waves from the events may be detected within the next few years.
Gamma-ray bursts (GRBs) are fleeting blasts of high-energy radiation that until recently appeared to fall into two classes. "Long" bursts last more than 2 seconds and are thought to occur when massive stars explode and their cores collapse into black holes. "Short" bursts typically last less than a second and are suspected to be caused by the merger of two neutron stars.
Now, two nearby GRBs have been found that refuse to fit neatly into either of those two categories. Both lasted more than two seconds, seemingly making them "long" bursts. But neither was seen with an accompanying supernova – something detected in all four previously known long bursts at similar distances.
One, dubbed GRB 060505 because it was detected on 5 May 2006, lasted for 4 seconds, while a second, called GRB 060614 for its discovery date, went on for a full 102 seconds.
First detected by NASA's Swift space telescope, a slew of other telescopes then turned their sights on them. Both bursts were found to lie within a few billion light years of Earth, inside galaxies that are still undergoing star formation. Because of this proximity – most long bursts lie 10 billion to 12 billion light years away – astronomers were able to take incredibly detailed measurements of their light, especially for GRB 060614. Time lag
"It was so bright and so well observed, it allowed us the best possible observations we've ever had," says Neil Gehrels, principal investigator of the Swift mission at NASA's Goddard Space Flight Center in Maryland, US.
The apparent lack of a supernova is surprising because any massive star that collapses into a black hole and produces enough energy to power a GRB would also be expected to produce a supernova. "That's hard to understand," Gehrels told New Scientist.
But the June burst is confounding in other ways, as well. High- and low-energy photons arrived at Earth at about the same time, as they do in typical short bursts (there is a time lag between the two for long GRBs). And the star-forming rate in its host galaxy is relatively low, which is characteristic of short bursts, which are thought to form from long-dead stars.
"This one really has us puzzled," says Gehrels. "Just when we thought we were starting to understand short and long bursts, this one pulled a fast one on us." Spiral arm
Nevertheless, astronomers have arrived at three possible explanations for the burst. First, it may actually be a long burst, but some property of the collapsing star may lead to weak – or nonexistent – supernovae.
Second, it may be created by a merger, like other short GRBs, but the merger process would somehow manage to last longer than 2 seconds. Third, it may have a completely different – and as yet unknown – provenance.
Johan Fynbo of the University of Copenhagen Dark Cosmology Centre in Denmark, who has studied both bursts, is partial to the first explanation – especially for the May burst. He led a team that observed the burst and found it went off in one of the star-forming spiral arms of its host galaxy – bolstering the idea that resulted from the death of a massive star, which tend to have short lives and thus would be found near stellar nurseries.
"I think the interesting thing is that some massive stars seem to die without making a supernova," Fynbo told New Scientist.
That could occur if the star imploded completely rather sending its outer layers shooting into space in a supernova, or if the energy at the centre of the dying star is not great enough to kick out radioactive nickel-56, which gives ordinary supernovae their glow. Imploding stars
If some massive stars implode rather than explode, that will alter astronomers' ideas of how heavy elements cycle through the universe. Nearly all of the elements heavier than hydrogen and helium – necessary for the building blocks of life – were created in stars and distributed throughout space via supernovae. "It seems not all massive stars contribute to this cycle," says Fynbo.
"We don't know how many of them there are, but GRBs might be a unique way of observing these underluminous supernovae," says Gehrels.
Alternatively, if the bursts are of the "short", merger type, they may not be caused by the collision of two neutron stars. Such a union is expected to produce a fleeting, "clean explosion, where nothing goes out and everything goes into the [resulting] black hole", says Gehrels.
Instead, the merger may be between a neutron star and a black hole. "The black hole would tear the neutron star apart and would spew gas around the region and could make radiation for 100 seconds, as observed," he says. Ripples in space-time
Such mergers had been predicted to occur just once for every 1000 or so mergers between two neutron stars. But if one or both of the newly observed nonconformist bursts involve a black hole-neutron star merger, that would mean these collisions are much more common.
That would be good news for gravitational wave detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO), which search for ripples in space-time caused by just such events. "The black hole-neutron star mergers produce a much stronger gravitational wave signal and would be seen to greater distance with instruments like LIGO," says Gehrels. "It could indicate a better potential for seeing gravitational wave signals in the next few years."
The exact nature of the bursts is likely to remain a mystery until more oddball GRBs are seen. Gehrels expects they will turn up at the rate of about one per year, with Swift expected to operate at least through 2015.
"We're going to redouble our efforts to look through the data and detect any of these that come along," says Gehrels. "Swift is getting us to ask questions we didn't even know existed half a year ago."