ODD BURST: A bright new explosion from June of this year doesn't fit into standard categories for such bursts, like the absorption of a neutron star into a black hole. Click on the picture for a short animation of this form of stellar demise. Image: COURTESY OF DANA BERRY/NASA
On June 14, the Burst Alert Telescope on NASA's Swift satellite was belted by gamma rays, the first warning that one of the most powerful explosions in the universe was taking place as a star went through its death throes. Within moments or hours a host of astronomers had trained telescopes all over the world on a galaxy 1.6 billion light-years away (relatively close in the cosmic scheme of things) toward the constellation Indus in anticipation of an incipient supernova--the brilliant demise of a massive star. As the flash lingered for 102 seconds, it fell into the "long" category of gamma-ray bursts typically associated with such epic stellar explosions. But even after months of monitoring no supernova appeared, bursting current theories for such events and potentially revealing a new type of stellar doom.
Current understanding divides gamma-ray bursts into those that last less than two seconds and those of longer duration. The latter occur when a massive but young star collapses in on itself and explodes, seeding the universe with its elemental remnants and leaving behind a central black hole of matter so dense that no light can escape. The former happen when an old neutron star spirals to its death in a preexisting black hole. In addition to differences in duration, the two types also differ in the type of energy released, with short ones emitting a quick burst of comparatively high energy.
But GRB060614 (after the date it was discovered) fits neither category. Its duration supported the long variety while the intensity of its emissions more closely matched the short one. "We have lots of data on this burst and have dedicated lots of observation time to it, and we just can't figure out what exploded," explains astronomer Neil Gehrels of NASA Goddard Space Flight Center, lead author of one of four papers in the December 21 Nature examining the phenomenon. "All the data seem to point to a new but perhaps not so uncommon kind of cosmic explosion."
In fact, observations of another gamma-ray burst (GRB060505 on May 5), which lasted four seconds, also lacked a supernova and 1990s data from the now-deorbited Compton Gamma Ray Observatory hint at similar "hybrid" bursts. "Two of the six long gamma-ray bursts seen at close distances appear to have no supernovae," notes astrophysicist Pall Jakobsson of the University of Hertfordshire. "This may be a more common type of explosion than we expected, possibly a new mechanism for star death."
Possible explanations for the hybrids could be the creation of black holes so powerful that the dying star gets no chance to supernova or that the massive stars in question differ significantly in their composition from their more spectacular peers. Or, it is possible that GRB060614 is witness to an entirely new form of stellar death. "This is brand-new territory," Gehrels adds. "We have no theories to guide us." Just the quick light of a dying star.