"We have lots of data on this event, have dedicated lots of observation time, and we just can't figure out what exploded," said Neil Gehrels of NASA Goddard Space Flight Center - lead author on one of four reports appearing in this week's edition of the journal Nature. "All the data seem to point to a new but perhaps not so uncommon kind of cosmic explosion."
Gamma-ray bursts typically fall into one of two categories, long or short. The long bursts last more than two seconds and appear to be from the core collapse of massive stars forming a black hole. Most such bursts come from the edge of the visible universe. Short bursts, which are under two seconds and often last just a few milliseconds, appear to be the merger of two neutron stars or a neutron star with a black hole, which subsequently creates a new or bigger black hole.
The burst in question lasted for 102 seconds, but it lacked the hallmark of a supernova, or star explosion, commonly seen shortly after long bursts. Also, the burst's host galaxy has a low star-formation rate with few massive stars that could produce supernovae and long gamma-ray bursts.
"This was close enough to detect a supernova if it existed," said Avishay Gal-Yam of Caltech, lead author on another Nature report. "Even Hubble didn't see anything."
Certain properties of the burst concerning its brightness and the arrival time of photons of various energies, called the lag-luminosity relationship, suggest that burst behaved more like a short burst (from a merger) than a long burst. Yet no theoretical model of mergers can support a sustained release of gamma-ray energy for 102 seconds.
"This is brand new territory; we have no theories to guide us," said Gehrels.