A new γ-ray burst classification scheme from GRB 060614

Gamma-ray bursts (GRBs) are known to come in two duration classes(1), separated at similar to 2 s. Long-duration bursts originate from star-forming regions in galaxies(2), have accompanying supernovae when these are near enough to observe and are probably caused by massive-star collapsars(3). Recent observations(4-10) show that short-duration bursts originate in regions within their host galaxies that have lower star-formation rates, consistent with binary neutron star or neutron star - black hole mergers(11,12). Moreover, although their hosts are predominantly nearby galaxies, no supernovae have been so far associated with short-duration GRBs. Here we report that the bright, nearby GRB 060614 does not fit into either class. Its similar to 102-s duration groups it with long-duration GRBs, while its temporal lag and peak luminosity fall entirely within the short-duration GRB subclass. Moreover, very deep optical observations exclude an accompanying supernova(13-15), similar to short-duration GRBs. This combination of a long-duration event without an accompanying supernova poses a challenge to both the collapsar and the merging-neutron-star interpretations and opens the door to a new GRB classification scheme that straddles both long- and short-duration bursts.