Making a Short Gamma-Ray Burst from a Long One: Implications for the Nature of GRB 060614

Abstract

The absence of a supernova accompanying the nearby long GRB 060614 poses a great puzzle about the progenitor of this event and challenges the current GRB classification scheme. This burst displays a short-hard emission episode followed by extended soft emission with strong spectral evolution. Noticing that this burst has an isotropic gamma-ray energy only ~8 times that of GRB 050724, a good candidate of merger-type short GRBs, we generate a "pseudo"burst that is ~8 times less energetic than GRB 060614 based on the spectral properties of GRB 060614 and the E p ∝ E (Amati) relation. We find that this pseudoburst would have been detected by BATSE as a marginal short-duration GRB and would have properties in the Swift BAT and XRT bands similar to GRB 050724. This suggests that GRB 060614 is likely a more intense event in the traditional short-hard GRB category. Events like GRB 060614 that seem to defy the traditional short versus long classification of GRBs may require modification of our classification terminology for GRBs. By analogy with supernova classifications, we suggest that GRBs be classified into Type I (typically short and associated with old populations) and Type II (typically long and associated with young populations). We propose that GRB 060614 belongs to Type I and predict that similar events will be detected in elliptical galaxies.