GRB060614: a "fake" short GRB from a merging binary system

TL;DR

GRB060614, a long-duration gamma-ray burst without an associated supernova, is explained as a 'disguised' short GRB originating from a binary neutron star or white dwarf merger within the fireshell model.

Contribution

This study demonstrates that GRB060614 can be interpreted as a 'fake' short GRB within the fireshell model, highlighting the role of the circumburst medium and binary progenitors.

Findings

GRB060614's light curves fit the fireshell model with a low-density galactic halo environment.

The initial spike is identified as the P-GRB, followed by an extended afterglow.

The progenitor is likely a white dwarf-neutron star binary, explaining the absence of a supernova.

Abstract

(Shortened) CONTEXT: [...] GRB060614 is the first nearby long duration GRB clearly not associated to a bright Ib/c supernova. Moreover, its duration (T_{90} ~ 100s) makes it hardly classifiable as a short GRB. It presents strong similarities with GRB970228, the prototype of the new class of "fake" short GRBs that appear to originate from the coalescence of binary neutron stars or white dwarfs spiraled out into the galactic halo. AIMS: Within the "canonical" GRB scenario based on the "fireshell" model, we test if GRB060614 can be a "fake" or "disguised" short GRB. [...] METHODS: We fit GRB060614 light curves in Swift's BAT (15-150keV) and XRT (0.2-10keV) energy bands. Within the fireshell model, light curves are formed by two well defined and different components: the Proper-GRB (P-GRB), emitted at the fireshell transparency, and the extended afterglow, due to the interaction between the leftover accelerated baryonic and leptonic shell and the CBM. RESULTS: We determine the two free parameters describing the GRB source within the fireshell model. [...] A small average CBM density [...] is inferred, typical of galactic halos. The first spikelike emission is identified with the P-GRB and the following prolonged emission with the extended afterglow peak.[...] CONCLUSIONS: The anomalous GRB060614 finds a natural interpretation within our canonical GRB scenario: it is a "disguised" short GRB. [...] This result points to an old binary system, likely formed by a white dwarf and a neutron star, as the progenitor of GRB060614 and well justify the absence of an associated SN Ib/c. Particularly important for further studies of the final merging process are the temporal structures in the P-GRB down to 0.1s.