GRB060614: a “fake” short GRB from a merging binary system

¹ Dipartimento di Fisica and ICRA, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy e-mail: [letizia.caito;maria.bernardini;bianco;dainotti;roberto.guida;ruffini]@icra.it
² ICRANet, Piazzale della Repubblica 10, 65122 Pescara, Italy
³ ICRANet, Université de Nice Sophia Antipolis, Grand Château, BP 2135, 28 avenue de Valrose, 06103 Nice Cedex 2, France

Received: 25 July 2008
Accepted: 13 January 2009

Abstract

Context. GRB060614 observations by VLT and by Swift have infringed the traditionally accepted gamma-ray burst (GRB) collapsar scenario that purports the origin of all long duration GRBs from supernovae (SN). GRB060614 is the first nearby long duration GRB clearly not associated with a bright Ib/c SN. Moreover, its duration (T₉₀ ~ 100 s) makes it hardly classifiable as a short GRB. It presents strong similarities with GRB970228, the prototype of a 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. We model the traditionally termed “prompt emission” and discriminate the signal originating from the gravitational collapse leading to the GRB from the process occurring in the circumburst medium (CBM).

Methods. We fit GRB060614 light curves in Swift's BAT (15–150 keV) and XRT (0.2–10 keV) energy bands. Within the fireshell model, light curves are formed by two well defined and different components: the proper-GRB (P-GRB), emitted when the fireshell becomes transparent, 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: the total plasma energy ( = 2.9410⁵¹ erg) and baryon loading (B = 2.810^-3). A small average CBM density ~ 10^-3 particles/cm³ 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. We obtain very good agreement in the BAT (15–150 keV) energy band, in what is traditionally called “prompt emission”, and in the XRT (0.2–10 keV) one.

Conclusions. The anomalous GRB060614 finds a natural interpretation within our canonical GRB scenario: it is a “disguised” short GRB. The total time-integrated extended afterglow luminosity is greater than the P-GRB one, but its peak luminosity is smaller since it is deflated by the peculiarly low average CBM density of galactic halos. 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 justifies 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 0.1 s.