QUARK-NOVAE IN LOW-MASS X-RAY BINARIES WITH MASSIVE NEUTRON STARS: A UNIVERSAL MODEL FOR SHORT-HARD GAMMA-RAY BURSTS

We show that several features reminiscent of short-hard gamma-ray bursts (GRBs) arise naturally when quark-novae (QNe) occur in low-mass X-ray binaries born with massive neutron stars (NS; >= 1.6 M-circle dot) and harboring a circumbinary disk (CD). Near the end of the first accretion phase, conditions are just right for the explosive conversion of the NS to a quark star (QS). In our model, the subsequent interaction of material from the NS's ejected crust with the CD explains the duration, variability, and near-universal nature of the prompt emission in short-hard GRBs. We also describe a statistical approach to ejecta breakup and collision to obtain the photon spectrum in our model, which turns out to be remarkably similar to the empirical Band function. We apply the model to the fluence and spectrum of GRB 000727, GRB 000218, and GRB980706A, obtaining excellent fits. Extended emission (EE; spectrum and duration) is explained by shock heating and ablation of the white dwarf by the highly energetic ejecta. Depending on the orbital separation when the QN occurs, we isolate interesting regimes within our model when both prompt emission and EE can occur. We find that the spectrum can carry signatures typical of Type Ib/c supernovae (SNe) although these should appear less luminous than normal type Ib/c SNe. Late X-ray activity is due to accretion onto the QS as well as its spin-down luminosity. Afterglow activity arises from the expanding shell of material from the shock-heated expanding CD. We find a correlation between the duration and spectrum of short-hard GRBs as well as modest hard-to-soft time evolution of the peak energy.