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

Abstract

We show that several features reminiscent of short-hard Gamma-ray Bursts (GRBs) arise naturally when Quark-Novae occur in low-mass X-ray binaries born with massive neutron stars (> 1.6M_sun) and harboring a circumbinary disk. Near the end of the first accretion phase, conditions are just right for the explosive conversion of the neutron star to a quark star (Quark-Nova). In our model, the subsequent interaction of material from the neutron star's ejected crust with the circumbinary disk explains the duration, variability and near-universal nature of the prompt emission in short-hard GRBs. We also describe a statistical approach to ejecta break-up and collision to obtain the photon spectrum in our model, which turns out remarkably similar to the empirical Band function (Band 1993). We apply the model to the fluence and spectrum of GRB 000727, GRB 000218, and GRB980706A obtaining excellent fits. Extended emission (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 Quark-Nova occurs, we isolate interesting regimes within our model when both prompt and extended emission can occur. We find that the spectrum can carry signatures typical of Type Ib/c SNe although these should appear less luminous than normal type Ib/c SNe. Late X-ray activity is due to accretion onto the quark star as well as its spin-down luminosity. Afterglow activity arise from the expanding shell of material from the shock-heated expanding circumbinary disk. 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.