Gamma-ray Bursts from Neutron Star Mergers

Binary neutron stars merger (NSM) at cosmological distances is probably the only γ-ray bursts model based on an independently observed phenomenon which is known to be taking place at a comparable rate. We describe this model, its predictions and some open questions. Cosmological γ-Ray Bursts and Fireballs Compton-GRO has demonstrated, quite convincingly, that γ-ray bursts (grbs) originate from cosmological sources. Evidence for the predicted correlations between the duration, the strength and the hardness of the bursts begins to emerge. Preliminary analysis suggests that the weakest bursts originate from z ≈ 1, in agreement with a cosmological C/Cmin distribution , corresponding to a local rate of ≈ 10/year/galaxy (depending on the cosmological model and on other factors). The energy released in each burst depends also on the cosmological model 1050∼E∼10 ergs if the energy emission is isotropic. The intense energy released in a small volume (evident by the rapid rise time of some of the pulses) implies that any cosmological grb source is initially optically thick to γγ → ee. The large initial optical depth prevent us from observing directly the photons released by the source regardless of the specific nature or the source. The sources produce an optically thick radiation-electronpositrons plasma “fireball”, which behaves like a fluid, expands and reaches relativistic velocities. The observed radiation emerges only after the fireball has expanded significantly and became optically thin. One should divide, therefore, the discussion of cosmological grbs to a discussion of the nature of the energy source (for which we present a model here) and a discussion of the fireball phase (which we address elsewhere in this volume). For the paper it is sufficient to recall that the fireball must reach ultra-relativistic velocities with a Lorentz factor γ∼10 2 to produce a grb. Since γ ≈ E/Mc (where E is the total energy of the fireball and M is the mass of the baryons in the fireball) the condition γ > 10 sets a strong upper limit on the amount of baryons: M < E/γc ≈ .510M⊙(E/10 ergs)(γ/10). This condition poses a strong constraint on grb models. NSM and GRBs Agreement at a Glance Neutron star binaries, such as the one observed in the famous binary pulsar PSR 1916+13, end their life in a catastrophic merge event (denoted here NSM). Using the three observed binary pulsars we can estimate the expected rate of NSM events as ≈ 10/year/galaxy. An energy comparable to a neutron star binding energy (∼5× 10 53 ergs) is released in NSMs mostly as neutrinos and gravitational radiation. The neutrino signal is comparable in its