Neutron-Star Merger Ejecta as Obstacles to Neutrino-Powered Jets of Gamma-Ray Bursts

Abstract

We present the first special relativistic, axisymmetric hydrodynamic simulations of black hole-torus systems (approximating general relativistic gravity) as remnants of binary-neutron star (NS-NS) and neutron star-black hole (NS-BH) mergers, in which the viscously driven evolution of the accretion torus is followed with selfconsistent energy-dependent neutrino transport and the interaction with the cloud of dynamical ejecta expelled during the NS-NS merging is taken into account. The modeled torus masses, BH masses and spins, and the ejecta masses, velocities, and spatial distributions are adopted from relativistic merger simulations. We find that energy deposition by neutrino annihilation can accelerate outflows with initially high Lorentz factors along polar lowdensity funnels, but only in mergers with extremely low baryon pollution in the polar regions. NS-BH mergers, where polar mass ejection during the merging phase is absent, provide sufficiently baryon-poor environments to enable neutrino-powered, ultrarelativistic jets with terminal Lorentz factors above 100 and considerable dynamical collimation, favoring short gamma-ray bursts (sGRBs), although their typical energies and durations might be too small to explain the majority of events. In the case of NS-NS mergers, however, neutrino emission of the accreting and viscously spreading torus is too short and too weak to yield enough energy for the outflows to break out from the surrounding ejecta shell as highly relativistic jets. We conclude that neutrino annihilation alone cannot power sGRBs from NS-NS mergers.

A Neutron Star Binary Merger Model for GW170817/GRB 170817A/SSS17a

“…This is because even a tiny mass of baryons polluting the jet will severely limit the maximum attainable Lorentz factor and effective jet triggering might have to wait until after black hole collapse. In scenario a, the wind emanating from the HMNS hampers the advancement of a relativistic jet, leading to a low luminosity event (Rosswog & RamirezRuiz 2002, 2003Nagakura et al 2014;Just et al 2016). In scenario b in Figure 1, the collapse to a black hole occurs (green) and L * = 0.7 × 10 51 erg/s, a = 0.6 promptly and a classical jetted sGRB is produced which we happen to view off-axis (such as Ramirez-Ruiz et al 2005).…”

Flares in gamma-ray bursts: disc fragmentation and evolution

“…As it is found that neutrinos might play an important role only at the highest mass accretion rates (e.g. Janiuk et al 2007, Janiuk et al 2013, Just et al 2016), we will not consider this mechanism here. Instead, we will focus on scenarios where the rotational energy of a spinning BH is extracted by a large scale magnetic field threading the disk (Blandford & Znajek 1977).…”

Impact of Neutrino Interactions on Outflows of Neutron-Star Mergers

“…In [12], we addressed the question if νν-annihilation in post-merger BH-torus systems is efficient enough for the resulting jet-like outflow to penetrate the cloud of dynamical ejecta and to explain sGRBs. Likewise to [8] we employed the ALCAR-AENUS code [13] but upgraded its Newtonian hydrodynamics module to a special relativistic version.…”

“…Color maps of density, ρ, energy deposition rate by νν-annihilation, Q ann , proton-to-baryon ratio, Y e , Lorentz factor, Γ, and terminal Lorentz factor, Γ times specific enthalpy density h, for a choked annihilationdriven jet at 300 ms after a 1.2 − 1.8 M ⊙ NS-NS merger, shortly before dissipating its remaining kinetic energy in the surrounding envelope of dynamical ejecta. Figure adopted from [12].…”