Assuming that the neutrino luminosity from the neutron star core is sufficiently high to drive supernova explosions by the neutrino-heating mechanism, we show that low-mode (l=1,2) convection can ...develop from random seed perturbations behind the shock. A slow onset of the explosion is crucial, requiring the core luminosity to vary slowly with time, in contrast to the burstlike exponential decay assumed in previous work. Gravitational and hydrodynamic forces by the globally asymmetric supernova ejecta were found to accelerate the remnant neutron star on a time scale of more than a second to velocities above 500 km s(-1), in agreement with observed pulsar proper motions.
ABSTRACT
We present three‐dimensional hydrodynamic simulations of the evolution of self‐gravitating, thick accretion discs around hyperaccreting stellar‐mass black holes. The black hole–torus systems ...are considered to be remnants of compact object mergers, in which case the disc is not fed by an external mass reservoir and the accretion is non‐stationary. Our models take into account viscous dissipation, described by an α‐law, a detailed equation of state for the disc gas, and an approximate treatment of general relativistic effects on the disc structure by using a pseudo‐Newtonian potential for the black hole including its possible rotation and spin‐up during accretion. Magnetic fields are ignored. The neutrino emission of the hot disc is treated by a neutrino‐trapping scheme, and the ‐annihilation near the disc is evaluated in a post‐processing step. Our simulations show that the neutrino emission and energy deposition by ‐annihilation increase sensitively with the disc mass, with the black hole spin in case of a disc in corotation, and in particular with the α‐viscosity. We find that for sufficiently large α‐viscosity, ‐annihilation can be a viable energy source for gamma‐ray bursts.
Based on recent models of relativistic jet formation by thermal energy deposition around black hole-torus systems, the relation between the on- and off-axis appearance of short, hard gamma-ray bursts ...(GRBs) is discussed in terms of energetics, duration, average Lorentz factor, and probability of observation, assuming that the central engines are remnants of binary neutron star (NS+NS) or neutron star-black hole (NS+BH) mergers. As a consequence of the interaction with the torus matter at the jet base and the subsequent expansion of the jets into an extremely low density environment, the collimated ultrarelativistic outflows possess flat core profiles with only little variation of radially averaged properties and are bounded by very steep lateral edges. Owing to the rapid decrease of the isotropic equivalent energy near the jet edges, the probability of observing the lateral, lower Lorentz factor wings is significantly reduced, and most short GRBs should be seen with on-axis-like properties. Taking into account cosmological and viewing angle effects, theoretical predictions are made for the short-GRB distributions with redshift z, fluence, and isotropic equivalent energy. The observational data for short bursts with determined redshifts are found to be compatible with the predictions only if either the intrinsic GRB rate density drops rapidly at z 1 or a large number of events at z > 1 are missed, implying that the subenergetic GRB 050509b was an extremely rare low-fluence event with detectable photon flux only because of its proximity and shortness. It appears unlikely that GRB 050509b can be explained as an off-axis event. The detection of short GRBs with small Lorentz factors is statistically disfavored, suggesting a possible reason for the absence of soft short bursts in the duration-hardness diagram.
Based on the shell model for Gamow-Teller and the random phase approximation for forbidden transitions, we calculate cross sections for inelastic neutrino-nucleus scattering (INNS) under supernova ...(SN) conditions, assuming a matter composition given by nuclear statistical equilibrium. The cross sections are incorporated into state-of-the-art stellar core-collapse simulations with detailed energy-dependent neutrino transport. While no significant effect on the SN dynamics is observed, INNS increases the neutrino opacities noticeably and strongly reduces the high-energy tail of the neutrino spectrum emitted in the neutrino burst at shock breakout. Relatedly the expected event rates for the observation of such neutrinos by earthbound detectors are reduced by up to about 60%.
Context. Mergers of neutron stars (NS) and black holes (BH) are among the strongest sources of gravitational waves and are potential central engines for short gamma-ray bursts. Aims. We aim to ...compare the general relativistic (GR) results of other groups with Newtonian calculations of models with equivalent parameters. We vary the mass ratio of the NS to BH and the compactness of the NS. The mass of the NS is 1.4 $M_\odot$. We compare the dynamics in the parameter-space regions where the NS is expected to reach the innermost stable circular orbit (ISCO) before being tidally disrupted (mass shedding, MS), and vice versa. Methods. The hydrodynamics is evolved by a Newtonian PPM scheme with four levels of nested grids. We use a polytropic EoS (Γ = 2), as adopted in the GR simulations. However, instead of full GR we use a Newtonian potential supplemented by a Paczyński-Wiita-Artemova potential for the BH, both disregarding and including rotation of the BH. Results. If the NS is compact (C = 0.18), it is accreted by the BH more quickly, and only a small amount of mass remains outside the BH. If the mass ratio is small (Q = 2 or 3) or the NS is less compact (C = 0.16 or less), the NS is tidally torn apart before being accreted. Although most of the mass is absorbed by the BH, some 0.1 $M_\odot$ remain in a tidal arm. For small mass ratios (Q = 2 and 3), the tidal arm can wrap around the BH to form a thick disk. When including the effects of either BH spin-up or spin-down by the accreted matter, more mass remains in the surroundings (0.2–0.3 $M_\odot$). Conclusions. Although details and quantitative results differ, the general trends of our Newtonian calculations are similar to the GR calculations. A clear delimiting line separating the ISCO from the MS cases is not found. Inclusion of BH rotation as well as sufficient numerical resolution are extremely important.
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