Neutron Star Binaries as Central Engines of GRBs

We describe the results high resolution, hydrodynamic calculations of neutron star mergers. The model makes use of a new, nuclear equation of state, accounts for multi-flavour neutrino emission and solves the equations of hydrodynamics using the smoothed particle hydrodynamics method with more than $10^6$ particles. The merger leaves behind a strongly differentially rotating central object of $\sim 2.5$ M$_{\odot}$ together with a distribution of hot debris material. For the most realistic case of initial neutron star spins, no sign of a collapse to a black hole can be seen. We argue that the differential rotation stabilizes the central object for $\sim 10^2$ s and leads to superstrong magnetic fields. We find the neutrino emission from the hot debris around the freshly-formed, supermassive neutron star to be substantially lower than predicted previously. Therefore the annihilation of neutrino anti-neutrino pairs will have difficulties to power very energetic bursts ($\gg 10^{49}$ erg).Comment: 3 pages, 2 figures; Proceedings of "Gamma-Ray Burst and Afterglow Astronomy 2001", Woods Hol

Fallback accretion in the aftermath of a compact binary merger

Recent observations of long and short gamma-ray bursts have revealed a puzzling X-ray activity that in some cases continues for hours after the burst. It is difficult to reconcile such time scales with the viscous time scales that an accretion disk can plausibly provide. Here I discuss the accretion activity expected from the material that is launched into eccentric, but gravitationally bound orbits during a compact binary merger coalescence. From a simple analytical model the time scales and accretion luminosities that result from fallback in the aftermath of a compact binary merger are derived. For the considered mass range, double neutron star binaries are relatively homogeneous in their fallback luminosities. Neutron star black hole systems show a larger spread in their fallback behaviour. While the model is too simple to make predictions about the detailed time structure of the fallback, it makes reasonable predictions about the gross properties of the fallback. About one hour after the coalescence the fallback accretion luminosity can still be as large as $\sim 10^{45}$ erg/s, a fraction of which will be transformed into X-rays. Large-scale amplitude variations in the X-ray luminosities can plausibly be caused by gravitational fragmentation, which for the high-eccentricity fallback should occur more easily than in an accretion disk.Comment: accepted for publication in MNRAS letters, minor changes due to referee comments, 3 figure

From Neutron Star Binaries to Gamma-ray bursts

I summarize recent results about how a neutron star binary coalescence can produce short gamma-ray bursts (GRBs). Two possibilities are discussed: the annihilation of neutrino anti-neutrino pairs above the merged remnant and the exponential amplification of magnetic fields in the central object up to values close to equipartition. We find that the neutrino annihilation drives bipolar, relativistic outflows with Lorentz-factors large enough to circumvent the GRB 'compactness problem'. The total energy within these outflows is moderate by GRB-standards ($\sim 10^{48}-10^{49}$ ergs), but the interaction with the baryonic material blown-off by the neutrinos collimates the outflows into opening angles of typically 0.1 sterad, yielding isotropic energies close to $10^{51}$ ergs. We further want to stress the plausibility of the central object resisting the immediate collapse to a black hole. In this case the central object will --similar to a proto-neutron star-- be subject to neutrino driven convection that --together with the rapid, differential rotation-- will lead to a drastic amplification of pre-existing magnetic fields. Within fractions of a second, field strengths comparable to equipartition field strength ($> 10^{17}$ G) will be reached. These will produce large torques that will spin-down the object within about 0.2 s, and would thus naturally explain the duration of short GRBs.Comment: Proceedings of the 4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome,18-22 October 200

Boosting the accuracy of SPH techniques: Newtonian and special-relativistic tests

We study the impact of different discretization choices on the accuracy of SPH and we explore them in a large number of Newtonian and special-relativistic benchmark tests. As a first improvement, we explore a gradient prescription that requires the (analytical) inversion of a small matrix. For a regular particle distribution this improves gradient accuracies by approximately ten orders of magnitude and the SPH formulations with this gradient outperform the standard approach in all benchmark tests. Second, we demonstrate that a simple change of the kernel function can substantially increase the accuracy of an SPH scheme. While the "standard" cubic spline kernel generally performs poorly, the best overall performance is found for a high-order Wendland kernel which allows for only very little velocity noise and enforces a very regular particle distribution, even in highly dynamical tests. Third, we explore new SPH volume elements that enhance the treatment of fluid instabilities and, last, but not least, we design new dissipation triggers. They switch on near shocks and in regions where the flow --without dissipation-- starts to become noisy. The resulting new SPH formulation yields excellent results even in challenging tests where standard techniques fail completely.Comment: accepted for publication in MNRA

Tidal disruption and ignition of white dwarfs by moderately massive black holes

We present a numerical investigation of the tidal disruption of white dwarfs by moderately massive black holes, with particular reference to the centers of dwarf galaxies and globular clusters. Special attention is given to the fate of white dwarfs of all masses that approach the black hole close enough to be disrupted and severely compressed to such extent that explosive nuclear burning can be triggered. Consistent modeling of the gas dynamics together with the nuclear reactions allows for a realistic determination of the explosive energy release. In the most favorable cases, the nuclear energy release may be comparable to that of typical type Ia supernovae. Although the explosion will increase the mass fraction escaping on hyperbolic orbits, a good fraction of the debris remains to be swallowed by the hole, causing a bright soft X-ray flare lasting for about a year. Such transient signatures, if detected, would be a compelling testimony for the presence of a moderately mass black hole (below $10^5 M_\odot$).Comment: 38 pages, 19 figures, further simulations adde

Nucleosynthesis Calculations for the Ejecta of Neutron Star Coalescences

We present the results of fully dynamical r-process network calculations for the ejecta of neutron star mergers (NSMs). The late stages of the inspiral and the final violent coalescence of a neutron star binary have been calculated in detail using a 3D hydrodynamics code (Newtonian gravity plus backreaction forces emerging from the emission of gravitational waves) and a realistic nuclear equation of state. The found trajectories for the ejecta serve as input for dynamical r-process calculations where all relevant nuclear reactions (including beta-decays depositing nuclear energy in the expanding material) are followed. We find that all the ejected material undergoes r-process. For an initial Ye close to 0.1 the abundance distributions reproduce very accurately the solar r-process pattern for nuclei with A above 130. For lighter nuclei strongly underabundant (as compared to solar) distributions are encountered. We show that this behaviour is consistent with the latest observations of very old, metal-poor stars, despite simplistic arguments that have recently been raised against the possibility of NSM as possible sources of Galactic r-process material.Comment: 5 pages, 2 figures, proceedings of Nuclei in the Cosmos 2000, to be published in Nucl. Phys. A; minor correctio