Stephan Rosswog

r-Process in Neutron Star Mergers

The production site of the neutron-rich heavy elements that are formed by rapid neutron capture (the r-process) is still unknown despite intensive research. Here we show detailed studies of a scenario that has been proposed earlier by Lattimer & Schramm, Symbalisty & Schramm, Eichler et al., and Davies et al., namely the merger of two neutron stars. The results of hydrodynamic and full network calculations are combined in order to investigate the relevance of this scenario for r-process nucleosynthesis. Sufficient material is ejected to explain the amount of r-process nuclei in the Galaxy by decompression of neutron star material. Provided that the ejecta consist of matter with a proton-to-nucleon ratio of Ye approximately 0.1, the calculated abundances fit the observed solar r-pattern excellently for nuclei that include and are heavier than the A approximately 130 peak.

High-resolution calculations of merging neutron stars - III. Gamma-ray bursts

Recent three-dimensional, high-resolution simulations of neutron star coalescences are analysed to assess whether short gamma-ray bursts (GRBs) could originate from such encounters. The two most popular modes of energy extraction - namely the annihilation of and magnetohydrodynamic processes - are explored in order to investigate their viability in launching GRBs. We find that annihilation can provide the necessary stresses to drive a highly relativistic expansion. However, unless the outflow is beamed into less than 1 per cent of the solid angle, this mechanism may fail to explain the apparent isotropized energies implied for short GRBs at cosmological distances. We argue that the energetic, neutrino-driven wind that accompanies the merger event will have enough pressure to provide adequate collimation to the -annihilation-driven jet, thereby comfortably satisfying constraints on event rate and apparent luminosity. We also assess magnetic mechanisms to transform the available energy into a GRB. If the central object does not collapse immediately into a black hole, it will be convective and it is expected to act as an effective large scale dynamo, amplifying the seed magnetic fields to a few times 1017 G within a small fraction of a second. The associated spindown time-scale is 0.2 s, coinciding with the typical duration of a short GRB. The efficiencies of the various assessed magnetic processes are high enough to produce isotropized luminosities in excess of 1052 erg s-1 even without beaming. (Less)

High-resolution calculations of merging neutron stars – II. Neutrino emission

The remnant resulting from the merger of two neutron stars produces neutrinos in copious amounts. In this paper we present the neutrino emission results obtained via Newtonian, high-resolution simulations of the coalescence event. These simulations use three-dimensional smoothed particle hydrodynamics together with a nuclear, temperature-dependent equation of state and a multiflavour neutrino leakage scheme. We present the details of our scheme, discuss the neutrino emission results from a neutron star coalescence and compare them with the core-collapse supernova case where neutrino emission has been studied for several decades. The average neutrino energies are similar to those in the supernova case, but contrary to the latter, the luminosities are dominated by electron-type antineutrinos that are produced in the hot, neutron-rich, thick disc of the merger remnant. The cooler parts of this disc contain substantial fractions of heavy nuclei, which, however, do not influence the overall neutrino emission results significantly. Our total neutrino luminosities from the merger event are considerably lower than those found in previous investigations. This imposes constraints on the ability of neutron star mergers to produce a gamma-ray burst via neutrino annihilation. The neutrinos are emitted preferentially along the initial binary rotation axis, an event seen ‘pole-on’ would appear much brighter in neutrinos than a similar event seen ‘edge-on’.

Accretion dynamics in neutron star-black hole binaries

We perform three-dimensional, Newtonian hydrodynamic simulations with a nuclear equation of state to investigate the accretion dynamics in neutron star black hole systems. We find as a general result that non-spinning donor stars yield larger circularization radii than corotating donors. Therefore, the matter from a neutron star without spin will more likely settle into an accretion disk outside the Schwarzschild radius. With the used stiff equation of state we find it hard to form an accretion disk that is promising to launch a gamma-ray burst. In all relevant cases the core of the neutron star survives and keeps orbiting the black hole as a mini neutron star for the rest of the simulation time (up to several hundred dynamical neutron star times scales). The existence of this mini neutron star leaves a clear imprint on the gravitational wave signal which thus can be used to probe the physics at supra-nuclear densities.

Jets, winds and bursts from coalescing neutron stars

Recent high-resolution calculations of neutron star coalescences are used to investigate whether ν ¯ ν annihilation can provide sufficient energy to power gamma-ray bursts, especially those belonging to the short duration category. Late time slices of the simulations, where the neutrino emission has reached a maximum, stationary level are analysed to address this question. We find that ν ¯ ν annihilation can provide the necessary driving stresses that lead to relativistic jet expansion. Maximum Lorentz factors of the order of 15 are found along the binary rotation axis, but larger values are expected to arise from higher numerical resolution. Yet the accompanying neutrino-driven wind must be absent from the axis when the burst occurs or prohibitive baryon loading may occur. We argue that even under the most favourable conditions, ν ¯ ν annihilation is unlikely to power a burst in excess of ∼10 48 erg. Unless the emission is beamed into less than one per cent of the solid angle, which we argue is improbable if it is collimated by gas pressure, this may fail to satisfy the apparent isotropic energies inferred at cosmological distances. This mechanism may none the less drive a precursor fireball, thereby evacuating a cavity into which a later magnetically driven jet could expand. A large range of time delays between the merger and the black hole formation are to be expected. If the magnetically driven jet occurs after the black hole has formed, a time span as long as weeks could pass between the neutrino powered precursor and the magnetically driven GRB.

An Adaptive Grid, Implicit Code for Spherically Symmetric, General Relativistic Hydrodynamics in Comoving Coordinates

We describe an implicit general relativistic hydrodynamics code. The evolution equations are formulated in comoving coordinates. A conservative finite differencing of the Einstein equations is outlined, and artificial viscosity and numerical diffusion are discussed. The time integration is performed with AGILE, an implicit solver for stiff algebrodifferential equations on a dynamical adaptive grid. We extend the adaptive grid technique, known from nonrelativistic hydrodynamics, to the general relativistic application and identify it with the concept of shift vectors in a 3+1 decomposition. The adaptive grid minimizes the number of required computational zones without compromising the resolution in physically important regions. Thus, the computational effort is greatly reduced when the zones are subject to computationally expensive additional processes, such as Boltzmann radiation transport or a nuclear reaction network. We present accurate results in the standard tests for supernova simulations: Sedovs point-blast explosion, the nonrelativistic and relativistic shock tube, the Oppenheimer-Snyder dust collapse, and homologous collapse.

On the diversity of short gamma‐ray bursts

Hydrodynamical simulations of the last inspiral stages and the final coalescence of a double neutron star system are used to investigate the power of the neutrino-driven wind, the energy and momentum of the fireball produced via νν annihilation, and the intensity and character of their interaction. It is argued that the outflow that derives from the debris will have enough pressure to collimate the relativistic fireball it surrounds. Then the low-luminosity relativistic jet will appear brighter to an observer within the beam, although most of the energy of the event is in the unseen, less collimated and slower wind. This model leads to a simple physical interpretation of the isotropic luminosities implied for short gamma-ray bursts at cosmological distances. A wide variety of burst phenomenology could be attributable to the dependence of the neutrino luminosity on the initial mass of the double neutron star binary.

Gamma-Ray Bursts, Supernova Kicks, and Gravitational Radiation

We suggest that the collapsing core of a massive rotating star may fragment to produce two or more compact objects. Their coalescence under gravitational radiation gives the resulting black hole or neutron star a significant kick velocity, which may explain those observed in pulsars. A gamma-ray burst can result only when this kick is small. Thus, only a small fraction of core-collapse supernovae produce gamma-ray bursts. The burst may be delayed significantly (hours to days) after the supernova, as suggested by recent observations. If our picture is correct, core-collapse supernovae should be significant sources of gravitational radiation with a chirp signal similar to a coalescing neutron star binary.

Neutron Star Binaries as Central Engines of GRBs

We describe the results of 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 106 particles. The merger leaves behind a strongly differentially rotating central object of ∼ 2.5 M⊙ 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 ∼ 102 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 powering very energetic bursts (≫ 1049 erg).

Nuclear physics issues of the r-process

The present paper aims at understanding r-process nucleosynthesis by addressing the nuclear physics involved, the necessary environment conditions in the (stellar) production sites, and the observational constraints. We also summarize the remaining challenges and uncertainties which need to be overcome for a full understanding of the nature of the r-process.