ALBERT

Beschreibung: We present the methodology and performance of the new Lagrangian hydrodynamics code magma2, a smoothed particle hydrodynamics (SPH) code that benefits from a number of non-standard enhancements. By default it uses high-order smoothing kernels and wherever gradients are needed, they are calculated via accurate matrix inversion techniques, but a more conventional formulation with kernel gradients has also been implemented for comparison purposes. We also explore a matrix inversion formulation of SPH with a symmetrization in the particle indices that is not frequently used. We find interesting advantages of this formulation in some of the tests, for example, a substantial reduction of surface tension effects for non-ideal particle setups and more accurate peak densities in Sedov blast waves.  magma2 uses artificial viscosity, but enhanced by techniques that are commonly used in finite-volume schemes such as reconstruction and slope limiting. While simple to implement, this approach efficiently suppresses particle noise, but at the same time drastically reduces dissipation in locations where it is not needed and actually unwanted. We demonstrate the performance of the new code in a number of challenging benchmark tests including, for example, multidimensional vorticity creating Schulz–Rinne-type Riemann problems and more astrophysical tests such as a collision between two stars to demonstrate its robustness and excellent conservation properties.

Beschreibung: We use 3D hydrodynamic simulations of the long-term evolution of neutron star merger ejecta to predict the light curves of electromagnetic transients that are powered by the decay of freshly produced r -process nuclei. For the dynamic ejecta that are launched by tidal and hydrodynamic interaction, we adopt grey opacities of 10 cm 2  g –1 , as suggested by recent studies. For our reference case of a 1.3–1.4  $\mathrm{{\rm M}}_{\odot }$ merger, we find a broad IR peak 2–4 d after the merger. The peak luminosity is 2 10 40 erg s –1 for an average orientation, but increased by up to a factor of 4 for more favourable binary parameters and viewing angles. These signals are rather weak and hardly detectable within the large error box (~100 deg 2 ) of a gravitational wave trigger. A second electromagnetic transient results from neutrino-driven winds. These winds produce ‘weak’ r -process material with 50 〈  A  〈 130 and abundance patterns that vary substantially between different merger cases. For an adopted opacity of 1 cm 2  g –1 , the resulting transients peak in the UV/optical about 6 h after the merger with a luminosity of 10 41 erg s –1 (for a wind of 0.01  $\mathrm{{\rm M}}_{\odot }$ ) These signals are marginally detectable in deep follow-up searches (e.g. using Hypersuprime camera on Subaru). A subsequent detection of the weaker but longer lasting IR signal would allow an identification of the merger event. We briefly discuss the implications of our results to the recent detection of a near infrared (nIR) transient accompanying GRB 130603B.

Beschreibung: Solitary stars that wander too close to their galactic centres can become tidally disrupted, if the tidal forces due to the supermassive black hole residing there overcome the self-gravity of the star. If the star is only partially disrupted, so that a fraction survives as a self-bound object, this remaining core will experience a net gain in specific orbital energy, which translates into a velocity ‘kick’ of up to ~10 3 km s –1 . In this paper, we present the result of smoothed particle hydrodynamics simulations of such partial disruptions, and analyse the velocity kick imparted on the surviving core. We compare  = 5/3 and  = 4/3 polytropes disrupted in both a Newtonian potential, and a generalized potential that reproduces most relativistic effects around a Schwarzschild black hole either exactly or to excellent precision. For the Newtonian case, we confirm the results of previous studies that the kick velocity of the surviving core is virtually independent of the ratio of the black hole to stellar mass, and is a function of the impact parameter β alone, reaching at most the escape velocity of the original star. For a given β, relativistic effects become increasingly important for larger black hole masses. In particular, we find that the kick velocity increases with the black hole mass, making larger kicks more common than in the Newtonian case, as low-β encounters are statistically more likely than high-β encounters. The analysis of the tidal tensor for the generalized potential shows that our results are robust lower limits on the true relativistic kick velocities, and are generally in very good agreement with the exact results.

Beschreibung: We explore the evolution of the different ejecta components generated during the merger of a neutron star and a black hole. Our focus is the interplay between material ejected dynamically during the merger, and the wind launched on a viscous time-scale by the remnant accretion disc. These components are expected to contribute to an electromagnetic transient and to produce r-process elements, each with a different signature when considered separately. Here we introduce a two-step approach to investigate their combined evolution, using two- and three-dimensional hydrodynamic simulations. Starting from the output of a merger simulation, we identify each component in the initial condition based on its phase-space distribution, and evolve the accretion disc in axisymmetry. The wind blown from this disc is injected into a three-dimensional computational domain where the dynamical ejecta is evolved. We find that the wind can suppress fallback accretion on time-scales longer than ~100 ms. Because of self-similar viscous evolution, the disc accretion at late times nevertheless approaches a power-law time dependence t –2.2 . This can power some late-time gamma-ray burst engine activity, although the available energy is significantly less than in traditional fallback models. Inclusion of radioactive heating due to the r-process does not significantly affect the fallback accretion rate or the disc wind. We do not find any significant modification to the wind properties at large radius due to interaction with the dynamical ejecta. This is a consequence of the different expansion velocities of the two components.

Beschreibung: We present a detailed, three-dimensional hydrodynamic study of the neutrino-driven winds emerging from the remnant of a neutron star merger. Our simulations are performed with the Newtonian, Eulerian code fish , augmented by a detailed, spectral neutrino leakage scheme that accounts for neutrino absorption. Consistent with earlier two-dimensional studies, a strong baryonic wind is blown out along the original binary rotation axis within 100 ms. From this model, we compute a lower limit on the expelled mass of 3.5 10 –3  M , relevant for heavy element nucleosynthesis. Because of stronger neutrino irradiation, the polar regions show substantially larger electron fractions than those at lower latitudes. The polar ejecta produce interesting r-process contributions from A 80 to about 130, while the more neutron-rich, lower latitude parts produce elements up to the third r-process peak near A 195. We calculate the properties of electromagnetic transients powered by the radioactivity in the wind, in addition to the ‘macronova’ transient stemming from the dynamic ejecta. The polar regions produce ultraviolet/optical transients reaching luminosities up to 10 41 erg s –1 , which peak around 1 d in optical and 0.3 d in bolometric luminosity. The lower latitude regions, due to their contamination with high-opacity heavy elements, produce dimmer and more red signals, peaking after ~2 d in optical and infrared.

Beschreibung: We perform hydro- and magnetohydrodynamical general-relativistic simulations of a tidal disruption of a 0.1 M red dwarf approaching a 10 5 M non-rotating massive black hole on a close (impact parameter β = 10) elliptical (eccentricity e  = 0.97) orbit. We track the debris self-interaction, circularization and the accompanying accretion through the black hole horizon. We find that the relativistic precession leads to the formation of a self-crossing shock. The dissipated kinetic energy heats up the incoming debris and efficiently generates a quasi-spherical outflow. The self-interaction is modulated because of the feedback exerted by the flow on itself. The debris quickly forms a thick, almost marginally bound disc that remains turbulent for many orbital periods. Initially, the accretion through the black hole horizon results from the self-interaction, while in the later stages it is dominated by the debris originally ejected in the shocked region, as it gradually falls back towards the hole. The effective viscosity in the debris disc stems from the original hydrodynamical turbulence, which dominates over the magnetic component. The radiative efficiency is very low because of low energetics of the gas crossing the horizon and large optical depth that results in photon trapping. Although the parameters of the simulated tidal disruption are probably not representative of most observed events, it is possible to extrapolate some of its properties towards more common configurations.

Beschreibung: A generalized Newtonian potential is derived from the geodesic motion of test particles in Schwarzschild space–time. This potential reproduces several relativistic features with higher accuracy than commonly used pseudo-Newtonian approaches. The new potential reproduces the exact location of the marginally stable, marginally bound and photon circular orbits, as well as the exact radial dependence of the binding energy and the angular momentum of these orbits. Moreover, it reproduces the orbital and epicyclic angular frequencies to better than 6 per cent. In addition, the spatial projections of general trajectories coincide with their relativistic counterparts, while the time evolution of parabolic-like trajectories and the pericentre advance of elliptical-like trajectories are both reproduced exactly. We apply this approach to a standard thin accretion disc and find that the efficiency of energy extraction agrees to within 3 per cent with the exact relativistic value, while the energy flux per unit area as a function of radius is reproduced everywhere to better than 7 per cent. As a further astrophysical application we implement the new approach within a smoothed particle hydrodynamics code and study the tidal disruption of a main-sequence star by a supermassive black hole. The results obtained are in very good agreement with previous relativistic simulations of tidal disruptions in Schwarzschild space–time. The equations of motion derived from this potential can be implemented easily within existing Newtonian hydrodynamics codes with hardly any additional computational effort.

Beschreibung: Compact binary mergers are prime sources of gravitational waves (GWs), targeted by current and next generation detectors. The question ‘what is the observable electromagnetic (EM) signature of a compact binary merger?’ is an intriguing one with crucial consequences to the quest for GWs. We present a large set of numerical simulations that focus on the EM signals that emerge from the dynamically ejected subrelativistic material. These outflows produce on a time-scale of a day macronovae – short-lived infrared (IR) to ultraviolet (UV) signals powered by radioactive decay. Like in regular supernovae the interaction of this outflow with the surrounding matter inevitably leads to a long-lasting remnant. We calculate the expected radio signals of these remnants on time-scales longer than a year, when the subrelativistic ejecta dominate the emission. We discuss their detectability in 1.4 GHz and 150 MHz and compare it with an updated estimate of the detectability of short gamma-ray bursts’ orphan afterglows (which are produced by a different component of this outflow). We find that mergers with characteristics similar to those of the Galactic neutron star binary population (similar masses and typical circummerger Galactic disc density of ~1 cm –3 ) that take place at the detection horizon of advanced GW detectors (300 Mpc) yield 1.4 GHz [150 MHz] signals of ~50 [300] μJy, for several years. The signal on time-scales of weeks is dominated by the mildly and/or ultrarelativistic outflow, which is not accounted for by our simulations, and is expected to be even brighter. Upcoming all sky surveys are expected to detect a few dozen, and possibly more, merger remnants at any given time thereby providing robust lower limits to the mergers rate even before the advanced GW detectors become operational. The macronovae signals from the same distance peak in the IR to UV range at an observed magnitude that may be as bright as 22–23 about 10 h after the merger but dimmer, redder and longer if the opacity is larger.