arXiv subjects

S. Rosswog

Publications and source records attributed to S. Rosswog.

At least 55 records · Page 3Linked to original sources

arXiv2012

On the point mass approximation to calculate the gravitational wave signal from white dwarf binaries

Double white dwarf binaries in the Galaxy dominate the gravitational wave sky and would be detectable for an instrument such as LISA. Most studies have calculated the expected gravitational wave signal under the assumption that the binary white dwarf system can be represented by two point masses in orbit. We discuss the accuracy of this approximation for real astrophysical systems. For non-relativistic binaries in circular orbit the gravitational wave signal can easily be calculated. We show that for these systems the point mass approximation is completely justified when the individual stars are axisymmetric irrespective of their size. We find that the signal obtained from Smoothed-Particle Hydrodynamics simulations of tidally deformed, Roche-lobe filling white dwarfs, including one case when an accretion disc is present, is consistent with the point mass approximation. The difference is typically at the level of one per cent or less in realistic cases, yielding small errors in the inferred parameters of the binaries.

astro-ph.HE↗

arXiv2010

Compact binary mergers: an astrophysical perspective

This paper reviews the current understanding of double neutron star and neutron star black hole binaries. It addresses mainly (nuclear) astrophysics aspects of compact binary mergers and thus complements recent reviews that have emphasized the numerical relativity viewpoint. In particular, the paper discusses different channels to release neutron-rich matter into the host galaxy, connections between compact binary mergers and short Gamma-ray bursts and accompanying electromagnetic signals.

astro-ph.HE↗

arXiv2010

Conservative, special-relativistic smoothed particle hydrodynamics

We present and test a new, special-relativistic formulation of Smoothed Particle Hydrodynamics (SPH). Our approach benefits from several improvements with respect to earlier relativistic SPH formulations. It is self-consistently derived from the Lagrangian of an ideal fluid and accounts for special-relativistic "grad-h terms". In our approach, we evolve the canonical momentum and the canonical energy per baryon and thus circumvent some of the problems that have plagued earlier formulations of relativistic SPH. We further use a much improved artificial viscosity prescription which uses the extreme local eigenvalues of the Euler equations and triggers selectively on a) shocks and b) velocity noise. The shock trigger accurately monitors the relative density slope and uses it to fine-tune the amount of artificial viscosity that is applied. This procedure substantially sharpens shock fronts while still avoiding post-shock noise. If not triggered, the viscosity parameter of each particle decays to zero. None of these viscosity triggers is specific to special relativity, both could also be applied in Newtonian SPH. The performance of the new scheme is explored in a large variety of benchmark tests where it delivers excellent results. Generally, the grad-h terms deliver minor, though worthwhile, improvements. The scheme performs close to perfect in supersonic advection tests, but also in strong relativistic shocks, usually considered a particular challenge for SPH, the method yields convincing results. For example, due to its perfect conservation properties, it is able to handle Lorentz-factors as large as $γ= 50 \; 000$ in the so-called wall shock test. Moreover, we find convincing results in a rarely shown, but challenging test that involves so-called relativistic simple waves and also in multi-dimensional shock tube tests.

astro-ph.HE↗

arXiv2010

Special-relativistic Smoothed Particle Hydrodynamics: a benchmark suite

In this paper we test a special-relativistic formulation of Smoothed Particle Hydrodynamics (SPH) that has been derived from the Lagrangian of an ideal fluid. Apart from its symmetry in the particle indices, the new formulation differs from earlier approaches in its artificial viscosity and in the use of special-relativistic ``grad-h-terms''. In this paper we benchmark the scheme in a number of demanding test problems. Maybe not too surprising for such a Lagrangian scheme, it performs close to perfectly in pure advection tests. What is more, the method produces accurate results even in highly relativistic shock problems.

astro-ph.IM↗

arXiv2009

Collisions of white dwarfs as a new progenitor channel for type Ia supernovae

We present the results of a systematic numerical study of an alternative progenitor scenario to produce type Ia supernova explosions, which is not restricted to the ignition of a CO white dwarf near the Chandrasekhar mass. In this scenario, a shock-triggered thermonuclear explosion ensues from the collision of two white dwarfs. Consistent modeling of the gas dynamics together with nuclear reactions using both a smoothed particle and a grid-based hydrodynamics code are performed to study the viability of this alternative progenitor channel. We find that shock-triggered ignition and the synthesis of Ni are in fact a natural outcome for moderately massive white dwarf pairs colliding close to head-on. We use a multi-dimensional radiative transfer code to calculate the emergent broadband light curves and spectral time-series of these events. The synthetic spectra and lightcurves compare well with those of normal type Ia supernovae over a similar B-band decline rate and are broadly consistent with the Phillips relation, although a mild dependence on viewing angle is observed due to the asymmetry of the ejected debris. While event rates within galactic centers and globular clusters are found to be much too low to explain the bulk of the type Ia supernovae population, they may be frequent enough to make as much as a one percent contribution to the overall rate. Although these rate estimates are still subject to substantial uncertainties, they do suggest that dense stellar systems should provide upcoming supernova surveys with hundreds of such collision-induced thermonuclear explosions per year.

astro-ph.HE↗

arXiv2008

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 $10^5 M_\odot$).

astro-ph↗

arXiv2008

Tidally-induced thermonuclear Supernovae

We discuss the results of 3D simulations of tidal disruptions of white dwarfs by moderate-mass black holes as they may exist in the cores of globular clusters or dwarf galaxies. Our simulations follow self-consistently the hydrodynamic and nuclear evolution from the initial parabolic orbit over the disruption to the build-up of an accretion disk around the black hole. For strong enough encounters (pericentre distances smaller than about 1/3 of the tidal radius) the tidal compression is reversed by a shock and finally results in a thermonuclear explosion. These explosions are not restricted to progenitor masses close to the Chandrasekhar limit, we find exploding examples throughout the whole white dwarf mass range. There is, however, a restriction on the masses of the involved black holes: black holes more massive than $2\times 10^5$ M$_\odot$ swallow a typical 0.6 M$_\odot$ dwarf before their tidal forces can overwhelm the star's self-gravity. Therefore, this mechanism is characteristic for black holes of moderate masses. The material that remains bound to the black hole settles into an accretion disk and produces an X-ray flare close to the Eddington limit of $L_{\rm Edd} \simeq 10^{41} {\rm erg/s} M_{\rm bh}/1000 M$_\odot$), typically lasting for a few months. The combination of a peculiar thermonuclear supernova together with an X-ray flare thus whistle-blows the existence of such moderate-mass black holes. The next generation of wide field space-based instruments should be able to detect such events.

astro-ph↗

arXiv2008

Mass transfer dynamics in double degenerate binary systems

We present a numerical study of the mass transfer dynamics prior to the gravitational wave-driven merger of a double white dwarf system. Recently, there has been some discussion about the dynamics of these last stages, different methods seemed to provide qualitatively different results. While earlier SPH simulations indicated a very quick disruption of the binary on roughly the orbital time scale, more recent grid-based calculations find long-lived mass transfer for many orbital periods. Here we demonstrate how sensitive the dynamics of this last stage is to the exact initial conditions. We show that, after a careful preparation of the initial conditions, the reportedly short-lived systems undergo mass transfer for many dozens of orbits. The reported numbers of orbits are resolution-biased and therefore represent only lower limits to what is realized in nature. Nevertheless, the study shows convincingly the convergence of different methods to very similar results.

astro-ph↗

arXiv2008

3D meshfree magnetohydrodynamics

We describe a new method to include magnetic fields into smooth particle hydrodynamics. The derivation of the self-gravitating hydrodynamics equations from a variational principle is discussed in some detail. The non-dissipative magnetic field evolution is instantiated by advecting so-called Euler potentials. This approach enforces the crucial $\nabla\cdot\vec{B}=0$-constraint by construction. These recent developments are implemented in our three-dimensional, self-gravitating magnetohydrodynamics code MAGMA. A suite of tests is presented that demonstrates the superiority of this new approach in comparison to previous implementations.

astro-ph↗

arXiv2008

Simulating Black Hole White Dwarf Encounters

The existence of supermassive black holes lurking in the centers of galaxies and of stellar binary systems containing a black hole with a few solar masses has been established beyond reasonable doubt. The idea that black holes of intermediate masses ($\sim 1000$ \msun) may exist in globular star clusters has gained credence over recent years but no conclusive evidence has been established yet. An attractive feature of this hypothesis is the potential to not only disrupt solar-type stars but also compact white dwarf stars. In close encounters the white dwarfs can be sufficiently compressed to thermonuclearly explode. The detection of an underluminous thermonuclear explosion accompanied by a soft, transient X-ray signal would be compelling evidence for the presence of intermediate mass black holes in stellar clusters. In this paper we focus on the numerical techniques used to simulate the entire disruption process from the initial parabolic orbit, over the nuclear energy release during tidal compression, the subsequent ejection of freshly synthesized material and the formation process of an accretion disk around the black hole.

astro-ph↗

arXiv2007

Atypical Thermonuclear Supernovae from Tidally Crushed White Dwarfs

Suggestive evidence has accumulated that intermediate mass black holes (IMBH) exist in some globular clusters. As stars diffuse in the cluster, some will inevitable wander sufficiently close to the hole that they suffer tidal disruption. An attractive feature of the IMBH hypothesis is its potential to disrupt not only solar-type stars but also compact white dwarf stars. Attention is given to the fate of white dwarfs that approach the hole close enough to be disrupted and compressed to such extent that explosive nuclear burning may be triggered. Precise modeling of the dynamics of the encounter coupled with a nuclear network allow for a realistic determination of the explosive energy release, and it is argued that ignition is a natural outcome for white dwarfs of all varieties passing well within the tidal radius. Although event rates are estimated to be significantly less than the rate of normal Type Ia supernovae, such encounters may be frequent enough in globular clusters harboring an IMBH to warrant a search for this new class of supernova.

astro-ph↗

arXiv2007

Enhancing SPH using moving least-squares and radial basis functions

In this paper we consider two sources of enhancement for the meshfree Lagrangian particle method smoothed particle hydrodynamics (SPH) by improving the accuracy of the particle approximation. Namely, we will consider shape functions constructed using: moving least-squares approximation (MLS); radial basis functions (RBF). Using MLS approximation is appealing because polynomial consistency of the particle approximation can be enforced. RBFs further appeal as they allow one to dispense with the smoothing-length -- the parameter in the SPH method which governs the number of particles within the support of the shape function. Currently, only ad hoc methods for choosing the smoothing-length exist. We ensure that any enhancement retains the conservative and meshfree nature of SPH. In doing so, we derive a new set of variationally-consistent hydrodynamic equations. Finally, we demonstrate the performance of the new equations on the Sod shock tube problem.

math.NA↗

arXiv2007

MAGMA: a 3D, Lagrangian magnetohydrodynamics code for merger applications

We present a new, completely Lagrangian magnetohydrodynamics code that is based on the SPH method. The equations of self-gravitating hydrodynamics are derived self-consistently from a Lagrangian and account for variable smoothing length (``grad-h''-) terms in both the hydrodynamic and the gravitational acceleration equations. The evolution of the magnetic field is formulated in terms of so-called Euler potentials which are advected with the fluid and thus guarantee the MHD flux-freezing condition. This formulation is equivalent to a vector potential approach and therefore fulfills the $\vec{\nabla}\cdot\vec{B}=0$-constraint by construction. Extensive tests in one, two and three dimensions are presented. The tests demonstrate the excellent conservation properties of the code and show the clear superiority of the Euler potentials over earlier magnetic SPH formulations.

astro-ph↗

arXiv2007

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.

astro-ph↗

arXiv2006

Last moments in the life of a compact binary system: gravitational waves, gamma-ray bursts and magnetar formation

The first detections of afterglows from short gamma-ray bursts (GRBs) have confirmed the previous suspicion that they are triggered by a different central engine than long bursts. In particular, the recent detections of short GRBs in galaxies without star formation lends support to the idea that an old stellar population is involved. Most prominent are mergers of either double neutron stars or of a neutron star with a stellar-mass black hole companion. Since the final identification of the central engine will only come from an integral view of several properties, we review the observable signatures that can be expected from both double neutron stars and neutron star black hole systems. We discuss the gravitational wave emission, the structure of the neutrino-cooled accretion disks, the resulting neutrino signal and possible mechanisms to launch a GRB. In addition, we address the speculative idea that in some cases a magnetar-like object may be the final outcome of a double neutron star merger. We also discuss possibilities to explain the late-time X-ray activity that has been observed in several bursts.

astro-ph↗

arXiv2005

Mergers of neutron star black hole binaries with small mass ratios: nucleosynthesis, gamma-ray bursts and electromagnetic transients

I discuss simulations of the coalescence of black hole neutron star binary systems with black hole masses between 14 and 20 \msun. The calculations use a three-dimensional smoothed particle hydrodynamics code, a temperature-dependent, nuclear equation of state and a multi-flavor neutrino scheme. General relativistic effects are mimicked using the \Pacz-Wiita pseudo-potential and gravitational radiation reaction forces. Opposite to previous, purely Newtonian calculations, in none of the explored cases episodic mass transfer occurs. The neutron star is always completely disrupted after most of its mass has been transferred directly into the hole. For black hole masses between 14 and 16 \Msun an accretion disk forms, large parts of it, however, are inside the last stable orbit and therefore falling with large radial velocities into the hole. These disks are (opposite to the neutron star merger case) thin and -apart from a spiral shock- essentially cold. For higher mass black holes ($M_{\rm BH} \ge 18$ \msun) almost the complete neutron star disappears in the hole without forming an accretion disk. In these cases the surviving material is spun up by tidal torques and ejected as a half-ring of neutron-rich matter. None of the investigated systems is a promising GRB central engine. We find between 0.01 and 0.2 \Msun of the neutron star to be dynamically ejected. Like in a type Ia supernova, the radioactive decay of this material will power a light curve with a peak luminosity of a few times $10^{44}$ erg/s. The maximum will be reached about three days after the coalescence and will be mainly visible in the optical/near infrared band. The coalescence itself may produce a precursor pulse with a thermal spectrum of $\sim 10$ ms duration.

astro-ph↗

arXiv2005

On the viability of neutron star black hole binaries as central engines of gamma-ray bursts

I discuss three-dimensional SPH simulations of neutron star black hole encounters. The calculations are performed using a nuclear equation of state and a multi-flavor neutrino treatment, general relativistic effects are mimicked using the Paczynski-Wiita pseudo-potential and gravitational radiation reaction forces. Most of the explored mass range (14 to 20 \msun) has not been considered before in numerical simulations. The neutron star is always disrupted during the first approach after most of its mass has been transferred directly into the hole. In none of the analyzed cases episodic mass transfer is found. If an accretion disk forms at all, it is relatively cold ($T < 2.5$ MeV), of only moderate density ($ρ\sim 10^{10}$ g/ccm) and substantially consumed by the hole on the simulation time scale of a few milliseconds. None of the investigated systems of this study yields conditions that are promising to launch a GRB. While we cannot completely exclude that a subset of neutron star black hole binaries, maybe black holes with low masses and very large initial spins, can produce a GRB, this seems to happen -if at all- only in a restricted region of the available parameter space. I argue that the difficulty to form promising disks together with the absence of any observed neutron star black hole binary may mean that they are insignificant as central engines of the observed, short-hard GRBs and that the vast majority of the latter ones is caused by double neutron star coalescences.

astro-ph↗

arXiv2005

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.

astro-ph↗