arXiv subjects

Stephan Rosswog

Publications and source records attributed to Stephan Rosswog.

83 records · Page 5Linked to original sources

arXiv2007

Remnant evolution after a carbon-oxygen white dwarf merger

We systematically explore the evolution of the merger of two carbon-oxygen (CO) white dwarfs. The dynamical evolution of a 0.9 Msun + 0.6 Msun CO white dwarf merger is followed by a three-dimensional SPH simulation. We use an elaborate prescription in which artificial viscosity is essentially absent, unless a shock is detected, and a much larger number of SPH particles than earlier calculations. Based on this simulation, we suggest that the central region of the merger remnant can, once it has reached quasi-static equilibrium, be approximated as a differentially rotating CO star, which consists of a slowly rotating cold core and a rapidly rotating hot envelope surrounded by a centrifugally supported disc. We construct a model of the CO remnant that mimics the results of the SPH simulation using a one-dimensional hydrodynamic stellar evolution code and then follow its secular evolution. The stellar evolution models indicate that the growth of the cold core is controlled by neutrino cooling at the interface between the core and the hot envelope, and that carbon ignition in the envelope can be avoided despite high effective accretion rates. This result suggests that the assumption of forced accretion of cold matter that was adopted in previous studies of the evolution of double CO white dwarf merger remnants may not be appropriate. Our results imply that at least some products of double CO white dwarfs merger may be considered good candidates for the progenitors of Type Ia supernovae. In this case, the characteristic time delay between the initial dynamical merger and the eventual explosion would be ~10^5 yr. (Abridged).

astro-ph↗

arXiv2006

Producing ultra-strong magnetic fields in neutron star mergers

We report an extremely rapid mechanism for magnetic field amplification during the merger of a binary neutron star system. This has implications for the production of the short class of Gamma-Ray Bursts, which recent observations suggest may originate in such mergers. In detailed magnetohydrodynamic simulations of the merger process, the fields are amplified via Kelvin-Helmholtz instabilities beyond magnetar field strength and may therefore represent the strongest magnetic fields in the Universe. The amplification occurs in the shear layer which forms between the neutron stars and on a time scale of only 1 millisecond, i.e. long before the remnant can collapse into a black hole.

astro-ph↗

arXiv2004

Gamma-ray Bursts and their Central Engines

Gamma-ray bursts are the most luminous and probably the most relativistic events in the universe. The last few years have seen a tremendous increase in our knowledge of these events, but the source of the bursts still remains elusive. I will summarise recent progress in this field with special emphasis on our understanding of the possible progenitor systems.

astro-ph↗

arXiv2003

Dynamos, Super-pulsars and Gamma-ray bursts

The remnant of a neutron star binary coalescence is expected to be temporarily stabilised against gravitational collapse by its differential rotation. We explore the possibility of dynamo activity in this remnant and assess the potential for powering a short-duration gamma-ray burst (GRB). We analyse our three-dimensional hydrodynamic simulations of neutron star mergers with respect to the flow pattern inside the remnant. If the central, newly formed super-massive neutron star remains stable for a good fraction of a second an efficient low-Rossby number $α-Ω$-dynamo will amplify the initial seed magnetic fields exponentially. We expect that values close to equipartition field strength will be reached within several tens of milliseconds. Such a super-pulsar could power a GRB via a relativistic wind, with an associated spin-down time scale close to the typical duration of a short GRB. Similar mechanisms are expected to be operational in the surrounding torus formed from neutron star debris.

astro-ph↗

arXiv2003

On the Central Engine of short Gamma-ray Bursts

We assess two mechanisms to produce a short GRB from a neutron star coalescence: the annihilation of neutrino and anti-neutrino pairs and magnetic processes within the remnant. We find that the neutrino-driven wind from the remnant can collimate the jets produced via neutrino annihilation into a small solid angle. This mechanism satisfies the observational constraints on the apparent luminosity and on the merger event rate. Moreover, the supermassive neutron star that is formed in the center of the remnant is expected to act as a convective dynamo reaching equipartition field strengths within fractions of a second. The associated spin-down time scale is similar to the typical duration of short GRBs.

astro-ph↗

arXiv2003

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 $ν\barν$ and magnetohydrodynamic processes- are explored in order to investigate their viability in launching GRBs. We find that $ν\barν$ annihilation can provide the necessary stresses to drive a highly relativistic expansion. But unless the outflow is beamed into less than one percent 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 $ν\barν$-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 $10^{17}$ 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 $10^{52}$ ergs/s even without beaming.

astro-ph↗

arXiv2003

On the diversity of short GRBs

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 $ν\barν$-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 that it surrounds. The low luminosity relativistic jet will then 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 GRBs 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 NS binary.

astro-ph↗

arXiv2002

Jets, winds and bursts from coalescing neutron stars

Recent high-resolution calculations of neutron star coalescences are used to investigate whether nu bar{nu} 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 analyzed to address this question. We find that nu bar{nu} 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 favorable conditions, nu bar{nu} annihilation is unlikely to power a burst in excess of ~ 10^{48} erg. Unless the emission is beamed into less than one percent 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 nonetheless drive a precursor fireball, thereby evacuating a cavity into which a later magnetic driven jet could expand. A large range of time delays between the merger and the black hole formation are to be expected. If the magnetic 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 magnetic driven GRB.

astro-ph↗

arXiv2002

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: Sedov's point-blast explosion, the nonrelativistic and relativistic shock tube, the Oppenheimer-Snyder dust collapse, and homologous collapse.

astro-ph↗

arXiv2002

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 - days) after the supernova, as suggested by recent observations. If this picture is correct, core-collapse supernovae should be significant sources of gravitational radiation with a chirp signal similar to a coalescing binary.

astro-ph↗

arXiv2002

Conformally Flat Smoothed Particle Hydrodynamics: Application to Neutron Star Mergers

We present a new 3D SPH code which solves the general relativistic field + hydrodynamics equations in the conformally flat approximation. Several test cases are considered to test different aspects of the code. We finally apply then the code to the coalescence of a neutron star binary system. The neutron stars are modeled by a polytropic equation of state (EoS) with adiabatic indices $Γ=2.0$, $Γ=2.6$ and $Γ=3.0$. We calculate the gravitational wave signals, luminosities and frequency spectra by employing the quadrupole approximation for emission and back reaction in the slow motion limit. In addition, we consider the amount of ejected mass.

gr-qc↗