arXiv subjects

William H. Lee

Publications and source records attributed to William H. Lee.

79 records · Page 5Linked to original sources

arXiv2001

Simulations of black hole-neutron star binary coalescence

We show the results of dynamical simulations of the coalescence of black hole-neutron star binaries. We use a Newtonian Smooth Particle Hydrodynamics code, and include the effects of gravitational radiation back reaction with the quadrupole approximation for point masses, and compute the gravitational radiation waveforms. We assume a polytropic equation of state determines the structure of the neutron star in equilibrium, and use an ideal gas law to follow the dynamical evolution. Three main parameters are explored: (i) The distribution of angular momentum in the system in the initial configuration, namely tidally locked systems vs. irrotational binaries; (ii) The stiffness of the equation of state through the value of the adiabatic index Gamma (ranging from Gamma=5/3 to Gamma=3); (iii) The initial mass ratio q=M(NS)/M(BH). We find that it is the value of Gamma that determines how the coalescence takes place, with immediate and complete tidal disruption for Gamma less than 2, while the core of the neutron star survives and stays in orbit around the black hole for Gamma=3. This result is largely independent of the initial mass ratio and spin configuration, and is reflected directly in the gravitational radiation signal. For a wide range of mass ratios, massive accretion disks are formed (M(disk)~0.2 solar masses), with baryon-free regions that could possibly give rise to gamma ray bursts.

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arXiv2000

Advection dominated flows around black holes in two dimensions

We show the results of two-dimensional, azimuthally symmetric simulations of advection dominated fluid flows around black holes. We use the SPH method, and the alpha-prescription for the viscosity, including all terms in the stress tensor. We have performed calculations with 0.001 < alpha < 0.2 and find strong circulation patterns in the flow, on spatial scales that depend on the magnitude of the viscosity.

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arXiv2000

Newtonian hydrodynamics of the coalescence of black holes with neutron stars III: Irrotational binaries with a stiff equation of state

We present a hydrodynamical study of the final stages of inspiral in a black hole-neutron star (NS) binary. We use a Newtonian 3D SPH code, and model the NS with a stiff (index G=3 and G=2.5) polytropic equation of state and the black hole as a Newtonian point mass. Our initial conditions correspond to irrotational binaries in equilibrium (approximating the NS as a compressible ellipsoid), and we have explored configurations with different initial mass ratios, 0.2< q=M_ns/M_bh<0.5. The dynamical evolution is followed for ~23ms. We include gravitational radiation losses in the quadrupole approximation for a point-mass binary. For G=3, after an initial episode of mass transfer, the NS is not completely disrupted and a remnant core remains in orbit about the black hole. For G=2.5 the disruption is more complex, with the NS being totally disrupted during a second periastron passage. The accretion disc formed around the black hole contains ~0.2 solar masses. A nearly baryon-free axis is always present in the system, and only modest beaming of a relativistic fireball that could give rise to a GRB would be sufficient to avoid baryon contamination. Around 0.01 solar masses may be dynamically ejected from the system, and could contribute substantially to the amount of observed galactic r-process material. We calculate the gravitational radiation waveforms in the quadrupole approximation. We also present the results of simulations that have used spherical NSs relaxed in isolation as initial conditions, in order to gauge the effect of using non-equilibrium initial conditions on the coalescence.

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arXiv1999

Newtonian hydrodynamics of the coalescence of black holes with neutron stars II: Tidally locked binaries with a soft equation of state

We present a numerical study of the hydrodynamics in the final stages of inspiral of a black hole-neutron star binary, when the binary separation becomes comparable to the stellar radius. We use a Newtonian three-dimensional Smooth Particle Hydrodynamics (SPH) code, and model the neutron star with a soft (adiabatic index Gamma=5/3) polytropic equation of state and the black hole as a Newtonian point mass which accretes matter via an absorbing boundary at the Schwarzschild radius. Our initial conditions correspond to tidally locked binaries in equilibrium, and we have explored configurations with different values of the mass ratio q=Mns/Mbh, ranging from q=1 to q=0.1. The dynamical evolution is followed for approximately 23 ms, and in every case studied here we find that the neutron star is tidally disrupted on a dynamical timescale, forming a dense torus around the black hole that contains a few tenths of a solar mass. A nearly baryon-free axis is present in the system throughout the coalescence, and only modest beaming of a fireball that could give rise to a gamma-ray burst would be sufficient to avoid excessive baryon contamination. We find that some mass (on the order of 0.01 to 0.001 solar masses) may be dynamically ejected from the system, and could thus contribute substantially to the amount of observed r-process material in the galaxy. We calculate the gravitational radiation waveforms and luminosity in the quadrupole approximation.

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arXiv1999

Hydrodynamics of black hole-neutron star coalescence

We present a numerical study of the hydrodynamics in the final stages of inspiral in a black hole-neutron star binary, when the separation becomes comparable to the stellar radius. We use a Newtonian three-dimensional Smooth Particle Hydrodynamics (SPH) code, and model the neutron star with a soft (adiabatic index Gamma=5/3) polytropic equation of state and the black hole as a Newtonian point mass which accretes matter via an absorbing boundary at the Schwarzschild radius. Our initial conditions correspond to tidally locked binaries in equilibirium. The dynamical evolution is followed for approximately 23 ms, and in every case for Gamma=5/3 we find that the neutron star is tidally disrupted on a dynamical timescale, forming a dense torus around the black hole that contains a few tenths of a solar mass. A nearly baryon-free axis is present in the system throughout the coalescence, a fireball expanding along that axis would avoid excessive baryon contamination and could give rise to a modestly beamed gamma-ray burst.

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arXiv1999

Creation of a dense torus in the coalescence of a black hole with a neutron star

We used a newtonian SPH (smooth-particle hydrodynamics) code to follow the final stages of evolution of a coalescing binary system of a neutron star and a black hole. We find that the outcome of the "merger" is very sensitive to the equation of state describing the neutron star. A neutron star with a soft equation of state (polytrope with index Gamma=5/3) is completely disrupted, and a fairly large and long-lived accretion torus is formed.

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arXiv1998

Newtonian Hydrodynamics of the Coalescence of Black Holes with Neutron Stars I: Tidally locked binaries with a stiff equation of state

We present a detailed study of the hydrodynamical interactions in a Newtonian black hole-neutron star binary during the last stages of inspiral. We consider close binaries which are tidally locked, use a stiff equation of state (with an adiabatic index Gamma=3) throughout, and explore the effect of different initial mass ratios on the evolution of the system. We calculate the gravitational radiation signal in the quadrupole approximation. Our calculations are carried out using a Smooth Particle Hydrodynamics (SPH) code.

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