Merger of white dwarf-neutron star binaries: Prelude to hydrodynamic simulations in general relativity (original) (raw)
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Monthly Notices of the Royal Astronomical Society
We investigate the effects of mass transfer and gravitational wave (GW) radiation on the orbital evolution of contact neutron-star–white-dwarf (NS–WD) binaries, and the detectability of these binaries by space GW detectors (e.g. Laser Interferometer Space Antenna, LISA; Taiji; Tianqin). A NS–WD binary becomes contact when the WD component fills its Roche lobe, at which the GW frequency ranges from ∼0.0023 to 0.72 Hz for WD with masses ∼0.05–1.4 M⊙. We find that some high-mass NS–WD binaries may undergo direct coalescence after unstable mass transfer. However, the majority of NS–WD binaries can avoid direct coalescence because mass transfer after contact can lead to a reversal of the orbital evolution. Our model can well interpret the orbital evolution of the ultra-compact X-ray source 4U 1820–30. For a 4-yr observation of 4U 1820–30, the expected signal-to-noise-ratio (SNR) in GW characteristic strain is ∼11.0/10.4/2.2 (LISA/Taiji/Tianqin). The evolution of GW frequencies of NS–WD b...
Evolution of Close White Dwarf Binaries
The Astrophysical Journal, 2007
We describe the evolution of double degenerate binary systems, consisting of components obeying the zero temperature mass radius relationship for white dwarf stars, from the onset of mass transfer to one of several possible outcomes including merger, tidal disruption of the donor, or survival as a semidetached AM CVn system. We use a combination of analytic solutions and numerical integrations of the standard orbit-averaged first-order evolution equations, including direct impact accretion and the evolution of the components due to mass exchange. We include also the effects of mass-loss during super-critical (super-Eddington) mass transfer and the tidal and advective exchanges of angular momentum between the binary components. We cover much the same ground as with the additional effects of the advective or consequential angular momentum from the donor and its tidal coupling to the orbit which is expected to be stronger than that of the accretor. With the caveat that our formalism does not include an explicit treatment of common envelope phases, our results suggest that a larger fraction of detached double white dwarfs than what has been hitherto assumed, survive the onset of mass transfer, even if this mass transfer is initially unstable and rises to super-Eddington levels. In addition, as a consequence of the tidal coupling, systems that come into contact near the mass transfer instability boundary undergo a phase of oscillation cycles in their orbital period (and other system parameters). Unless the donor star has a finite entropy such that the effective mass-radius relationship deviates significantly from that of a zero temperature white dwarf, we expect our results to be valid. Much of the formalism developed here would also apply to other mass-transferring binaries, and in particular to cataclysmic variables and Algol systems. A system of two point masses orbiting around each other, in circular orbits, radiates gravitationally . The loss of orbital angular momentum as a result is given by J * LRS2 show that for Riemann-S and Roche-Riemann sequences dE = ΩdJ + ΛdC, where Λ is the angular velocity of internal motions and C is the equatorial circulation. Thus, as a binary evolves driven by gravitational wave radiation, circulation is conserved and the minima of E and J will coincide. However tidal dissipation does not preserve C and thus in general the minima will not coincide in the presence of tidal spin-orbit coupling. † In the range 0 < q ≤ 1, the function ζr L takes values between 0.32 and 0.46, and is well approximated by ζr L ≈ 0.30 + 0.16q for 0.1 ≤ q ≤ 1
Gravitational wave radiation from the coalescence of white dwarfs
Monthly Notices of the Royal Astronomical Society, 2005
We compute the emission of gravitational radiation from the merging of a close white dwarf binary system. This is done for a wide range of masses and compositions of the white dwarfs, ranging from mergers involving two He white dwarfs, mergers in which two CO white dwarfs coalesce to mergers in which a massive ONe white dwarf is involved. In doing so we follow the evolution of binary system using a Smoothed Particle Hydrodynamics code. Even though the coalescence process of the white dwarfs involves considerable masses, moving at relatively high velocities with a high degree of asymmetry we find that the signature of the merger is not very strong. In fact, the most prominent feature of the coalescence is that in a relatively small time scale (of the order of the period of the last stable orbit, typically a few minutes) the sources stop emitting gravitational waves. We also discuss the possible implications of our calculations for the detection of the coalescence within the framework of future spaceborne interferometers like LISA.
Accurate evolutions of inspiralling neutron-star binaries: prompt and delayed collapse to black hole
2008
Binary neutron-star (BNS) systems represent primary sources for the gravitational-wave (GW) detectors. We present a systematic investigation in full GR of the dynamics and GW emission from BNS which inspiral and merge, producing a black hole (BH) surrounded by a torus. Our results represent the state of the art from several points of view: (i) We use HRSC methods for the hydrodynamics equations and high-order finite-differencing techniques for the Einstein equations; (ii) We employ AMR techniques with "moving boxes"; (iii) We use as initial data BNSs in irrotational quasi-circular orbits; (iv) We exploit the isolated-horizon formalism to measure the properties of the BHs produced in the merger; (v) Finally, we use two approaches, based either on gauge-invariant perturbations or on Weyl scalars, to calculate the GWs. These techniques allow us to perform accurate evolutions on timescales never reported before (ie ~30 ms) and to provide the first complete description of the i...
Merging White Dwarf/Black Hole Binaries and Gamma‐Ray Bursts
The Astrophysical Journal, 1999
The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRB's). In this paper, we present three dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed and the energy release is negligible. Peak accretion rates are ∼0.05 M ⊙ s −1 (less for lower mass white dwarfs) lasting for approximately a minute. Many of the disk parameters can be explained by a simple analytic model which we derive and compare to our simulations. This model can be used to predict accretion rates for white dwarf and black hole (or neutron star) masses which are not simulated in this paper. Although the mergers studied here create disks with larger radii, and longer accretion times than those from the merger of double neutron stars, a larger fraction of the merging star's mass becomes part of the disk. Thus the merger of a white dwarf and a black hole could produce a long duration GRB. The even rate of these mergers may be as high as 10 −4 yr −1 per galaxy.
Population of Merging Compact Binaries Inferred Using Gravitational Waves through GWTC-3
Physical Review X, 2023
We report on the population properties of compact binary mergers inferred from gravitational-wave observations of these systems during the first three LIGO-Virgo observing runs. The Gravitational-Wave Transient Catalog 3 (GWTC-3) contains signals consistent with three classes of binary mergers: binary black hole, binary neutron star, and neutron star-black hole mergers. We infer the binary neutron star merger rate to be between 10 and 1700 Gpc −3 yr −1 and the neutron star-black hole merger rate to be between 7.8 and 140 Gpc −3 yr −1 , assuming a constant rate density in the comoving frame and taking the union of 90% credible intervals for methods used in this work. We infer the binary black hole merger rate, allowing for evolution with redshift, to be between 17.9 and 44 Gpc −3 yr −1 at a fiducial redshift (z ¼ 0.2). The rate of binary black hole mergers is observed to increase with redshift at a rate proportional to ð1 þ zÞ κ with κ ¼ 2.9 þ1.7 −1.8 for z ≲ 1. Using both binary neutron star and neutron star-black hole binaries, we obtain a broad, relatively flat neutron star mass distribution extending from 1.2 þ0.1 −0.2 to 2.0 þ0.3 −0.3 M ⊙. We confidently determine that the merger rate as a function of mass sharply declines after the expected maximum neutron star mass, but cannot yet confirm or rule out the existence of a lower mass gap between neutron stars and black holes. We also find the binary black hole mass distribution has localized over-and underdensities relative to a power-law distribution, with peaks emerging at chirp masses of 8.3 þ0.3 −0.5 and 27.9 þ1.9 −1.8 M ⊙. While we continue to find that the mass distribution of a binary's more massive component strongly decreases as a function of primary mass, we observe no evidence of a strongly suppressed merger rate above approximately 60M ⊙ , which would indicate the presence of a upper mass gap. Observed black hole spins are small, with half of spin magnitudes below χ i ≈ 0.25. While the majority of spins are preferentially aligned with the orbital angular momentum, we infer evidence of antialigned spins among the binary population. We observe an increase in spin magnitude for systems with more unequal-mass ratio. We also observe evidence of misalignment of spins relative to the orbital angular momentum.
Non-dissipative tidal synchronization in accreting binary white dwarf systems
Monthly Notices of the Royal Astronomical Society, 2007
We study a non-dissipative hydrodynamical mechanism that can stabilize the spin of the accretor in an ultra-compact double white dwarf binary. This novel synchronization mechanism relies on a nonlinear wave interaction spinning down the background star. The essential physics of the synchronization mechanism is summarized as follows. As the compact binary coalesces due to gravitational wave emission, the largest star eventually fills its Roche lobe and accretion starts. The accretor then spins up due to infalling material and eventually reaches a spin frequency where a normal mode of the star is resonantly driven by the gravitational tidal field of the companion. If the resonating mode satisfies a set of specific criteria, which we elucidate in this paper, it exchanges angular momentum with the background star at a rate such that the spin of the accretor locks at this resonant frequency, even though accretion is ongoing. Some of the accreted angular momentum that would otherwise spin up the accretor is fed back into the orbit through this resonant tidal interaction. In this paper we solve analytically a simple dynamical system that captures the essential features of this mechanism. Our analytical study allows us to identify two candidate Rossby modes that may stabilize the spin of an accreting white dwarf in an ultra-compact binary. These two modes are the l = 4, m = 2 and l = 5, m = 3 CFS unstable hybrid r-modes, which stabilize the spin of the accretor at frequency 2.6 ω orb and 1.54 ω orb respectively, where ω orb is the binary's orbital frequency. Since the stabilization mechanism relies on continuously driving a mode at resonance, its lifetime is limited since eventually the mode amplitude saturates due to non-linear mode-mode coupling. Rough estimates of the lifetime of the effect lie from a few orbits to possibly thousands of years. We argue that one must include this hydrodynamical stabilization effect to understand stability and survival rate of ultra-compact binaries, which is relevant in predicting the galactic white dwarf gravitational background that LISA will observe.
Newtonian and post-Newtonian binary neutron star mergers
We present two of our efforts directed toward the numerical analysis of neutron star mergers, which are the most plausible sources for gravitational wave detectors that should begin operating in the near future. First we present Newtonian 3D simulations including radiation reaction (2.5PN) effects. We discuss the gravitational wave signals and luminosity from the merger with/without radiation reaction effects. Second we present the matching problem between post-Newtonian formulations and general relativity in numerical treatments. We prepare a spherical, static neutron star in a post-Newtonian matched spacetime, and find that discontinuities at the matching surface become smoothed out during fully relativistic evolution if we use a proper slicing condition.