Accurate evolutions of unequal-mass neutron-star binaries: properties of the torus and short GRB engines (1001.3074v2)

Abstract: We present new results from accurate and fully general-relativistic simulations of the coalescence of unmagnetized binary neutron stars with various mass ratios. The evolution of the stars is followed through the inspiral phase, the merger and prompt collapse to a black hole, up until the appearance of a thick accretion disk, which is studied as it enters and remains in a regime of quasi-steady accretion. Although a simple ideal-fluid equation of state with \Gamma=2 is used, this work presents a systematic study within a fully general relativistic framework of the properties of the resulting black-hole--torus system produced by the merger of unequal-mass binaries. More specifically, we show that: (1) The mass of the torus increases considerably with the mass asymmetry and equal-mass binaries do not produce significant tori if they have a total baryonic mass M_tot >~ 3.7 M_sun; (2) Tori with masses M_tor ~ 0.2 M_sun are measured for binaries with M_tot ~ 3.4 M_sun and mass ratios q ~ 0.75-0.85; (3) The mass of the torus can be estimated by the simple expression M_tor(q, M_tot) = c_1 (1-q) + c_2, involving the maximum mass for the binaries and coefficients constrained from the simulations, and suggesting that the tori can have masses as large as M_tor ~ 0.35 M_sun for M_tot ~ 2.8 M_sun and q ~ 0.75-0.85; (4) Using a novel technique to analyze the evolution of the tori we find no evidence for the onset of non-axisymmetric instabilities and that very little, if any, of their mass is unbound; (5) Finally, for all the binaries considered we compute the complete gravitational waveforms and the recoils imparted to the black holes, discussing the prospects of detection of these sources for a number of present and future detectors.

• The paper simulates unequal-mass neutron-star binary mergers, finding that the resulting torus mass increases with mass asymmetry, capable of producing significant masses (0.2-0.35 Msun) for accretion.
• Unequal-mass binaries show distinct post-merger dynamics and gravitational waveforms compared to equal-mass systems, including less symmetric accretion and earlier ringdown.
• The simulations reveal that the resulting accretion disks maintain stable, quasi-Keplerian profiles on short timescales, supporting potential SGRB engine activity.

Unequal-Mass Neutron-Star Binaries and their Astrophysical Implications

The paper of unequal-mass neutron-star (NS) binaries provides significant insights into the models of short gamma-ray bursts (SGRBs) and the physics of dense matter under extreme conditions. The paper "Accurate evolutions of unequal-mass neutron-star binaries: properties of the torus and short GRB engines" presents fully general-relativistic simulations of NS binaries with different mass ratios. The focus is on understanding the dynamics from the inspiral phase through merger and the subsequent formation of a black hole-accretion torus system. These simulations aid in exploring the central engines thought to power SGRBs.

Summary of Key Findings

1. Mass of the Torus: The paper indicates that the torus mass resulting from the merger of unequal-mass binaries increases with mass asymmetry, particularly for systems with lower mass ratios. Systems with mass ratios between 0.75 and 0.85 can produce tori with masses approximately 0.2 to 0.35 solar masses, highlighting a significant portion of mass available for accretion and possible GRB activity.
2. Dependence on Mass Ratios: Unequal-mass binaries demonstrate distinct dynamics compared to equal-mass systems. The remnants from unequal mass systems are characterized by less symmetric accretion flows and are capable of maintaining quasi-Keplerian disks. Additionally, their gravitational waveforms show earlier onset of the ringdown stage and tend to have lower post-merger amplitudes.
3. Axial Symmetry and Dynamical Stability: The paper's simulations reveal that, in spite of large-scale mass asymmetries, the resulting accretion disks do not display non-axisymmetric instabilities on short timescales. The specific angular momentum profile often becomes Keplerian, supporting the potential for stable accretion processes essential for powering SGRB engines.
4. Gravitational Waveforms and Recoil Velocities: The paper achieves complete gravitational waveforms for various binary configurations, offering vital insight into the gravitational signature related to such astronomical events, crucial for identification by present and future detectors. The recoil velocities, though lower than those expected from binary black holes, might still exceed escape velocities in globular clusters, influencing stellar dynamics in such environments.

Implications and Future Considerations

The findings have profound implications for both theoretical understanding and observational strategies concerning SGRBs and gravitational waves. These results provide a valuable data set that complements gravitational wave detections, aiding the identification of NS mergers and enhancing our understanding of NS equations of state.

The paper paves the way for more refined simulations incorporating realistic equations of state and magnetic fields, potentially affecting torus dynamics and mass ejection processes. Utilizing advanced gravitational wave detectors like LIGO and Virgo, and future setups like the Einstein Telescope, will enable the detection of these events at greater distances, providing a broader data set for constraining astrophysical models.

Further research should consider a broader range of binary masses and spins to encompass the diversity of NS merger scenarios and improve the statistical significance of these phenomena. Additionally, accounting for neutrino transport and a more complete interaction model could refine understanding of post-merger evolution and SGRB central engines.

In conclusion, the insights drawn from the simulation of unequal-mass NS binaries offer transformative perspectives on high-energy astrophysics and provide a substantial foundation for detailing the violent and energetic universe through comprehensive computational and observational synergy.