Numerical simulations of low-mass black hole-neutron star mergers with moderate eccentricities

The gist A confident detection of orbital eccentricity in gravitational-wave signals from black hole–neutron star systems would be a strong indicator of their dynamical formation origin This paper performs numerical simulations of low-mass black hole–neutron star mergers with moderate eccentricities to test the validity of eccentric waveform models against numerical relativity simulations

Simulation Setup and Methodology

The researchers performed numerical-relativity simulations of low-mass BHNS mergers with orbital eccentricities around five times larger than those inferred for GW200105 They simulated a subset of these systems with orbital eccentricities around five times larger than those inferred for GW200105 Additionally, they performed simulations with magnetized matter to assess whether trapped g-mode oscillations persist in strongly magnetized disks

The initial data construction utilized FUKA, which is based on the Kadath spectral library To make the eccentric waveforms of comparable length to their QC counterparts studied in their previous work, they equated the mean motions of the binaries to those of the corresponding QC cases They leveraged this ability to adjust these parameters to introduce eccentricity instead of reducing it The eccentricity, eGW, was calculated shortly after the beginning of the simulations at the reference dimensionless frequency frefM = 0.0087

Waveform Model Comparison

The study employed two eccentric waveform families: the effective-one-body model SEOBNRv5EHM and the inspiral-only PN waveform models pyEFPE/pyEFPEHM They aligned each waveform model to the NR data by minimizing the time-domain mismatch of the (2, 2) mode over a fixed window in retarded time u ∈ [u0, u1]

For the equal-mass system, which has the largest effective tidal deformability, eccentric tidal corrections have a small but noticeable impact on the phase They found an almost negligible impact of eccentricity on the properties of the remnant black hole and differences of up to a factor of two in the remnant disk mass

Remnant Properties and Accretion

The mass and the spin of the remnant BH are unaffected by the initial eccentricity considered here, which is consistent with conclusions from Ref. [16] for BBH mergers Out of all unmangetized cases, only the Q2 DD2 cases have a non-negligible mass of dynamical ejecta There is no clear dependency of the dynamical ejecta mass and disk mass on the eccentricity for these systems

For Type B systems, Q2 SLy, the accretion rate is consistently higher for higher eccentricities, which can be attributed to larger disk masses allowing more matter to be accreted Neither the frequency nor the phase of the trapped gmode oscillations changes with eccentricity

Effects of Strong Magnetic Fields

In simulations with strongly magnetized disks, they become less dense and have less pronounced structure than in the unmagnetized case The trapped g modes previously identified in the unmagnetized cases are efficiently damped when accretion disks are strongly magnetized

The simulations with purely poloidal and toroidal magnetic fields showed that the trapped g modes might be destroyed for strongly magnetized disks The presence of quasi-periodic oscillations in the observables of confirmed BHNS mergers would indicate that the accretion disks might not be sufficiently magnetized

The simulations with magnetized NS matter in poloidal and toroidal topologies show that when disks are strongly magnetized (1015 G), the trapped g modes are successfully damped The accretion disks formed in their simulations are less dense and have less pronounced structure than in the unmagnetized case

Conclusion

The SEOBNRv5EHM model can describe the inspiral of such eccentric BHNS systems as long as the tidal contributions are negligible The inclusion of eccentric tidal contributions to pyEFPEHM has a small but noticeable impact on the waveform in cases with larger effective tidal deformability They identify global trapped g-mode oscillations in the disk, consistent with observations in QC cases The frequencies of trapped g modes match closely with those in the QC cases and do not change with eccentricity If these oscillations are present and capable of modulating the observables, such as gamma-ray burst prompt emission, they would serve as a clear, eccentricity-independent probe for the remnant BH properties The authors suggest that these changes are likely to depend on the value of the phase of the radial period (true anomaly) at the time of tidal disruption and matter ejection Further studies specifically targeting the effect of varying true anomaly would be necessary to confirm this hypothesis

The study provides a comprehensive assessment of eccentric waveform models against numerical relativity simulations for low-mass BHNS mergers. It demonstrates that while the SEOBNRv5EHM model performs reasonably well unless tidal contributions become important, the inspiral-only post-Newtonian models show larger disagreement in the late inspiral The simulations confirm that trapped g modes depend mainly on the properties of the remnant black hole and are damped when accretion disks are strongly magnetized. Overall, this work is instrumental in populating the parameter space with long NR waveforms of eccentric BHNS systems.

1. Bold header: Eccentricity-Aware Waveform Model Calibration

This system can calibrate effective-one-body waveform models by utilizing numerical relativity simulations of low-mass black hole–neutron star mergers with orbital eccentricities around five times larger than those inferred for GW200105, as the paper notes that the effective one-body waveform model SEOBNRv5EHM shows reasonable agreement unless tidal contributions become important.

2. Bold header: Dynamical Formation Origin Assessment

The improved system can provide a strong indicator of dynamical formation origin by detecting orbital eccentricity in gravitational-wave signals, as the text states, A confident detection of orbital eccentricity in gravitational-wave signals from black hole–neutron star systems would be a strong indicator of their dynamical formation origin.

3. Bold header: Remnant Property Prediction Under Eccentricity

The system can predict remnant properties by assessing how eccentricity impacts the final state, noting that we find an almost negligible impact of eccentricity on the properties of the remnant black hole, and differences of up to a factor of two in the remnant disk mass.

4. Bold header: Accretion Disk State Analysis

The system can analyze accretion disk behavior by distinguishing between merger types; for instance, it can predict that For the Type B systems, Q2 SLy, the accretion rate is consistently higher for higher eccentricities, and confirm that Neither the frequency nor the phase of the trapped gmode oscillations changes with eccentricity.

5. Bold header: Magnetic Field Impact on Disk Structure

The system can model strong magnetic fields to predict disk dynamics, learning that In Fig. 3, Matter density in the equatorial plane for different stages of merger and disk evolution for unmagnetized, magnetized with the initial NS magnetic field topology being either poloidal or toroidal, and that the magnetized disks have visually very similar morphologies and densities overall.

A confident detection of orbital eccentricity in gravitational-wave signals from black hole-neutron star systems would be a strong indicator of their dynamical formation origin. While eccentric waveform models have recently been developed, their validity for analyzing black hole--neutron star signals has yet to be tested against numerical-relativity simulations. Building upon our previous study of quasicircular black hole--neutron star mergers, we perform simulations of a subset of these systems with orbital eccentricities around five times larger than those inferred for GW200105 162426. Additionally, we perform simulations with magnetized matter to assess whether trapped g-mode oscillations persist in strongly magnetized disks. Regarding the emitted gravitational-wave signal, we find that the effective one-body waveform model SEOBNRv5EHM shows reasonable agreement unless tidal contributions become important, and the inspiral-only post-Newtonian models pyEFPE/pyEFPEHM show larger disagreement in the late inspiral. For the equal-mass system, which has the largest effective tidal deformability, eccentric tidal corrections have a small but noticeable impact on the phase, about0.1 rad. We find an almost negligible impact of eccentricity on the properties of the remnant black hole, and differences of up to a factor of two in the remnant disk mass. Trapped g modes in the accretion disks have the same frequency regardless of the eccentricity, confirming that they depend mainly on the properties of the remnant black hole. We find that when accretion disks are strongly magnetized, they become less dense and have less pronounced structure, and that the trapped g modes previously identified in the unmagnetized cases are efficiently damped.