Black hole-neutron star binaries with high spins and large mass asymmetries: III. Properties of the ejected material and its electromagnetic signatures

Technical Summary: Black hole–neutron star binaries with high spins and large mass asymmetries: III. Properties of the ejected material and its electromagnetic signatures

Problem and Context
Black hole–neutron star (BHNS) binaries are prime candidates for multi-messenger astronomy, offering signals in both gravitational waves (GW) and electromagnetic (EM) channels. While the GW170817 event provided unprecedented constraints on binary neutron star (BNS) mergers, secure EM counterparts for BHNS mergers remain unconfirmed despite several GW detections (e.g., GW200105, GW200115). Understanding the properties of dynamical ejecta in BHNS systems is crucial for constraining the equation of state (EOS) of dense nuclear matter, verifying relativistic jet launching mechanisms, and evaluating the contribution of BHNS mergers to the cosmic abundance of heavy elements via the $r$-process.

Existing literature often relies on idealized ejecta distributions derived from analytical models or simplified numerical results. This paper, the third in a series, addresses this by adopting an ab-initio approach. It investigates the geometric, thermodynamic, and kinematic properties of purely dynamical ejecta from general-relativistic magnetohydrodynamical (GRMHD) simulations of BHNS systems characterized by high black hole (BH) spins and large mass ratios ($Q = M_{BH}/M_{NS}$).

Methodology
The study utilizes a comprehensive parameter space covering six BHNS configurations:

1. Fixed mass ratio $Q=4$ with varying BH spins $\chi_{BH} \in \{0.4, 0.6, 0.8\}$.
2. Fixed BH spin $\chi_{BH}=0.8$ with varying mass ratios $Q \in \{4, 5, 6, 7\}$.

The simulations employ the DD2 EOS for the neutron star (NS) with a baryonic mass of $1.53 M_\odot$. The numerical infrastructure includes the EinsteinToolkit with the Antelope code for metric evolution (using CCZ4 or Z4c formulations) and the Frankfurt-IllinoisGRMHD (FIL) code for the magnetized fluid. Initial data is generated using the FUKA solver.

The analysis proceeds in three main stages:

1. Dynamical Ejecta Characterization: The authors analyze energy and angular momentum transfer to the disrupted matter, specifically examining the phase-space distribution of specific energy ($\tilde{E}$) and normalized specific angular momentum ($\tilde{J}$). They define dynamical ejecta using both geodesic ($u_t < -1$) and Bernoulli ($h u_t < -1$) unboundness criteria.
2. Nucleosynthesis: Artificial tracer particles are extracted from the simulation data at a detector radius of $400 M_\odot$. These tracers, weighted by mass, serve as inputs for the SkyNet nuclear-reaction network to compute elemental abundances resulting from the $r$-process. The network runs for $\sim 32$ years to capture the evolution of heavy nuclei.
3. Electromagnetic Signatures: The resulting ejecta properties (mass, velocity, entropy, geometry) are fed into the radiative-transfer code POSSIS to generate kilonova (KN) light curves. These are compared against the AT2017gfo data and upper limits from S190814bv.

Key Contributions and Results

• Mechanism of Mass Ejection: The study details the mechanism of dynamical mass ejection for high-mass-ratio, high-spin binaries. Unlike lower-mass-ratio cases, high-$Q$ binaries with high spins exhibit a delayed formation of an accretion disk. The tidal tail performs more rotations around the BH before disruption is complete, facilitating angular momentum transfer and delaying the accumulation of bound material. In the $Q=7, \chi_{BH}=0.8$ case, the ISCO is pushed outward, and matter closer than $\approx 120$ km is depleted within 5 ms post-merger.

• Bulk Properties of Ejecta:

  • Mass: Strong tidal disruption (STD) cases (e.g., $Q=4, \chi_{BH}=0.8$) eject substantial mass ($\sim 0.04 - 0.06 M_\odot$), which decreases as the mass ratio increases. The $Q=4, \chi_{BH}=0.8$ and $Q=5, \chi_{BH}=0.8$ configurations represent a plateau of maximum ejecta mass for the given spin.
  • Thermodynamics: Stronger tidal disruptions lead to higher mean entropy due to more frequent collisions in the tidal tail. Entropy values can exceed $50 k_B$/baryon in the tails of the distribution.
  • Kinematics: Increasing mass asymmetry generally increases the mean velocity of the ejecta. The kinetic energy is amplified by the system's angular momentum. The average velocity $\langle v \rangle$ ranges from $\sim 0.18c$ to $0.29c$, with a clear trend of increasing velocity with higher $Q$.
  • Composition: The electron fraction ($Y_e$) remains low ($\approx 0.04 - 0.06$) across all simulations, consistent with the absence of neutrino radiation effects in the simulations. This low $Y_e$ drives the production of heavy elements.

• Spatial and Angular Distribution:

  • The ejecta exhibits significant anisotropy. Strong tidal disruptions result in more collimated ejecta with a polar half-opening angle of $\sim 5^\circ$, whereas weak disruptions are broader ($\sim 10^\circ$).
  • The azimuthal extent varies: strong disruptions cover $\sim 120^\circ$, while weak disruptions and high-$Q$ cases can approach $360^\circ$ due to the wrapping of the tidal tail.
  • The 90%-confinement region of the ejecta spans between 4% and 8% of the sky, depending on the disruption strength.

• Nucleosynthesis Yields:

  • The $r$-process yields are dominated by very neutron-rich material ($Y_e \approx 0.05$).
  • The resulting abundance patterns show a classic signature of neutron-rich dynamical ejecta: the second and third $r$-process peaks are overproduced by an order of magnitude compared to solar $r$-process data, while the first peak is severely depleted.
  • Variations in the abundance patterns between simulations are most pronounced between the second and third peaks ($140 < A < 190$) and in the actinide region, correlating with differences in entropy and expansion timescales.

• Electromagnetic Signatures:

  • Kilonova light curves computed with POSSIS show that the bolometric and optical/near-infrared evolution depends on the viewing angle and binary parameters ($Q, \chi_{BH}$).
  • The models are generally consistent with the AT2017gfo data and the upper limits for S190814bv, suggesting that BHNS mergers with these parameters can produce observable transients, though they may be fainter and more enduring than BNS counterparts.

Significance
The paper claims significance by providing a detailed, ab-initio link between the underlying binary parameters (mass ratio and BH spin) and the observable properties of the ejecta and kilonova. By moving beyond idealized assumptions, the authors demonstrate that:

1. High BH spins and large mass ratios significantly alter the dynamics of tidal disruption, delaying disk formation and altering the angular distribution of ejecta.
2. The geometric and thermodynamic properties of the ejecta are directly tied to the binary configuration, influencing the resulting $r$-process yields and kilonova light curves.
3. The detailed description of the angular extent (polar and azimuthal) provided here offers necessary input for refining synthetic kilonova models, which often rely on fixed geometries.
4. The findings support the viability of BHNS mergers as sources of heavy elements and multi-messenger signals, even in the absence of a confirmed EM counterpart to date.

The work concludes that while the dynamical ejecta alone cannot fully reproduce the solar abundance pattern (missing the first peak and secular outflow contributions), it provides a robust baseline for understanding the heavy-element production in the most extreme BHNS mergers.