Black Hole-neutron Star Research Articles

Forecast analysis of astrophysical stochastic gravitational wave background beyond general relativity: a case study on brans-dicke gravity

Scalar-tensor gravity, exemplified by Brans-Dicke (BD) gravity, introduces additional scalar polarization modes that contribute scalar radiation alongside tensor modes.We conduct a comprehensive analysis of how gravitational wave generation and propagation effects under Brans-Dicke gravity are encoded into the astrophysical stochastic gravitational wave background (AGWB).We perform end-to-end analyses of realistic populations of simulated coalescing binary systems to generate AGWB mock data with third-generation gravitational wave detectors and conducted a complete Bayesian analysis for the first time.We find the uncertainties in the population properties of binary black holes (BBH) significantly affect the ability to constrain BD gravity.Furthermore, we explore the detectability of potential scalar backgrounds that originates from binary neutron star (BNS) and neutron-star-black-hole (NSBH) mergers, with NSBH systems expected to modify the spectral index of the scalar background and introduce oscillatory behavior.We show that the observations of the AGWB enable the separation of mixed tensor and scalar polarization modes with comparable sensitivity to each mode.However, the scalar background is expected to remain substantially weaker than the tensor background, even in scenarios where BD gravity exhibits significant deviations from general relativity (GR), resulting only upper limits can be placed on the scalar background.We conclude that for ambiguous populations, employing waveform matching with individual sources provides a more robust approach to constrain BD gravity.

Not Just Winds: Why Models Find That Binary Black Hole Formation Is Metallicity-dependent, while Binary Neutron Star Formation Is Not

Abstract Both detailed and rapid population studies alike predict that binary black hole (BHBH) formation is orders of magnitude more efficient at low metallicity than high metallicity, while binary neutron star (NSNS) formation remains mostly flat with metallicity, and black hole–neutron star mergers show intermediate behavior. This finding is a key input to employ double compact objects as tracers of low-metallicity star formation, as spectral sirens, and for merger rate calculations. Yet the literature offers various (sometimes contradicting) explanations for these trends. We investigate the dominant cause for the metallicity dependence of double compact object formation. We find that the BHBH formation efficiency at low metallicity is set by initial condition distributions, and conventional simulations suggest that about one in eight interacting binary systems with sufficient mass to form black holes will lead to a merging BHBH. We further find that the significance of metallicities in double compact object formation is a question of formation channel. The stable mass transfer and chemically homogeneous evolution channels mainly diminish at high metallicities due to changes in stellar radii, while the common envelope channel is primarily impacted by the combined effects of stellar winds and mass-scaled natal kicks. Outdated giant wind prescriptions exacerbate the latter effect, suggesting that BHBH formation may be much less metallicity-dependent than previously assumed. NSNS formation efficiency remains metallicity-independent, as they form exclusively through the common envelope channel, with natal kicks that are assumed to be uncorrelated with mass. Forthcoming gravitational-wave observations will provide valuable constraints on these findings.

What is the nature of GW230529? An exploration of the gravitational lensing hypothesis

Abstract On the 29th of May 2023, the LIGO–Virgo–KAGRA Collaboration observed a compact binary coalescence event consistent with a neutron star – black hole merger, though the heavier object of mass 2.5 − 4.5 M⊙ would fall into the purported lower mass gap. An alternative explanation for apparent observations of events in this mass range has been suggested as strongly gravitationally lensed binary neutron stars. In this scenario, magnification would lead to the source appearing closer and heavier than it really is. Here, we investigate the chances and possible consequences for the GW230529 event to be gravitationally lensed. We find this would require high magnifications and we obtain low rates for observing such an event, with a relative fraction of lensed versus unlensed observed events of 2 × 10−3 at most. When comparing the lensed and unlensed hypotheses accounting for the latest rates and population model, we find a 1/58 chance of lensing, disfavoring this option. Moreover, when the magnification is assumed to be strong enough to bring the mass of the heavier binary component below the standard upper limits on neutron star masses, we find high probability for the lighter object to have a sub-solar mass, making the binary even more exotic than a mass-gap neutron star–black hole system. Even when the secondary is not sub-solar, its tidal deformability would likely be measurable, which is not the case for GW230529. Finally, we do not find evidence for extra lensing signatures such as the arrival of additional lensed images, type-II image dephasing, or microlensing. Therefore, we conclude it is unlikely for GW230529 to be a strongly gravitationally lensed binary neutron star signal.

Mass Transfer in Eccentric Black Hole–Neutron Star Mergers

Abstract Black hole–neutron star binaries are of interest in many ways: they are intrinsically transient, radiate gravitational waves detectable by LIGO, and may produce γ-ray bursts. Although it has long been assumed that their late-stage orbital evolution is driven entirely by gravitational wave emission, we show here that in certain circumstances, mass transfer from the neutron star onto the black hole can both alter the binary's orbital evolution and significantly reduce the neutron star's mass: when the fraction of its mass transferred per orbit is ≳10−2, the neutron star's mass diminishes by order unity, leading to mergers in which the neutron star mass is exceptionally small. The mass transfer creates a gas disk around the black hole before merger that can be comparable in mass to the debris remaining after merger, i.e., ~0.1 M ⊙. These processes are most important when the initial neutron star–black hole mass ratio q is in the range ≈0.2–0.8, the orbital semimajor axis is 40 ≲ a 0/r g ≲ 300 (r g ≡ GM BH/c 2), and the eccentricity is large at e 0 ≳ 0.8. Systems of this sort may be generated through the dynamical evolution of a triple system, as well as by other means.

Comparison of the Origin of Short Gamma-Ray Bursts with or without Extended Emission

The merger of compact binary stars produces short gamma-ray bursts (sGRBs), involving channels such as neutron star–neutron star (BNS) and neutron star–black hole (NS–BH). The association between sGRB 170817A and gravitational wave GW170817 provides reliable evidence for the BNS channel. Some speculations suggest that sGRBs with extended emission (EE) may represent another distinct population. The offset is the distance between the GRB sky localization and the host galaxy center. We compared the offset distributions of these two types of samples (46 sGRBs with EE and 9 without EE samples) and found that they follow the same distribution. Utilizing nonparametric methods, we examined the luminosity function and formation rate of sGRBs without any assumptions. The luminosity function can be described as ψ(L0)∝L0−0.12±0.01 for L0<L0b ( ψ(L0)∝L0−0.73±0.02 and for L0>L0b ) for sGRBs without EE and ψ(L0)∝L0−0.13±0.003 for L0<L0b ( ψ(L0)∝L0−0.61±0.01 and for L0>L0b ) for sGRBs with EE. The formation rate is characterized as ρ(z) ∝ (1 + z)−3.04±0.10 for z < 1 and as ρ(z) ∝ (1 + z)−0.29±0.38 for 1 < z < 3 for sGRBs without EE, while for sGRBs with EE, it is ρ(z) ∝ (1 + z)−3.85±0.15 for z < 1 and ρ(z) ∝ (1 + z)−0.40±1.11 for 1 < z < 3. Our findings suggest no significant difference in the progenitors of sGRBs with and without EE when considered in terms of spatial offsets, formation rates, and luminosity function.

PyMerger: Detecting Binary Black Hole Mergers from the Einstein Telescope Using Deep Learning

We present PyMerger, a Python tool for detecting binary black hole (BBH) mergers from the Einstein Telescope (ET), based on a deep residual neural network (ResNet) model. ResNet was trained on data combined from all three proposed subdetectors of ET (TSDCD) to detect BBH mergers. Five different lower-frequency cutoffs (F low)—5 Hz, 10 Hz, 15 Hz, 20 Hz, and 30 Hz—with the match-filter signal-to-noise ratio (MSNR) ranges 4–5, 5–6, 6–7, 7–8, and >8 were employed in the data simulation. Compared to previous work that utilized data from a single subdetector, the detection accuracy from TSDCD has shown substantial improvements, increasing from 60%, 60.5%, 84.5%, 94.5%, and 98.5% to 78.5%, 84%, 99.5%, 100%, and 100% for sources with MSNRs of 4–5, 5–6, 6–7, 7–8, and >8, respectively. The ResNet model is evaluated on the first ET mock data challenge (ET-MDC1) data set, where the model demonstrates strong performance in detecting BBH mergers, identifying 5566 out of 6578 BBH events, with optimal SNRs starting from 1.2 and a minimum and maximum D L of 0.5 Gpc and 148.95 Gpc, respectively. Despite being trained only on BBH mergers without overlapping sources, the model achieves high BBH detection rates. Notably, even though the model was not trained on binary neutron star (BNS) and black hole-neutron star (BHNS) mergers, it successfully detected 11,477 BNS and 323BHNS mergers in ET-MDC1, with optimal SNRs starting from 0.2 and 1, respectively, indicating its potential for broader applicability.

Scalar emission from neutron star-black hole binaries in scalar-tensor theories with kinetic screening

Scalar emission from neutron star-black hole binaries in scalar-tensor theories with kinetic screening

Compact object populations over cosmic time II. Compact object merger rates and masses over redshift from varying initial conditions

ABSTRACT We perform a first study of the impact of varying two components of the initial conditions in binary population synthesis of compact binary mergers – the initial mass function, which is made metallicity- and star formation rate-dependent, and the orbital parameter (orbital period, mass ratio, and eccentricity) distributions, which are assumed to be correlated – within a larger grid of initial condition models also including alternatives for the primary mass-dependent binary fraction and the metallicity-specific cosmic star formation history. We generate the initial populations with the sampling code bossa and evolve them with the rapid population synthesis code compas. We find strong suggestions that the main role of initial conditions models is to set the relative weights of key features defined by the evolution models. In the two models we compare, black hole–black hole (BHBH) mergers are the most strongly affected, which we connect to a shift from the common envelope to the stable Roche lobe overflow formation channels with decreasing redshift. We also characterize variations in the black hole–neutron star (BHNS) and neutron star–neutron star (NSNS) final parameter distributions. We obtain the merger rate evolution for BHBH, BHNS, and NSNS mergers up to $z=10$, and find a variation by a factor of $\sim 50\textnormal {--}60$ in the local BHBH and BHNS merger rates, suggesting a more important contribution from initial conditions than previously thought, and calling for a complete exploration of the initial conditions model permutations.

Late-time non-thermal emission from mildly relativistic tidal ejecta of compact objects merger

ABSTRACT Mergers of compact objects [binary neutron stars or neutron star-black hole (NSBH)] with a substantial mass ratio ($q\gt 1.5$) are expected to produce a mildly relativistic ejecta within $\sim 20^\circ$ from the equatorial plane. We present a semi-analytic approach to calculate the expected synchrotron emission observed from various viewing angles, along with the corresponding radio maps, that are produced by a collisionless shock driven by such ejecta into the interstellar medium. This method reproduces well (up to $\sim 30~{{\ \rm per\ cent}}$ deviations) the observed emission produced by 2D numerical calculations of the full relativistic hydrodynamics. We consider a toroidal ejecta with an opening angle of $15^\circ \le \theta _ \text{open}\le 30^\circ$ and broken power-law mass distribution, $M(\gt \gamma \beta)\propto (\gamma \beta)^{-s}$ with $s=s_{\rm KN}$ at $\gamma \beta \lt \gamma _0\beta _0$ and $s=s_{\rm ft}$ at $\gamma \beta \gt \gamma _0\beta _0$ (where $\gamma$ is the Lorentz factor). The parameter values are chosen to characterize merger calculation results – a ‘shallow’ mass distribution, $1\lt s_{\rm KN}\lt 3$, for the bulk of the ejecta (at $\gamma \beta \approx 0.2$), and a steep, $s_{\rm ft}\gt 5$, ‘fast tail’ mass distribution. While the peak flux is dimmer by a factor of $\sim$2–3, and the peak time remains roughly the same (within 20 per cent), for various viewing angles compared to isotropic equivalent ejecta ($\theta _\text{open}=90^\circ$) considered in preceding papers, the radio maps are significantly different from the spherical case. The semi-analytic method can provide information on the ejecta geometry and viewing angle from future radio map observations and, consequently, constrain the ejection mechanism. For NSBH mergers with a significant mass ejection ($\sim 0.1\,{\rm M}_\odot$), this late non-thermal signal can be observed to distances of $\lesssim 200$ Mpc for typical parameter values.

Origin of the black hole spin in lower-mass-gap black hole-neutron star binaries

During the fourth observing run, the LIGO-Virgo-KAGRA Collaboration reported the detection of a coalescing compact binary (GW230529$_ $181500) with component masses estimated at $2.5-4.5\, M_ and $1.2-2.0\, M_ with 90&lt;!PCT!&gt; credibility. Given the current constraints on the maximum neutron star (NS) mass, this event is most likely a lower-mass-gap (LMG) black hole-neutron star (BHNS) binary. The spin magnitude of the BH, especially when aligned with the orbital angular momentum, is critical in determining whether the NS is tidally disrupted. An LMG BHNS merger with a rapidly spinning BH is an ideal candidate for producing electromagnetic counterparts. However, no such signals have been detected. In this study, we employ a detailed binary evolution model that incorporates new dynamical tide implementations to explore the origin of BH spin in an LMG BHNS binary. If the NS forms first, the BH progenitor (He-rich star) must begin in orbit shorter than 0.35 days to spin up efficiently, potentially achieving a spin magnitude of $ BH &gt; 0.3$. Alternatively, if a nonspinning BH (e.g., $M_ BH = 3.6\, M_ forms first, it can accrete up to $ 0.2\, M_ via case BA mass transfer (MT), reaching a spin magnitude of $ BH 0.18$ under Eddington-limited accretion. With a higher Eddington accretion limit (i.e., 10.0 $ M Edd $), the BH can attain a significantly higher spin magnitude of $ BH by accreting approximately $1.0\, M_ during case BA MT phase.

The impact of astrophysical priors on parameter inference for GW230529

ABSTRACT We investigate the effects of prior selection on the inferred mass and spin parameters of the neutron star–black hole merger GW230529_181500. Specifically, we explore models motivated by astrophysical considerations, including massive binary and pulsar evolution. We examine mass and spin distributions of neutron stars constrained by radio pulsar observations, alongside black hole spin observations from previous gravitational-wave detections. We show that the inferred mass distribution highly depends upon the spin prior. Specifically, under the most restrictive, binary stellar evolution models, we obtain narrower distributions of masses with a black hole mass of $4.3^{+0.1}_{-0.1}\ {\rm M}_{\odot }$ and neutron star mass of $1.3^{+0.03}_{-0.03}\ {\rm M}_{\odot }$ where, somewhat surprisingly, it is the prior on component spins that has the greatest impact on the inferred mass distributions. Re-weighting using neutron star mass and spin priors from observations of radio pulsars, with black hole spins from observations of gravitational waves, yields the black hole and the neutron star masses to be $3.8^{+0.5}_{-0.6}$ and $1.4^{+0.2}_{-0.1} \ \mathrm{ M}_\odot$, respectively. The sequence of compact object formation – whether the neutron star or the black hole formed first – cannot be determined at the observed signal-to-noise ratio. However, there is no evidence that the black hole was tidally spun up.

Rates and Beaming Angles of Gamma-Ray Bursts Associated with Compact Binary Coalescences

Some, if not all, binary neutron star (BNS) coalescences, and a fraction of neutron star–black hole (NSBH) mergers, are thought to produce sufficient mass ejection to power gamma-ray bursts (GRBs). However, this fraction, as well as the distribution of beaming angles of BNS-associated GRBs, is poorly constrained from observation. Recent work applied machine learning tools to analyze GRB light curves observed by Fermi/Gamma-Ray Burst Monitor (GBM) and Swift/Burst Alert Telescope (BAT). GRBs were segregated into multiple distinct clusters, with the tantalizing possibility that one of them (BNS cluster) could be associated with BNSs and another (NSBH cluster) with NSBHs. As a proof of principle, assuming that all GRBs detected by Fermi/GBM and Swift/BAT associated with BNSs (NSBHs) lie in the BNS (NSBH) cluster, we estimate their rates (Gpc−3 yr−1). We compare these rates with corresponding BNS and NSBH rates estimated by the LIGO–Virgo–KAGRA (LVK) collaboration from the first three observing runs (O1, O2, O3). We find that the BNS rates are consistent with LVK’s rate estimates, assuming a uniform distribution of beaming fractions (f b ∈ [0.01, 0.1]). Conversely, using the LVK’s BNS rate estimates, assuming all BNS mergers produce GRBs, we are able to constrain the beaming angle distribution to θ j ∈ [0.°8, 33.°5] at 90% confidence. We similarly place limits on the fraction of GRB-bright NSBHs as f B ∈ [1.3%, 63%] (f B ∈ [0.4%, 15%]) with Fermi/GBM (Swift/BAT) data.

Searching for Gravitational Wave Optical Counterparts with the Zwicky Transient Facility: Summary of O4a

During the first half of the fourth observing run (O4a) of the International Gravitational Wave Network, the Zwicky Transient Facility (ZTF) conducted a systematic search for kilonova (KN) counterparts to binary neutron star (BNS) and neutron star–black hole (NSBH) merger candidates. Here, we present a comprehensive study of the five high-significance (False Alarm Rate less than 1 yr−1) BNS and NSBH candidates in O4a. Our follow-up campaigns relied on both target-of-opportunity observations and re-weighting of the nominal survey schedule to maximize coverage. We describe the toolkit we have been developing, Fritz, an instance of SkyPortal, instrumental in coordinating and managing our telescope scheduling, candidate vetting, and follow-up observations through a user-friendly interface. ZTF covered a total of 2841 deg2 within the skymaps of the high-significance GW events, reaching a median depth of g ≈ 20.2 mag. We circulated 15 candidates, but found no viable KN counterpart to any of the GW events. Based on the ZTF non-detections of the high-significance events in O4a, we used a Bayesian approach, nimbus, to quantify the posterior probability of KN model parameters that are consistent with our non-detections. Our analysis favors KNe with initial absolute magnitude fainter than −16 mag. The joint posterior probability of a GW170817-like KN associated with all our O4a follow-ups was 64%. Additionally, we use a survey simulation software, simsurvey, to determine that our combined filtered efficiency to detect a GW170817-like KN is 36%, when considering the 5 confirmed astrophysical events in O3 (1 BNS and 4 NSBH events), along with our O4a follow-ups. Following Kasliwal et al., we derived joint constraints on the underlying KN luminosity function based on our O3 and O4a follow-ups, determining that no more than 76% of KNe fading at 1 mag day−1 can peak at a magnitude brighter than −17.5 mag.

From spherical stars to disk-like structures: 3D common-envelope evolution of massive binaries beyond inspiral

Self-consistent three-dimensional modeling of the entire common-envelope phase of gravitational wave progenitor systems until full envelope ejection is challenged by the vast range of spatial and temporal scales involved in the problem. Previous attempts were either terminated shortly after the rapid spiral-in with significant amounts of gravitationally bound material left in the system or they omitted this plunge-in phase and modeled the system afterward. We investigated the common-envelope interactions of a 10 M⊙ red supergiant primary star with a black hole and a neutron star companion, respectively, until full envelope ejection (≳97% of the envelope mass). In contrast to the expectation from e.g. population synthesis models, we find that the dynamical plunge-in of the systems determines (to leading order) the orbital separations of the core binary system, while the envelope ejection by recombination acts only at later stages of the evolution and fails to harden the core binaries down to orbital frequencies where they qualify as progenitors of gravitational-wave-emitting double-compact object mergers. Diverging from the conventional picture of an expanding common envelope that is ejected more or less spherically, our simulations show a new mechanism: The rapid plunge-in of the companion transforms the spherical morphology of the giant primary star into a disk-like structure. During this process, magnetic fields are amplified, and the subsequent transport of material through the disk around the core binary system drives a fast jet-like outflow in the polar directions. While most of the envelope material is lost through a recombination-driven wind from the outer edge of the disk, about 7% of the envelope leaves the system via the magnetically driven outflows. We further explored the potential evolutionary pathways of the post-common-envelope systems in light of the expected remaining lifetime of the primary core (2.97 M⊙) until core collapse (6 × 104 yr), most likely forming a neutron star. We find that the interaction of the core binary system with the circumbinary disk substantially increases the likelihood of giving rise to a double-neutron star merger (55%) or a neutron star black hole (5%) merger event.

Investigating the Cosmological Rate of Compact Object Mergers from Isolated Massive Binary Stars

Gravitational-wave (GW) detectors are observing compact object mergers from increasingly far distances, revealing the redshift evolution of the binary black hole (BBH)—and soon the black hole–neutron star (BHNS) and binary neutron star (BNS)—merger rate. To help interpret these observations, we investigate the expected redshift evolution of the compact object merger rate from the isolated binary evolution channel. We present a publicly available catalog of compact object mergers and their accompanying cosmological merger rates from population synthesis simulations conducted with the COMPAS software. To explore the impact of uncertainties in stellar and binary evolution, our simulations use two-parameter grids of binary evolution models that vary the common-envelope efficiency with mass transfer accretion efficiency and supernova (SN) remnant mass prescription with SN natal kick velocity, respectively. We quantify the redshift evolution of our simulated merger rates using the local (z ∼ 0) rate, the redshift at which the merger rate peaks, and the normalized differential rates (as a proxy for slope). We find that although the local rates span a range of ∼103 across our model variations, their redshift evolutions are remarkably similar for BBHs, BHNSs, and BNSs, with differentials typically within a factor 3 and peaks of z ≈ 1.2–2.4 across models. Furthermore, several trends in our simulated rates are correlated with the model parameters we explore. We conclude that future observations of the redshift evolution of the compact object merger rate can help constrain binary models for stellar evolution and GW formation channels.

Occurrence of Gravitational Collapse in the Accreting Neutron Stars of Binary-driven Hypernovae

The binary-driven hypernova (BdHN) model proposes long gamma-ray bursts (GRBs) originate in binaries composed of a carbon–oxygen (CO) star and a neutron star (NS) companion. The CO core collapse generates a newborn NS and a supernova that triggers the GRB by accreting onto the NSs, rapidly transferring mass and angular momentum to them. This article aims to determine the conditions under which a black hole (BH) forms from NS collapse induced by the accretion and the impact on the GRB’s observational properties and taxonomy. We perform three-dimensional, smoothed particle hydrodynamics simulations of BdHNe using up-to-date NS nuclear equations of state, with and without hyperons, and calculate the structure evolution in full general relativity. We assess the binary parameters leading either NS in the binary to the critical mass for gravitational collapse into a BH and its occurrence time, t col. We include a nonzero angular momentum of the NSs and find that t col ranges from a few tens of seconds to hours for decreasing NS initial angular momentum values. BdHNe I are the most compact (about 5 minute orbital period), promptly form a BH, and release ≳1052 erg of energy. They form NS–BH binaries with tens of kiloyears merger timescales by gravitational-wave emission. BdHNe II and III do not form BHs, and release ∼1050–1052 erg and ≲1050 erg of energy, respectively. They form NS–NS binaries with a range of merger timescales larger than for NS–BH binaries. In some compact BdHNe II, either NS can become supramassive, i.e., above the critical mass of a nonrotating NS. Magnetic braking by a 1013 G field can delay BH formation, leading to BH–BH or NS–BH with tens of kiloyears merger timescales.

The Binary Black Hole Merger Rate Deviates from the Cosmic Star Formation Rate: A Tug of War between Metallicity and Delay Times

Gravitational-wave detectors are now making it possible to investigate how the merger rate of binary black holes (BBHs) evolves with redshift. In this study, we examine whether the BBH merger rate of isolated binaries deviates from a scaled star formation rate density (SFRD)—a frequently used model in state-of-the-art research. To address this question, we conduct population synthesis simulations using COMPAS with a grid of stellar evolution models, calculate their cosmological merger rates, and compare them to a scaled SFRD. We find that our simulated rates deviate by factors up to 3.5 at z ∼ 0 and 5 at z ∼ 9 due to two main phenomena: (i) the formation efficiency of BBHs is an order of magnitude higher at low metallicities than at solar metallicity, and (ii) BBHs experience a wide range of delays (from a few megayears to many gigayears) between formation and merger. The deviations are similar when comparing to a delayed SFRD, and even larger (up to ∼10×) when comparing to SFRD-based models scaled to the local merger rate. Interestingly, our simulations find that the BBH delay time distribution is redshift dependent, increasing the complexity of the redshift distribution of mergers. We find similar results for simulated merger rates of black hole–neutron stars (BHNSs) and binary neutron stars (BNSs). We conclude that the rate of BBH, BHNS, and BNS mergers from the isolated channel can significantly deviate from a scaled SFRD, and that future measurements of the merger rate will provide insights into the formation pathways of gravitational-wave sources.

Black hole-neutron star mergers in Einstein-scalar-Gauss-Bonnet gravity

Gravitational wave observations of black hole-neutron star binaries, particularly those where the black hole has a lower mass compared to other observed systems, have the potential to place strong constraints on modifications to general relativity that arise at small curvature length scales. Here we study the dynamics of black hole-neutron star mergers in shift-symmetric Einstein-scalar-Gauss-Bonnet gravity, a representative example of such a theory, by numerically evolving the full equations of motion. We consider quasi-circular binaries with different mass-ratios that are consistent with recent gravitational wave observations, including cases with and without tidal disruption of the star, and quantify the impact of varying the coupling controlling deviations from general relativity on the gravitational wave signal and scalar radiation. We find that the main effect on the late inspiral is the accelerated frequency evolution compared to general relativity, and that—even considering Gauss-Bonnet coupling values approaching those where the theory breaks down—the impact on the merger gravitational wave signal is mild, predominately manifesting as a small change in the amplitude of the ringdown. We compare our results to current post-Newtonian calculations and find consistency throughout the inspiral. Published by the American Physical Society 2024

Corrections of the Distribution of Nuclei Due to Neutron Degeneracy and Their Effects in the R-Process in Neutron Stars—Black Hole Mergers

The r-process is one of the processes that produces heavy elements in the Universe. One of its possible astrophysical sites is the neutron star–black hole (NS-BH) merger. We first show that the neutrons can degenerate before and during the r-process in these mergers. Previous studies assumed neutrons were non-degenerate and the related rates were calculated under Maxwell–Boltzmann approximations. Hence, we corrected the related rates with neutron degeneracy put in the network code and calculated with the trajectories of NS-BH mergers. We show that there are differences in the nuclei distributions. The heating rates and the temperature at most can be two times larger. The change in heating rates and temperature can affect the light curves of the kilonovae. However, this has little effect on the final abundances.

Black hole—neutron star binary mergers: the impact of stellar compactness

Abstract Recent gravitational wave (GW) observations include possible detections of black hole—neutron star binary mergers. As with binary black hole mergers, numerical simulations help characterize the sources. For binary systems with neutron star components, the simulations help to predict the imprint of tidal deformations and disruptions on the GW signals. In a previous study, we investigated how the mass of the black hole has an impact on the disruption of the neutron star and, as a consequence, on the shape of the GWs emitted. We extend these results to study the effects of varying the compactness of the neutron star. We consider neutron star compactness in the 0.113–0.2 range for binaries with mass ratios of 3 and 5. As the compactness and the mass ratio increase, the binary system behaves during the late inspiral and merger more like a black hole binary. For the cases with the least compact neutron star, the GWs emitted, in terms of mismatches, are the most distinguishable from those by a binary black hole. The disruption of the star significantly suppresses the kicks on the final black hole. The disruption also affects, although not dramatically, the spin of the final black hole. Lastly, for neutron stars with low compactness, the quasi-normal ringing of the black hole after the merger does not show a clean quasi-normal ringing because of the late accretion of debris from the neutron star.