Binary Neutron Star Merger Research Articles

XMM-Newton-discovered Fast X-ray Transients: host galaxies and limits on contemporaneous detections of optical counterparts

ABSTRACT Extragalactic fast X-ray transients (FXTs) are a class of soft (0.3–10 keV) X-ray transients lasting a few hundred seconds to several hours. Several progenitor mechanisms have been suggested to produce FXTs, including supernova shock breakouts, binary neutron star mergers, or tidal disruptions involving an intermediate-mass black hole and a white dwarf. We present detailed host studies, including spectroscopic observations of the host galaxies of seven XMM-Newton-discovered FXTs. The candidate hosts lie at redshifts 0.0928 <z < 0.645 implying peak X-ray luminosities of 1043 erg s−1<LX < 1045 erg s−1 and physical offsets of 1 kpc < rproj < 22 kpc. These observations increase the number of FXTs with a spectroscopic redshift measurement by a factor of 2, although we note that one event is re-identified as a Galactic flare star. We infer host star formation rates and stellar masses by fitting the combined spectroscopic and archival photometric data. We also report on a contemporaneous optical counterpart search to the FXTs in Pan-STARRS and ATLAS by performing forced photometry at the position of the FXTs. We do not find any counterpart in our search. Given our constraints, including peak X-ray luminosities, optical limits, and host properties, we find that XRT 110 621 is consistent with an supernova shock breakout (SN SBO) event. Spectroscopic redshifts of likely host galaxies for four events imply peak X-ray luminosities that are too high to be consistent with SN SBOs, but we are unable to discard either the binary neutron star or white dwarf–intermediate-mass black hole tidal disruption event scenarios for these FXTs.

An updated nuclear-physics and multi-messenger astrophysics framework for binary neutron star mergers

The multi-messenger detection of the gravitational-wave signal GW170817, the corresponding kilonova AT2017gfo and the short gamma-ray burst GRB170817A, as well as the observed afterglow has delivered a scientific breakthrough. For an accurate interpretation of all these different messengers, one requires robust theoretical models that describe the emitted gravitational-wave, the electromagnetic emission, and dense matter reliably. In addition, one needs efficient and accurate computational tools to ensure a correct cross-correlation between the models and the observational data. For this purpose, we have developed the Nuclear-physics and Multi-Messenger Astrophysics framework NMMA. The code allows incorporation of nuclear-physics constraints at low densities as well as X-ray and radio observations of isolated neutron stars. In previous works, the NMMA code has allowed us to constrain the equation of state of supranuclear dense matter, to measure the Hubble constant, and to compare dense-matter physics probed in neutron-star mergers and in heavy-ion collisions, and to classify electromagnetic observations and perform model selection. Here, we show an extension of the NMMA code as a first attempt of analyzing the gravitational-wave signal, the kilonova, and the gamma-ray burst afterglow simultaneously. Incorporating all available information, we estimate the radius of a 1.4M⊙ neutron star to be R=11.98−0.40+0.35\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R=11.9{8}_{-0.40}^{+0.35}$$\\end{document} km.

Nuclear physics constraints from binary neutron star mergers in the Einstein Telescope era

Nuclear physics constraints from binary neutron star mergers in the Einstein Telescope era

Effects of onset of phase transition on binary neutron star mergers

ABSTRACT Quantum Chromodynamics predicts phase transition from hadronic matter to quark matter at high density, which is highly probable in astrophysical systems like binary neutron star mergers. To explore the critical density where such phase transition can occur, we performed numerical relativity simulations of binary neutron star mergers with various masses (equal and unequal binaries). We aim to understand the effect of the onset of phase transition on the merger dynamics and gravitational wave spectra. We generated a set of equations of states by agnostically changing the onset of phase transition, having the hadronic matter part and quark matter part fixed. This particular arrangement of the equation of states explores the scenario of mergers where mixed phases of matter are achieved before or during the merger. Under these circumstances, if the matter properties with hadronic and quark degrees differ significantly, it is reflected in the stability of the final merger product for the intermediate mass binary. We performed a case study on mixed species merger, where one of the binary companions is hybrid star. If quark matter appears at low densities, we observe significant change in post-merger gravitational wave analysis in terms of higher peak frequencies and post-merger frequencies in power spectral density. We report indications expressed as spikes in phase difference plots at merger time for mixed mergers. We found that the expression of phase transition in post-merger gravitational wave signals is more significant for unequal mass binary than for equal mass binary having the same total baryonic mass.

Jets from Neutron-Star Merger Remnants and Massive Blue Kilonovae.

We perform three-dimensional general-relativistic magnetohydrodynamic simulations with weak interactions of binary neutron-star (BNS) mergers resulting in a long-lived remnant neutron star, with properties typical of galactic BNS and consistent with those inferred for the first observed BNS merger GW170817. We demonstrate self-consistently that within ≲30 ms postmerger magnetized (σ∼5-10) incipient jets emerge with asymptotic Lorentz factor Γ∼5-10, which successfully break out from the merger debris within ≲20 ms. A fast (v≲0.6c), magnetized (σ∼0.1) wind surrounds the jet core and generates a UV/blue kilonova precursor on timescales of hours, similar to the precursor signal due to free neutron decay in fast dynamical ejecta. Postmerger ejecta are quickly dominated by magnetohydrodynamically driven outflows from an accretion disk. We demonstrate that, within only 50ms postmerger, ≳2×10^{-2}M_{⊙} of lanthanide-free, quasispherical ejecta with velocities ∼0.1-0.2c is launched, yielding a kilonova signal consistent with GW170817 on timescales of ≲5 d.

Differential rotation in compact objects with hyperons and delta isobars

AbstractNeutron stars may experience differential rotation on short, dynamical timescales following extreme astrophysical events like binary neutron star mergers. In this work, the masses and radii of differentially rotating neutron star models are computed. We employ a set of equations of states for dense hypernuclear and ‐admixed‐hypernuclear matter obtained within the framework of CDF theory in the relativistic Hartree‐Fock (RHF) approximation. Results are shown for varying meson‐ couplings, or equivalently the ‐potential in nuclear matter. A comparison of our results with those obtained for nonrotating stars shows that the maximum mass difference between differentially rotating and static stars is independent of the underlying particle composition of the star. We further find that the decrease in the radii and increase in the maximum masses of stellar models when ‐isobars are added to hyperonuclear matter (as initially observed for static and uniformly rotating stars) persist also in the case of differentially rotating neutron stars.

Premerger Sky Localization of Gravitational Waves from Binary Neutron Star Mergers Using Deep Learning

The simultaneous observation of gravitational waves (GW) and prompt electromagnetic counterparts from the merger of two neutron stars can help reveal the properties of extreme matter and gravity during and immediately after the final plunge. Rapid sky localization of these sources is crucial to facilitate such multimessenger observations. As GWs from binary neutron star (BNS) mergers can spend up to 10–15 minutes in the frequency bands of the detectors at design sensitivity, early-warning alerts and premerger sky localization can be achieved for sufficiently bright sources, as demonstrated in recent studies. In this work, we present premerger BNS sky localization results using GW-SkyLocator, a deep-learning model capable of inferring sky location posterior distributions of GW sources at orders of magnitude faster speeds than standard Markov Chain Monte Carlo methods. We test our model’s performance on a catalog of simulated injections from Sachdev, recovered at 0–60 s before the merger, and obtain comparable sky localization areas to the rapid localization tool BAYESTAR. These results show the feasibility of our model for premerger sky localization and the possibility of follow-up observations for precursor emissions from BNS mergers.

The short gamma-ray burst population in a quasi-universal jet scenario

We present a model of the short gamma-ray burst (SGRB) population under a ‘quasi-universal jet’ scenario in which jets can differ somewhat in their on-axis peak prompt emission luminosity, Lc, but share a universal angular luminosity profile, ℓ(θv) = L(θv)/Lc, as a function of the viewing angle, θv. The model was fitted, through a Bayesian hierarchical approach inspired by gravitational wave (GW) population analyses, to three observed SGRB samples simultaneously: the Fermi/GBM sample of SGRBs with spectral information available in the catalogue (367 events); a flux-complete sample of 16 Swift/BAT SGRBs that are also detected by the GBM and have a measured redshift; and a sample of SGRBs with a binary neutron star (BNS) merger counterpart, which only includes GRB 170817A at present. Particular care was put into modelling selection effects. The resulting model, which reproduces the observations, favours a narrow jet ‘core’ with half-opening angle θc = 2.1−1.4+2.4 deg (uncertainties hereon refer to 90% credible intervals from our fiducial ‘full sample’ analysis) whose peak luminosity, as seen on-axis, is distributed as a power law, p(Lc) ∝ Lc−A with A = 3.2−0.4+0.7, above a minimum isotropic-equivalent luminosity, Lc⋆ = 5−2+11 × 1051 erg s−1. For viewing angles larger than θc, the luminosity profile scales as a single power law, l ∝ θv−αL with αL = 4.7−1.4+1.2, with no evidence of a break, despite the model allowing for it. While the model implies an intrinsic ‘Yonetoku’ correlation between L and the peak photon energy, Ep, of the spectral energy distribution, its slope is somewhat shallower, Ep ∝ L0.4 ± 0.2, than the apparent one, and the normalisation is offset towards larger Ep due to selection effects. The implied local rate density of SGRBs (regardless of the viewing angle) is between about one hundred up to several thousand events per cubic gigaparsec per year, in line with the BNS merger rate density inferred from GW observations. Based on the model, we predict 0.2 to 1.3 joint GW+SGRB detections per year by the advanced GW detector network and Fermi/GBM during the O4 observing run.

Predictions for electromagnetic counterparts to Neutron Star mergers discovered during LIGO-Virgo-KAGRA observing runs 4 and 5

ABSTRACT We present a comprehensive, configurable open-source software framework for estimating the rate of electromagnetic detection of kilonovae (KNe) associated with gravitational wave detections of binary neutron star (BNS) mergers. We simulate the current LIGO-Virgo-KAGRA (LVK) observing run (O4) using current sensitivity and uptime values as well as using predicted sensitivites for the next observing run (O5). We find the number of discoverable kilonovae during LVK O4 to be ${ 1}_{- 1}^{+ 4}$ or ${ 2 }_{- 2 }^{+ 3 }$, (at 90 per cent confidence) depending on the distribution of NS masses in coalescing binaries, with the number increasing by an order of magnitude during O5 to ${ 19 }_{- 11 }^{+ 24 }$. Regardless of mass model, we predict at most five detectable KNe (at 95 per cent confidence) in O4. We also produce optical and near-infrared light curves that correspond to the physical properties of each merging system. We have collated important information for allocating observing resources for search and follow-up observations, including distributions of peak magnitudes in several broad-bands and time-scales for which specific facilities can detect each KN. The framework is easily adaptable, and new simulations can quickly be produced in response to updated information such as refined merger rates and NS mass distributions. Finally, we compare our suite of simulations to the thus-far completed portion of O4 (as of 2023, October 14), finding a median number of discoverable KNe of 0 and a 95 percentile upper limit of 2, consistent with no detections so far in O4.

The Jet Opening Angle and Event Rate Distributions of Short Gamma-Ray Bursts from Late-time X-Ray Afterglows

We present a comprehensive study of 29 short gamma-ray bursts (SGRBs) observed ≈0.8−60 days postburst using Chandra and XMM-Newton. We provide the inferred distributions of the SGRB jet opening angles and true event rates to compare against neutron star merger rates. We perform a uniform analysis and modeling of their afterglows, obtaining 10 opening angle measurements and 19 lower limits. We report on two new opening angle measurements (SGRBs 050724A and 200411A) and eight updated values, obtaining a median value of 〈θ j〉 ≈ 6.°1 [−3.°2, +9.°3] (68% confidence on the full distribution) from jet measurements alone. For the remaining events, we infer θ j ≳ 0.°5–26°. We uncover a population of SGRBs with wider jets of θ j ≳ 10° (including two measurements of θ j ≳ 15°), representing ∼28% of our sample. Coupled with multiwavelength afterglow information, we derive a total true energy of 〈E true,tot〉 ≈ 1049–1050 erg, which is consistent with magnetohydrodynamic jet launching mechanisms. Furthermore, we determine a range for the beaming-corrected event rate of Gpc−3 yr−1, set by the inclusion of a population of wide jets on the low end, and the jet measurements alone on the high end. From a comparison with the latest merger rates, our results are consistent with the majority of SGRBs originating from binary neutron star mergers. However, our inferred rates are well above the latest neutron star–black hole merger rates, consistent with at most a small fraction of SGRBs originating from such mergers.

Effects of nuclear matter properties in neutron star mergers

ABSTRACT The dynamics in mergers of binary neutron star (BNS) systems depend sensitively on the equation of state (EOS) of dense matter. This has profound implications on the emission of gravitational waves (GWs) and the ejection of matter in the merger and post-merger phases and is thus of high interest for multimessenger astronomy. Today, a variety of nuclear EOSs are available with various underlying microphysical models. This calls for a study to focus on EOS effects from different physical nuclear matter properties and their influence on BNS mergers. We perform simulations of equal-mass BNS mergers with a set of nine different EOSs based on Skyrme density functionals. In the models, we systematically vary the effective nucleon mass, incompressibility, and symmetry energy at saturation density. This allows us to investigate the influence of specific nuclear matter properties on the dynamics of BNS mergers. We analyse the impact of these properties on the merger dynamics, the fate of the remnant, disc formation, ejection of matter, and GW emission. Our results indicate that some aspects of the merger, such as the frequencies of the post-merger GW spectrum and the shock-heated ejecta mass, are sensitive to the EOS around saturation density while others, such as the contraction of the remnant and the tidal ejecta mass, are sensitive to the behaviour towards higher densities, e.g. characterized by the slope of the pressure versus density. The detailed density dependence of the EOS thus needs to be taken into account to describe its influence on BNS mergers.

Impact of anisotropic ejecta on jet dynamics and afterglow emission in binary neutron-star mergers

ABSTRACT Binary neutron-stars mergers widely accepted as potential progenitors of short gamma-ray bursts. After the remnant of the merger has collapsed to a black hole, a jet is powered and may breakout from the the matter expelled during the collision and the subsequent wind emission. The interaction of the jet with the ejecta may affect its dynamics and the resulting electromagnetic counterparts. We here examine how an inhomogeneous and anisotropic distribution of ejecta affects such dynamics, dictating the properties of the jet-ejecta cocoon and of the afterglow radiated by the jet upon deceleration. More specifically, we carry out general-relativistic hydrodynamical simulations of relativistic jets launched within a variety of geometrically inhomogeneous and anisotropic distributions of ejected matter. We find that different anisotropies impact the variance of the afterglow light curves as a function of the jet luminosity and ejected mass. A considerable amount of the jet energy is deposited in the cocoon through the jet-ejecta interaction with a small but important dependence on the properties of the ejecta. Furthermore, all configurations show a two-component behaviour for the polar structure of the jet, with a narrow core at large energies and Lorentz factors and a shallow segment at high latitudes from the jet axis. Hence, afterglows measured on off-axis lines of sight could be used to deduce the properties of the ejected matter, but also that the latter need to be properly accounted for when modelling the afterglow signal and the jet-launching mechanisms.

General relativistic simulations of high-mass binary neutron star mergers: rapid formation of low-mass stellar black holes

ABSTRACT Almost a hundred compact binary mergers have been detected via gravitational waves by the LIGO-Virgo-KAGRA collaboration in the past few years providing us with a significant amount of new information on black holes and neutron stars. In addition to observations, numerical simulations using newly developed modern codes in the field of gravitational wave physics will guide us to understand the nature of single and binary degenerate systems and highly energetic astrophysical processes. We here presented a set of new fully general relativistic hydrodynamic simulations of high-mass binary neutron star systems using the open-source EINSTEIN TOOLKIT and LORENE codes. We considered systems with total baryonic masses ranging from 2.8$\, \mathrm{M}_\odot$ to 4.0$\, \mathrm{M}_\odot$ and used the SLy equation of state. We analysed the gravitational wave signal for all models and report potential indicators of systems undergoing rapid collapse into a black hole that could be observed by future detectors like the Einstein Telescope and the Cosmic Explorer. The properties of the post-merger black hole, the disc and ejecta masses, and their dependence on the binary parameters were also extracted. We also compared our numerical results with recent analytical fits presented in the literature and provided parameter-dependent semi-analytical relations between the total mass and mass ratio of the systems and the resulting black hole masses and spins, merger frequency, BH formation time, ejected mass, disc mass, and radiated gravitational wave energy.

Gamma-ray burst precursors from tidally resonant neutron star oceans: potential implications for GRB 211211A

ABSTRACT Precursors have been observed seconds to minutes before some short gamma-ray bursts. While the precursor origins remain unknown, one explanation relies on the resonance of neutron star pulsational modes with the tidal forces during the inspiral phase of a compact binary merger. In this paper, we present a model for short gamma-ray burst precursors that relies on tidally resonant neutron star oceans. In this scenario, the onset of tidal resonance in the crust–ocean interface mode ignites the precursor flare, possibly through the interaction between the excited neutron star ocean and the surface magnetic fields. From just the precursor total energy, the time before the main event, and a detected quasi-periodic oscillation frequency, we may constrain the binary parameters and neutron star ocean properties. Our model can immediately distinguish neutron star–black hole mergers from binary neutron star mergers without gravitational wave detection. We apply our model to GRB 211211A, the recently detected long duration short gamma-ray burst with a quasi-periodic precursor, and explore the parameters of this system. The precursor of GRB 211211A is consistent with a tidally resonant neutron star ocean explanation that requires an extreme mass ratio neutron star–black hole merger and a high-mass neutron star. While difficult to reconcile with the main gamma-ray burst and associated kilonova, our results constrain the possible precursor mechanisms in this system. A systematic study of short gamma-ray burst precursors with the model presented here can test precursor origin and probe the possible connection between gamma-ray bursts and neutron star–black hole mergers.

Updated Observing Scenarios and Multimessenger Implications for the International Gravitational-wave Networks O4 and O5

An advanced LIGO and Virgo’s third observing run brought another binary neutron star merger (BNS) and the first neutron-star black hole mergers. While no confirmed kilonovae were identified in conjunction with any of these events, continued improvements of analyses surrounding GW170817 allow us to project constraints on the Hubble Constant (H 0), the Galactic enrichment from r-process nucleosynthesis, and ultra-dense matter possible from forthcoming events. Here, we describe the expected constraints based on the latest expected event rates from the international gravitational-wave network and analyses of GW170817. We show the expected detection rate of gravitational waves and their counterparts, as well as how sensitive potential constraints are to the observed numbers of counterparts. We intend this analysis as support for the community when creating scientifically driven electromagnetic follow-up proposals. During the next observing run O4, we predict an annual detection rate of electromagnetic counterparts from BNS of () for the Zwicky Transient Facility (Rubin Observatory).

Constraining the long-lived supramassive neutron stars by magnetar boosted kilonovae

ABSTRACT Kilonovae are optical transients following the merger of neutron star binaries, which are powered by the r-process heating of merger ejecta. However, if a merger remnant is a long-lived supramassive neutron star supported by its uniform rotation, it will inject energy into the ejecta through spin-down power. The energy injection can boost the peak luminosity of a kilonova by many orders of magnitudes, thus significantly increasing the detectable volume. Therefore, even if such events are only a small fraction of the kilonova population, they could dominate the detection rates. However, after many years of optical sky surveys, no such event has been confirmed. In this work, we build a boosted kilonova model with rich physical details, including the description of the evolution and stability of a proto neutron star, and the energy absorption through X-ray photoionization. We simulate the observation prospects and find the only way to match the absence of detection is to limit the energy injection by the newly born magnetar to only a small fraction of the neutron star rotational energy, thus they should collapse soon after the merger. Our result indicates that most supramassive neutron stars resulting from binary neutron star mergers are short lived and they are likely to be rare in the Universe.

On the “Loose” Constraint from IceCube Neutrino Nondetection of GRB 230307A

The recent extremely bright gamma-ray burst (GRB) from a binary neutron star merger, GRB 230307A, may offer a good probe for the production of GRB neutrinos. Within the constraint from IceCube neutrino nondetection, the limits for key physical parameters of this burst are extracted in different scenarios, including the fireball, Poynting-flux-dominated, and hybrid jet. Different from the former nearby “monsters” and due to its smaller isotropic equivalent radiated energy (E γ,iso ∼ 4 × 1052 erg), the constraint seems loose if nonthermal neutrinos produced from photomeson interactions are the only consideration. However, a quasi-thermal neutrino emission from hadronuclear processes is constrained in this neutron-rich postmerger environment, and the upper limit of the allowed nucleon loading factor is ∼a few. Based on this, a discussion is presented on the possible prompt emission mechanism and jet composition for GRB 230307A in the context of multimessenger astrophysics. It is worth noting that until now, no GRB neutrinos have been ever detected, even for the two brightest nearby GRBs ever observed (GRB 221009A and GRB 230307A), which have different dissipation mechanisms.

Exploring the multiband gravitational wave background with a semi-analytic galaxy formation model

ABSTRACT An enormous number of compact binary systems, spanning from stellar to supermassive levels, emit substantial gravitational waves during their final evolutionary stages, thereby creating a stochastic gravitational wave background (SGWB). We calculate the merger rates of stellar compact binaries and massive black hole binaries using a semi-analytic galaxy formation model – Galaxy Assembly with Binary Evolution (GABE) – in a unified and self-consistent approach, followed by an estimation of the multiband SGWB contributed by those systems. We find that the amplitudes of the principal peaks of the SGWB energy density are within one order of magnitude ΩGW ∼ 10−9–10−8. This SGWB could easily be detected by the Square Kilometre Array (SKA), as well as by the planned interferometric detectors, such as the Einstein Telescope and the Laser Interferometer Space Antenna (LISA). The energy density of this background varies as ΩGW ∝ f2/3 in the SKA band. The shape of the SGWB spectrum in the frequency range ∼[10−4,1] Hz could allow the LISA to distinguish the black hole seed models. The amplitude of the SGWB from merging stellar binary black holes at ∼100 Hz is approximately 10 and 100 times greater than those from merging binary neutron stars and neutron-star–black-hole mergers, respectively. Note that, since the cosmic star formation rate density predicted by GABE is somewhat lower than the observational results by ∼0.2 dex at z < ∼2, the amplitude of the SGWB in the frequency range ∼[1, 104] Hz may be underestimated by a similar factor at most.

Neutron Star Constraints on Neutron Dark Decays

Motivated by the neutron lifetime puzzle, it is proposed that neutrons may decay into new states yet to be observed. We review the neutron star constraints on dark fermions carrying unit baryon number with masses around 939 MeV, and discuss the interaction strengths required for the new particle. The possibility of neutrons decaying into three dark fermions is investigated. While up to six flavors of dark quarks with masses around 313 MeV can be compatible with massive pulsars, any such exotic states lighter than about 270 MeV are excluded by the existence of low-mass neutron stars around ∼1.2M⊙. Light dark quarks in the allowed mass range may form a halo surrounding normal neutron stars. We discuss the potential observable signatures of the halo during binary neutron star mergers.

On the possibility to detect gravitational waves from post-merger supermassive neutron stars with a kilohertz detector

ABSTRACT The detection of a secular post-merger gravitational wave (GW) signal in a binary neutron star (BNS) merger serves as strong evidence for the formation of a long-lived post-merger neutron star (NS), which can help constrain the maximum mass of NSs and differentiate NS equations of state. We specifically focus on the detection of GW emissions from rigidly rotating NSs formed through BNS mergers, using several kilohertz GW detectors that have been designed. We simulate the BNS mergers within the detecting limit of LIGO-Virgo-KARGA O4 and attempt to find out on what fraction the simulated sources may have a detectable secular post-merger GW signal. For kilohertz detectors designed in the same configuration of LIGO A+, we find that the design with peak sensitivity at approximately 2 kHz is most appropriate for such signals. The fraction of sources that have a detectable secular post-merger GW signal would be approximately $0.94{\!-\!}11~{{ \rm per\ cent}}$ when the spin-downs of the post-merger rigidly rotating NSs are dominated by GW radiation, while it would be approximately $0.46{\!-\!}1.6~{{ \rm per\ cent}}$ when the contribution of electromagnetic (EM) radiation to the spin-down processes is non-negligible. We also estimate this fraction based on other well-known proposed kilohertz GW detectors and find that, with advanced design, it can reach approximately $12{\!-\!}45~{{ \rm per\ cent}}$ for the GW-dominated spin-down case and $4.7{\!-\!}16~{{\rm per\ cent}}$ when both the GW and EM radiations are considered.