Binary Neutron Star Merger Research Articles

A Channel to Form Fast-spinning Black Hole–Neutron Star Binary Mergers as Multimessenger Sources. II. Accretion-induced Spin-up

In this work, we investigate an alternative channel for the formation of fast-spinning black hole–neutron star (BHNS) binaries, in which super-Eddington accretion is expected to occur in accreting BHs during the stable mass transfer phase within BH-stripped helium (BH–He-rich) star binary systems. We evolve intensive MESA grids of close-orbit BH–He-rich star systems to systematically explore the projected aligned spins of BHs in BHNS binaries, as well as the impact of different accretion limits on the tidal disruption probability and electromagnetic (EM) signature of BHNS mergers. Most of the BHs in BHNS mergers cannot be effectively spun up through accretion if the accretion rate is limited to ≲10ṀEdd , where ṀEdd is the standard Eddington accretion limit. In order to reach high spins (e.g., χ BH ≳ 0.5), the BHs are required to be born less massive (e.g., ≲3.0 M ⊙) in binary systems with initial periods of ≲0.2–0.3 days and accrete material at ∼100ṀEdd . However, even under this high accretion limit, ≳6 M ⊙ BHs are typically challenging to significantly spin up and generate detectable associated EM signals. Our population simulations suggest that different accretion limits have a slight impact on the ratio of tidal disruption events. However, as the accretion limit increases, the EM counterparts from the cosmological BHNS population can become bright overall.

Secular Outflows from Long-Lived Neutron Star Merger Remnants

We study mass ejection from a binary neutron star merger producing a long-lived massive neutron star remnant with general-relativistic neutrino-radiation hydrodynamics simulations. In addition to outflows generated by shocks and tidal torques during and shortly after the merger, we observe the appearance of a wind driven by spiral density waves in the disk. This spiral-wave-driven outflow is predominantly located close to the disk orbital plane and have a broad distribution of electron fractions. At higher latitudes, a high electron-fraction wind is driven by neutrino radiation. The combined nucleosynthesis yields from all the ejecta components is in good agreement with Solar abundance measurements.

Maximally local two-nucleon interactions at N3LO in Δ -less chiral effective field theory

We present new maximally local two-nucleon interactions derived in Δ-less chiral effective field theory up to next-to-next-to-next-to-leading order (N3LO), which include all contact and pion-exchange contributions to the nuclear Hamiltonian up to this order. Our interactions are fit to nucleon-nucleon phase shifts using a Bayesian statistical approach, and explore a wide cutoff range from 0.6–0.9 fm (≈660–440MeV). These interactions can be straightforwardly employed in quantum Monte Carlo methods, such as the auxiliary field diffusion Monte Carlo method. Together with local three-nucleon forces, calculations with these new interactions will provide improved benchmarks for the structure of atomic nuclei and serve as crucial input to analyses of astrophysical phenomena of neutron stars, such as binary neutron-star mergers. Published by the American Physical Society 2024

Achieving the Fundamental Quantum Limit of Linear Waveform Estimation.

Sensing a classical signal using a linear quantum device is a pervasive application of quantum-enhanced measurement. The fundamental precision limits of linear waveform estimation, however, are not fully understood. In certain cases, there is an unexplained gap between the known waveform-estimation quantum Cramér-Rao bound and the optimal sensitivity from quadrature measurement of the outgoing mode from the device. We resolve this gap by establishing the fundamental precision limit, the waveform-estimation Holevo Cramér-Rao bound, and how to achieve it using a nonstationary measurement. We apply our results to detuned gravitational-wave interferometry to accelerate the search for postmerger remnants from binary neutron-star mergers. If we have an unequal weighting between estimating the signal's power and phase, then we propose how to further improve the signal-to-noise ratio by a factor of sqrt[2] using this nonstationary measurement.

Formation of lower mass-gap black hole–neutron star binary mergers through super-Eddington stable mass transfer

ABSTRACT Super-Eddington accretion of neutron stars (NSs) has been suggested both observationally and theoretically. In this paper, we propose that NSs in close-orbit binary systems with companions of helium (He) stars, most of which systems form after the common-envelope phase, could experience super-Eddington stable Case BB/BC mass transfer (MT), and can sometimes undergo accretion-induced collapse (AIC), resulting in the formation of lower mass-gap black holes (mgBHs). Our detailed binary evolution simulations reveal that AIC events tend to happen if the primary NSs have an initial mass ${\gtrsim} 1.7\, {\rm M}_\odot$ with a critical accretion rate of ≳300 times the Eddington limit. These mgBHs would have a mass nearly equal to or slightly higher than the NS maximum mass. The remnant mgBH–NS binaries after the core collapses of He stars are potential progenitors of gravitational-wave (GW) sources. Multimessenger observations between GW and kilonova signals from a population of high-mass binary NS and mgBH–NS mergers formed through super-Eddington stable MT are helpful in constraining the maximum mass and equation of state of NSs.

Independent evidence in multi-messenger astrophysics

In this paper I discuss the first “multi-messenger” observations of a binary neutron star merger and kilonova. These observations, touted as “revolutionary,” included both gravitational-wave and electromagnetic observations of a single source. I draw on analogies between astrophysics and historical sciences (e.g., paleontology) to explain the significance of this for (gravitational-wave) astrophysics. In particular, I argue that having independent lines of evidence about a target system enables the use of argumentative strategies—the “Sherlock Holmes” method and consilience—that help overcome the key challenges astrophysics faces as an observational and historical science.

Gravitational-wave imprints of nonconvex dynamics in binary neutron star mergers

Gravitational-wave imprints of nonconvex dynamics in binary neutron star mergers

Kilohertz gravitational waves from binary neutron star mergers: Numerical-relativity informed postmerger model

Kilohertz gravitational waves from binary neutron star mergers: Numerical-relativity informed postmerger model

Probing a magnetar origin for the population of extragalactic fast X-ray transients detected by Chandra

Context. Twenty-two extragalactic fast X-ray transients (FXTs) have now been discovered from two decades of Chandra data (analyzing ∼259 Ms of data), with 17 associated with distant galaxies (≳100 Mpc). Different mechanisms and progenitors have been proposed to explain their properties; nevertheless, after analyzing their timing, spectral parameters, host-galaxy properties, luminosity function, and volumetric rates, their nature remains uncertain. Aims. We interpret a sub-sample of nine FXTs that show a plateau or a fast-rise light curve within the framework of a binary neutron star (BNS) merger magnetar model. Methods. We fit their light curves and derive magnetar (magnetic field and initial rotational period) and ejecta (ejecta mass and opacity) parameters. This model predicts two zones: an orientation-dependent free zone (where the magnetar spin-down X-ray photons escape freely to the observer) and a trapped zone (where the X-ray photons are initially obscured and only escape freely once the ejecta material becomes optically thin). We argue that six FXTs show properties consistent with the free zone and three FXTs with the trapped zone. Results. This sub-sample of FXTs has a similar distribution of magnetic fields and initial rotation periods to those inferred for short gamma-ray bursts, suggesting a possible association. We compare the predicted ejecta emission fed by the magnetar emission (called merger-nova) to the optical and near-infrared upper limits of two FXTs, XRT 141001 and XRT 210423 where contemporaneous optical observations are available. The non-detections place lower limits on the redshifts of XRT 141001 and XRT 210423 of z ≳ 1.5 and ≳0.1, respectively. Conclusions. If the magnetar remnants lose energy via gravitational waves (GWs), it should be possible to detect similar objects with the current advanced LIGO detectors out to a redshift z ≲ 0.03, while future GW detectors will be able to detect them out to z ≈ 0.5.

Gravitational wave triggered searches for high-energy neutrinos from binary neutron star mergers: Prospects for next generation detectors

Gravitational wave triggered searches for high-energy neutrinos from binary neutron star mergers: Prospects for next generation detectors

Towards inferring the geometry of kilonovae

ABSTRACT Recent analysis of the kilonova, AT2017gfo, has indicated that this event was highly spherical. This may challenge hydrodynamics simulations of binary neutron star mergers, which usually predict a range of asymmetries, and radiative transfer simulations show a strong direction dependence. Here we investigate whether the synthetic spectra from a 3D kilonova simulation of asymmetric ejecta from a hydrodynamical merger simulation can be compatible with the observational constraints, suggesting a high degree of sphericity in AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line profile model leads to a value for the photospheric velocity that is consistent with the value obtained from the expanding photosphere method. We would infer that our kilonova simulation is highly spherical at early times, when the spectra resemble a blackbody distribution. The two independently inferred photospheric velocities can be very similar, implying a high degree of sphericity, which can be as spherical as inferred for AT2017gfo, demonstrating that the photosphere can appear spherical even for asymmetrical ejecta. The last-interaction velocities of radiation escaping the simulation show a high degree of sphericity, supporting the inferred symmetry of the photosphere. We find that when the synthetic spectra resemble a blackbody, the expanding photosphere method can be used to obtain an accurate luminosity distance (within 4–7 per cent).

A large-scale magnetic field produced by a solar-like dynamo in binary neutron star mergers

The merger of two neutron stars launches a relativistic jet, which must be driven by a strong large-scale magnetic field. However, the magnetohydrodynamical mechanism required to build up this magnetic field remains uncertain. By performing an ab initio super-high-resolution neutrino-radiation magnetohydrodynamics merger simulation in full general relativity, we show that the αΩ dynamo mechanism, driven by the magnetorotational instability, builds up the large-scale magnetic field inside the long-lived remnant of the binary neutron star merger. As a result, the magnetic field induces a Poynting-flux-dominated relativistic outflow with an isotropic equivalent luminosity of ~1052 erg s−1 and a magnetically driven post-merger mass ejection of ~0.1 M⊙. Therefore, the magnetar hypothesis, in which an ultra-strongly magnetized neutron star drives a relativistic jet in binary neutron star mergers, is possible. Magnetars can be the engines of short, hard gamma-ray bursts, and they should be associated with very bright kilonovae, which current telescopes could observe. Therefore, this scenario is testable in future observations.

Neutron star mergers as the dominant contributor to the production of heavy r-process elements

ABSTRACT The discovery of the radioactively powered kilonova AT2017gfo, associated with the short-duration gamma-ray burst GRB 170817A and the gravitational wave source GW170817, has provided the first direct evidence supporting binary neutron star mergers as crucial astrophysical sites for the synthesis of heavy elements beyond iron through r-process nucleosysthesis in the universe. However, recent identification of kilonovae following long-duration gamma-ray bursts, such as GRB 211211A and GRB 230307A, has sparked discussions about the potential of neutron star–white dwarf mergers to also produce neutron-rich ejecta and contribute to the production of heavy r-process elements. In this work, we estimate the contribution of binary neutron star mergers to the total mass of r-process elements in the Milky Way and investigate the possibility of neutron star–white dwarf mergers as alternative astrophysical sites for r-process nucleosynthesis through an analysis of the total mass of the r-process elements in the Milky Way. Our results reveal that binary neutron star mergers can sufficiently account for the Galactic heavy r-process elements, suggesting that these events are the dominant contributor to the production of heavy r-process elements in the Milky Way. Considering the total mass of r-process elements in the Milky Way and the higher occurrence rate of neutron star–white dwarf mergers, it is unlikely that such mergers can produce a significant amount of neutron-rich ejecta, with the generated mass of r-process elements being lower than $0.005\, {\rm M}_{\odot }$.

The S-PLUS Transient Extension Program: imaging pipeline, transient identification, and survey optimization for multimessenger astronomy

ABSTRACT We present the S-PLUS Transient Extension Program (STEP): a supernova and fast transient survey conducted in the southern hemisphere using data from the Southern Photometric Local Universe Survey (S-PLUS) Main Survey and the T80-South telescope. Transient astrophysical phenomena have a range of interest that goes through different fields of astrophysics and cosmology. With the detection of an electromagnetic counterpart to the gravitational wave (GW) event GW170817 from a binary neutron stars merger, new techniques and resources to study fast astrophysical transients in the multimessenger context have increased. In this paper, we present the STEP overview, the SN follow-up data obtained, data reduction, analysis of new transients and deep learning algorithms to optimize transient candidate selection. Additionally, we present prospects and optimized strategy for the search of gravitational wave counterparts in the current LIGO/Virgo/Kagra observational run (O4) in the context of T80-South telescope.

Nuclear Matter Properties and Neutron Star Phenomenology Using the Finite Range Simple Effective Interaction

The saturation properties of symmetric and asymmetric nuclear matter have been computed using the finite range simple effective interaction with Yukawa form factor. The results of higher-order derivatives of the energy per particle and the symmetry energy computed at saturation, namely, Q0, Ksym, Kτ, Qsym, are compared with the corresponding values extracted from studies involving theory, experiment and astrophysical observations. The overall uncertainty in the values of these quantities, which results from a wide spectrum of studies described in earlier literature, lies in the ranges −1200≲Q0≲400 MeV, −400≲Ksym≲100 MeV, −840≲Kτ≲−126 MeV and −200≲Qsym≲800 MeV, respectively. The ability of the equations of state computed with this simple effective interaction in predicting the threshold mass for prompt collapse in binary neutron star merger and gravitational redshift has been examined in terms of the compactness of the neutron star and the incompressibility at the central density of the maximum mass star. The correlations existing between neutron star properties and the nuclear matter saturation properties have been analyzed and compared with the predictions of other model calculations.

Thermal behavior as indicator for hyperons in binary neutron star merger remnants

Thermal behavior as indicator for hyperons in binary neutron star merger remnants

What if GW190425 did not produce a black hole promptly?

ABSTRACT It is widely believed that the binary neutron star merger GW190425 produced a black hole promptly upon merger. Motivated by the potential association with the fast radio burst FRB 20190425A, which took place 2.5 h after the merger, we revisit the question of the outcome of GW190425 by means of numerical relativity simulations. We show that current laboratory and astrophysical constraints on the equation of state of dense matter do not rule out the formation of a long-lived remnant. However, the formation of a stable remnant would have produced a bright kilonova, in tension with upper limits by ZTF at the location and time of FRB 20190425A. Moreover, the ejecta would have been optically thick to radio emission for days to months, preventing a putative FRB from propagating out. The predicted dispersion measure is also several orders of magnitude larger than that observed for FRB 20190425A. Our results indicate that FRB 20190425A and GW190425 are not associated. However, we cannot completely rule out the formation of a long-lived remnant, due to the incomplete coverage of the relevant sky regions. More observations of GW190425-like events, including potential upper limit, have the potential to constrain nuclear physics. To this aim, it is important that follow-up observational campaigns of gravitational wave events are informed by the properties of the source, such as their chirp mass, and we urge the LIGO-Virgo-KAGRA collaboration to promptly release them publicly.

What Can We Learn about the Unstable Equation-of-state Branch from Neutron Star Mergers?

The equation of state (EOS) of dense strongly interacting matter can be probed by astrophysical observations of neutron stars (NS), such as X-ray detections of pulsars or the measurement of the tidal deformability of NSs during the inspiral stage of NS mergers. These observations constrain the EOS at most up to the density of the maximum-mass configuration, n TOV, which is the highest density that can be explored by stable NSs for a given EOS. However, under the right circumstances, binary neutron star (BNS) mergers can create a postmerger remnant that explores densities above n TOV. In this work, we explore whether the EOS above n TOV can be measured from gravitational-wave or electromagnetic observations of the postmerger remnant. We perform a total of 25 numerical-relativity simulations of BNS mergers for a range of EOSs and find no case in which different descriptions of the matter above n TOV have a detectable impact on postmerger observables. Hence, we conclude that the EOS above n TOV can likely not be probed through BNS merger observations for the current and next generation of detectors.

Did a Kilonova Set Off in Our Galactic Backyard 3.5 Myr ago?

The recent detection of the live isotopes 60Fe and 244Pu in deep ocean sediments dating back to the past 3–4 Myr poses a serious challenge to the identification of their production site(s). While 60Fe is usually attributed to standard core-collapse supernovae, actinides are r-process nucleosynthesis yields, which are believed to be synthesized in rare events, such as special classes of supernovae or binary mergers involving at least one neutron star. Previous works concluded that a single binary neutron star merger cannot explain the observed isotopic ratio. In this work, we consider a set of numerical simulations of binary neutron star mergers producing long-lived massive remnants expelling both dynamical and spiral-wave wind ejecta. The latter, due to a stronger neutrino irradiation, also produce iron-group elements. Assuming that large-scale mixing is inefficient before the fading of the kilonova remnant and that the spiral-wave wind is sustained over a 100–200 ms timescale, the ejecta emitted at mid-high latitudes provide a 244Pu over 60Fe ratio compatible with observations. The merger could have happened 80–150 pc away from the Earth and between 3.5 and 4.5 Myr ago. We also compute expected isotopic ratios for eight other live radioactive nuclides showing that the proposed binary neutron star merger scenario is distinguishable from other scenarios proposed in the literature.

On the Maximum Mass and Oblateness of Rotating Neutron Stars with Generic Equations of State

A considerable effort has been dedicated recently to the construction of generic equations of state (EOSs) for matter in neutron stars. The advantage of these approaches is that they can provide model-independent information on the interior structure and global properties of neutron stars. Making use of more than 106 generic EOSs, we assess the validity of quasi-universal relations of neutron-star properties for a broad range of rotation rates, from slow rotation up to the mass-shedding limit. In this way, we are able to determine with unprecedented accuracy the quasi-universal maximum-mass ratio between rotating and nonrotating stars and reveal the existence of a new relation for the surface oblateness, i.e., the ratio between the polar and equatorial proper radii. We discuss the impact that our findings have on the imminent detection of new binary neutron-star mergers and how they can be used to set new and more stringent limits on the maximum mass of nonrotating neutron stars, as well as to improve the modeling of the X-ray emission from the surface of rotating stars.