Aims. The mass discrepancy between the observed population of double neutron star binaries by radio pulsar observations and gravitational-wave observation requires an explanation.
Methods. Binary population synthesis calculations are performed, and their results are compared with the radio and the gravitational-wave observations simultaneously.
Results. Simulations of binary evolution are used to link different observations of double neutron star binaries with each other. The progenitor of GW190425 is investigated in more detail. A distribution of masses and merger times of the possible progenitors is presented.
Conclusions. A mass discrepancy between the radio pulsars in the Milky Way with another neutron star companion and the inferred masses from gravitational-wave observations of those kind of merging systems is naturally found in binary evolution.
Although the multimessenger detection of the neutron star merger event GW170817 confirmed that mergers are promising sites producing the majority of nature's heavy elements via the rapid neutron-capture process ($r$-process), a number of issues related to the production of translead nuclei -- the actinides -- remain to be answered. In this short review paper, we summarize the general requirements for actinide production in $r$-process and the impact of nuclear physics inputs. We also discuss recent efforts addressing the actinide production in neutron star mergers from different perspectives, …
The oscillations of a merger remnant forming after the coalescence of two neutron stars are very characteristic for the high-density equation of state. The dominant oscillation frequency occurs as a pronounced peak in the kHz range of the gravitational-wave spectrum. We describe how the dominant oscillation frequency of the remnant can be employed to infer the radii of non-rotating neutron stars.
Half of all the elements in the universe heavier than iron were created by rapid neutron capture. The theory for this astrophysical `$r$-process' was worked out six decades ago and requires an enormous neutron flux to make the bulk of these elements. Where this happens is still debated. A key piece of missing evidence is the identification of freshly-synthesised $r$-process elements in an astrophysical site. Current models and circumstantial evidence point to neutron star mergers as a probable $r$-process site, with the optical/infrared `kilonova' emerging in the days after the merger a likely…
We demonstrate by a large set of merger simulations for symmetric binary neutron stars (NSs) that there is a tight correlation between the frequency peak of the postmerger gravitational-wave (GW) emission and the physical properties of the nuclear equation of state (EoS), e.g. expressed by the radius of the maximum-mass Tolman-Oppenheimer-Volkhoff configuration. Therefore, a single measurement of the peak frequency of the postmerger GW signal will constrain the NS EoS significantly. For plausible optimistic merger-rate estimates a corresponding detection with Advanced LIGO is likely to happen …
A supra-massive neutron star (NS) spinning extremely rapidly could survive from a merger of NS-NS binary. The spin-down of this remnant NS that is highly magnetized would power the isotropic merger ejecta to produce a bright mergernova emission in ultraviolet/optical bands. Before the mergernova, the early interaction between the NS wind and the ejecta can drive a forward shock propagating outwards into the ejecta. As a result, a remarkable amount of heat can be accumulated behind the shock front and the final escaping of this heat can produce a shock breakout emission. We describe the dynamic…
A massive millisecond magnetar may survive a merger of a neutron star (NS) binary, which would continuously power the merger ejecta. We develop a generic dynamic model for the merger ejecta with energy injection from the central magnetar. The ejecta emission (the "merger-nova") powered by the magnetar peaks in the UV band and the peak of lightcurve progressively shifts to an earlier epoch with increasing frequency. A magnetar-powered mergernova could have an optical peak brightness comparable to a supernova, which is a few tens or hundreds times brighter than the radioactive-powered merger-nov…
The maximum offsets of binary neutron star mergers from host galaxies
Monash University（蒙纳士大学）；The ARC Centre of Excellence for Gravitational Wave Discovery – OzGrav（ARC引力波发现卓越中心 – OzGrav）；INAF – Osservatorio Astronomico di Brera（意大利国家天体物理研究所 – 布雷拉天文台）；INFN – sezione di Milano-Bicocca（意大利国家核物理研究所 – 米兰-比可卡分部）；Department of Astrophysics/IMAPP, Radboud University（拉德堡德大学天体物理学系/IMAPP）；Department of Physics, University of Warwick（华威大学物理系）
The existence of neutron star mergers has been supported since the discovery of the binary pulsar and the observation of its orbital energy loss, consistent with General Relativity. They are considered nucleosynthesis sites of the rapid neutron-capture process (r-process), which is responsible for creating approximately half of all heavy elements beyond Fe and is the only source of elements beyond Pb and Bi. Detailed nucleosynthesis calculations based on the decompression of neutron star matter are consistent with solar r-process abundances of heavy nuclei. Neutron star mergers have also been …
The coalescence of a neutron-star binary is likely to result in the formation of a neutron-star merger remnant for a large range of binary mass configurations. The massive merger remnant shows strong oscillations, which are excited by the merging process, and emits gravitational waves. Here we discuss possibilities and prospects of inferring unknown stellar properties of neutron stars by the detection of postmerger gravitational-wave emission, which thus leads to constraints of the equation of state of high-density matter. In particular, the dominant oscillation frequency of the postmerger rem…
Although the main features of the evolution of binary neutron star systems are now well established, many details are still subject to debate, especially regarding the post-merger phase. In particular, the lifetime of the hyper-massive neutron stars formed after the merger is very hard to predict. In this work, we provide a simple analytic relation for the lifetime of the merger remnant as function of the initial mass of the neutron stars. This relation results from a joint fit of data from observational evidence and from various numerical simulations. In this way, a large range of collapse ti…
We describe the first observations of the same celestial object with gravitational waves and light. * GW170817 was the first detection of a neutron star merger with gravitational waves. * The detection of a spatially coincident weak burst of $γ$-rays (GRB 170817A) 1.7 s after the merger constituted the first electromagnetic detection of a gravitational wave source and established a connection between at least some cosmic short gamma-ray bursts (SGRBs) and binary neutron star mergers. * A fast-evolving optical and near-infrared transient (AT 2017gfo) associated with the event can be interpreted…
Using general-relativistic hydrodynamical simulations, we show that merging binary neutron stars can form hypermassive neutrons stars that undergo the one-arm spiral instability. We study the particular case of a dynamical capture merger where the stars have a small spin, as may arise in globular clusters, and focus on an equal-mass scenario where the spins are aligned with the orbital angular momentum. We find that this instability develops when post-merger fluid vortices lead to the generation of a toroidal remnant - a configuration whose maximum density occurs in a ring around the center-of…
We perform high-resolution magnetohydrodynamics simulations of binary neutron star mergers in numerical relativity on the Japanese supercomputer K. The neutron stars and merger remnants are covered by a grid spacing of 70\,m, which yields the highest-resolution results among those derived so far. By an in-depth resolution study, we clarify several amplification mechanisms of magnetic fields during the binary neutron star merger for the first time. First, the Kelvin-Helmholtz instability developed in the shear layer at the onset of the merger significantly amplifies the magnetic fields. A hyper…
The rapid-neutron-capture ("r") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far fail to account for the observed over-enhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger and explore if varying levels of neutron richness can reproduce the actinide boost. We also in…
GW200115 was the second merger of a black hole and a neutron star confidently detected through gravitational waves. Inference on the signal allows for a large black hole spin misaligned with the orbital angular momentum, but shows little support for aligned spin values. We show that this is a natural consequence of measuring the parameters of a black hole -- neutron star binary with non-spinning components while assuming the priors used in the LIGO-Virgo-KAGRA analysis. We suggest that, a priori, a non-spinning binary is more consistent with current astrophysical understanding.
The connection between short gamma-ray bursts (SGRBs) and binary neutron star (BNS) mergers was recently confirmed by the association of GRB 170817A with the merger event GW170817. However, no conclusive indications were obtained on whether the merger remnant that powered the SGRB jet was an accreting black hole (BH) or a long-lived massive neutron star (NS). Here, we explore the latter case via BNS merger simulations covering up to 250 ms after merger. We report, for the first time in a full merger simulation, the formation of a magnetically-driven collimated outflow along the spin axis of th…
Employing a simplified version of the Israel-Stewart formalism of general-relativistic shear-viscous hydrodynamics, we explore the evolution of a remnant massive neutron star of binary neutron star merger and pay special attention to the resulting gravitational waveforms. We find that for the plausible values of the so-called viscous alpha parameter of the order $10^{-2}$, the degree of the differential rotation in the remnant massive neutron star is significantly reduced in the viscous timescale, $\alt 5$\,ms. Associated with this, the degree of non-axisymmetric deformation is also reduced qu…
Comparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if at low metallicities such mergers have very short delay times and significant ejecta masses that are facilitated by the masses of the compact objects. Namely, black hole-neutron star mergers, depending on the black-hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this paper, i.e., COMPAS, StarTrack, Brussels, C…
We perform hydrodynamical simulations of neutron-star mergers for a large sample of temperature-dependent, nuclear equations of state, and determine the threshold mass above which the merger remnant promptly collapses to form a black hole. We find that, depending on the equation of state, the threshold mass is larger than the maximum mass of a non-rotating star in isolation by between 30 and 70 per cent. Our simulations also show that the ratio between the threshold mass and maximum mass is tightly correlated with the compactness of the non-rotating maximum-mass configuration. We speculate on …
Conformally flat, quasi-circular numerical simulations of the gravitational wave chirp from binary neutron star merger GW170817
Center for Astrophysics, Department of Physics, University of Notre Dame（圣母大学天体物理中心物理系）；Center for Research Computing, University of Notre Dame（圣母大学计算研究中心）
The mergers of binaries containing neutron stars and stellar-mass black holes are the most promising sources for direct detection in gravitational waves by the interferometers Advanced LIGO and Virgo over the next few years. The concurrent detection of electromagnetic emission from these events would greatly enhance the scientific return of these discoveries. Here we review the state of the art in modeling the electromagnetic signal of neutron star binary mergers across different phases of the merger and multiple wavelengths. We focus on those observables which provide the most sensitive diagn…
We introduce a classification scheme of the post-merger dynamics and gravitational-wave emission in binary neutron star mergers, after identifying a new mechanism by which a secondary peak in the gravitational-wave spectrum is produced. It is caused by a spiral deformation, the pattern of which rotates slower with respect to the double-core structure in center of the remnant. This secondary peak is typically well separated in frequency from the secondary peak produced by a nonlinear interaction between a quadrupole and a quasi-radial oscillation. The new mechanism allows for an explanation of …
Neutron star mergers are believed to occur in accretion disks around supermassive black holes. Here we show that a putative jet launched from the merger of a binary neutron star (BNS) or a neutron star--black hole (NSBH) merger occurring at the migration trap in an active galactic nucleus (AGN) disk would be choked. The jet energy is deposited within the disk materials to power a hot cocoon. The cocoon is energetic enough to break out from the AGN disk and produce a bright X-ray shock breakout transient peaking at $\sim0.15\,{\rm d}$ after the merger. The peak luminosity is estimated as $\sim …
We perform a general-relativistic magnetohydrodynamics simulation for $\approx 30$ ms after merger of a binary neutron star to a remnant massive neutron star (RMNS) with a high spatial resolution of the finest grid resolution $12.5$ m. First, we estimate that the Kelvin-Helmholtz instability at merger could amplify the magnetic-field energy up to $\sim 1\%$ of the thermal energy. Second, we find that the magnetorotational instability in the RMNS envelope and torus with $ρ< 10^{13}~{\rm g~cm^{-3}}$ sustains magneto-turbulent state and the effective viscous parameter in these regions is likely t…
The first combined detection of gravitational waves and electromagnetic signals from a binary neutron star (BNS) merger in August 2017 (event named GW170817) represents a major landmark for the ongoing investigation on these extraordinary systems. In this short review, we introduce BNS mergers as events of the utmost importance for astrophysics and fundamental physics and discuss the main discoveries enabled by this first multimessenger observation, which include compelling evidence that such mergers produce a copious amount of heavy r-process elements and can power short gamma-ray bursts. We …
The two neutron star-black hole mergers (GW200105 and GW200115) observed in gravitational waves by advanced LIGO and Virgo, mark the first ever discovery of such binaries in nature. We study these two neutron star-black hole systems through isolated binary evolution, using a grid of population synthesis models. Using both mass and spin observations (chirp mass, effective spin and remnant spin) of the binaries, we probe their different possible formation channels in different metallicity environments. Our models only support LIGO data when assuming the black hole is non spinning. Our results sh…
In this paper, we discuss the damping of density oscillations in dense nuclear matter in the temperature range relevant to neutron star mergers. This damping is due to bulk viscosity arising from the weak interaction ``Urca'' processes of neutron decay and electron capture. The nuclear matter is modelled in the relativistic density functional approach. The bulk viscosity reaches a resonant maximum close to the neutrino trapping temperature, then drops rapidly as temperature rises into the range where neutrinos are trapped in neutron stars. We investigate the bulk viscous dissipation timescales…
A compact accretion disk may be formed in the merger of two neutron stars or of a neutron star and a stellar-mass black hole. Outflows from such accretion disks have been identified as a major site of rapid neutron-capture (r-process) nucleosynthesis and as the source of 'red' kilonova emission following the first observed neutron-star merger GW170817. We present long-term general-relativistic radiation magnetohydrodynamic simulations of a typical post-merger accretion disk at initial accretion rates of $\dot{M}\sim 1\,M_\odot\,\text{s}^{-1}$ over 400ms post-merger. We include neutrino radiati…
We study equal and unequal-mass neutron star mergers by means of new numerical relativity simulations in which the general relativistic hydrodynamics solver employs an algorithm that guarantees mass conservation across the refinement levels of the computational mesh. We consider eight binary configurations with total mass $M=2.7\,M_\odot$, mass-ratios $q=1$ and $q=1.16$, and four different equation of states (EOSs), and one configuration with a stiff EOS, $M=2.5M_\odot$ and $q=1.5$. We focus on the post-merger dynamics and study the merger remnant, dynamical ejecta and the postmerger gravitati…
Neutron tunneling between neutron-rich nuclei in inhomogeneous dense matter encountered in neutron star crusts can release enormous energy on a short-timescale to power explosive phenomena in neutron stars. In this work we clarify aspects of this process that can occur in the outer regions of neutron stars when oscillations or cataclysmic events increase the ambient density. We use a time-dependent Hartree-Fock-Bogoliubov formalism to determine the rate of neutron diffusion and find that large amounts of energy can be released rapidly. The role of nuclear binding, the two-body interaction and …
We explore magnetic-field amplification due to the Kelvin-Helmholtz instability during binary neutron star mergers. By performing high-resolution general relativistic magnetohydrodynamics simulations with a resolution of $17.5$ m for $4$--$5$ ms after the onset of the merger on the Japanese supercomputer "K", we find that an initial magnetic field of moderate maximum strength $10^{13}$ G is amplified at least by a factor of $\approx 10^3$. We also explore the saturation of the magnetic-field energy and our result shows that it is likely to be $\gtrsim 4 \times 10^{50}$ erg, which is $\gtrsim 0…
