We investigate the significance of isomers on a kilonova associated with neutron star mergers (NSMs) for the first time. We calculate the evolution of abundances and energy generation rates ($\dotε_{\rm nuc}$) of ejecta from NSMs, taking into account $β^-$ decay through isomers. We find that for ejecta with electron fraction ($Y_e$) of $0.2-0.3$, $\dotε_{\rm nuc}$ is appreciably changed from those without isomers, due to the large change in timing of $β^-$ decay through the isomeric states of ${}^{123, 125, 127} \rm Sn$ and ${}^{128} \rm Sb$. In particular, the effects of the isomers on $\dotε_{\rm nuc}$ are prominent for ejecta of $Y_e \sim 0.25$, which could emit a fraction of an early, blue component of a kilonova observed in GW170817. When the excitation from a ground state to its isomeric state is important, the isomers of ${}^{129} \rm Sb$ and ${}^{129, 131} \rm Te$ also cause additional and appreciable change in $\dotε_{\rm nuc}$. Furthermore, we demonstrate that larger amounts of lanthanide-free ejecta result in a better fit of an observed light curve of the kilonova in GW170817 if the isomers are taken into account.
We review current understanding of kilonova/macronova emission from compact binary mergers (mergers of two neutron stars or a neutron star and a black hole). Kilonova/macronova is optical and near-infrared emission powered by radioactive decays of r-process nuclei. Emission from the dynamical ejecta with ~0.01 Msun is likely to have a luminosity of ~10^{40}-10^{41} erg s^{-1} with a characteristic timescale of about 1 week. The spectral peak is located in red optical or near-infrared wavelengths. A subsequent accretion disk wind may provide an additional luminosity, or an earlier/bluer emissio…
We identify new astrophysical signatures of dark matter that implodes neutron stars (NSs), which could decisively test whether NS-imploding dark matter is responsible for missing pulsars in the Milky Way galactic center, the source of some $r$-process elements, and the origin of fast-radio bursts. First, NS-imploding dark matter forms $\sim 10^{-10}$ solar mass or smaller black holes inside neutron stars, which proceed to convert neutron stars into $\sim$1.5 solar mass BHs. This decreases the number of neutron star mergers seen by LIGO/Virgo (LV) and associated merger kilonovae seen by telesco…
The gravitational wave event GW170817 associated with the short gamma-ray burst (GRB) 170817A confirms that binary neutron star (BNS) mergers are one of the origins of short GRBs. The associated kilonova emission, radioactively powered by nucleosynthesized heavy elements, was also detected. Black hole-neutron star (BH-NS) mergers have been argued to be another promising origin candidate of short GRBs and kilonovae. Numerical simulations show that the ejecta in BH-NS mergers is geometrically much more anisotropic than the BNS merger case. In this paper, we investigate observational signatures o…
Outflows launched from a remnant disc of compact binary merger may have essential contribution to the kilonova emission. Numerical calculations are conducted in this work to study the structure of accretion flows and outflows. By the incorporation of limited-energy advection in the hyper-accretion discs, outflows occur naturally from accretion flows due to imbalance between the viscous heating and the sum of the advective and radiative cooling. Following this spirit, we revisit the properties of the merger outflow ejecta. Our results show that around $10^{-3} \sim 10^{-1} M_\odot$ of the disc …
The observation of a radioactively-powered kilonova associated with the first binary neutron star (BNS) merger detected in gravitational waves proved that these events are ideal sites for the production of heavy r-process elements. However, the physical origin of the ejected material responsible for the early ("blue") and late ("red") components of this kilonova is still debated. Here, we investigate the possibility that the early/blue kilonova originated from the magnetically driven baryon wind launched after merger by the metastable neutron star remnant. Exploiting a magnetized BNS merger si…
In the first work of this series, we adopt a GW170817-like, viewing-angle-dependent kilonova model and the standard afterglow model with the lightcurve distribution based on the properties of cosmological short gamma-ray bursts afterglows to simulate the luminosity functions and color evolution of both kilonovae and optical afterglow emissions from binary neutron star (BNS) mergers. {We find that $\sim10\%$ afterglows are brighter than the associated kilonovae at the peak time, most of which are on-axis or nearly-on-axis. These kilonovae would be significantly polluted by the associated afterg…
One of the most promising electromagnetic signatures of compact object mergers are kilonovae: approximately isotropic radioactively-powered transients that peak days to weeks post-merger. Key uncertainties in modeling kilonovae include the emission profiles of the radioactive decay products---non-thermal beta- and alpha-particles, fission fragments, and gamma-rays---and the efficiency with which they deposit their energy in the ejecta. The total radioactive energy and the efficiency of its thermalization sets the luminosity budget and is therefore necessary for predicting kilonova light curves…
Here we collect three unique bursts, GRBs\,060614, 211211A and 211227A, all characterized by a long-duration main emission (ME) phase and a rebrightening extended emission (EE) phase, to study their observed properties and the potential origin as neutron star-black hole (NSBH) mergers. NS-first-born (BH-first-born) NSBH mergers tend to contain fast-spinning (non-spinning) BHs that more easily (hardly) allow tidal disruption to happen with (without) forming electromagnetic signals. We find that NS-first-born NSBH mergers can well interpret the origins of these three GRBs, supported by that: (1)…
The neutron star merger, GW170817, was followed by an optical-infrared transient (a kilonova) which indicated that a substantial ejection of mass at trans-relativistic velocities occurred during the merger. Modeling of the kilonova is able to constrain the kinetic energy of the ejecta and its characteristic velocity, but not the high-velocity distribution of the ejecta. Yet, this distribution contains crucial information on the merger dynamics. In this work, we assume a power-law distribution of the form $E(>βΓ)\propto(βΓ)^{-α}$ for the energy of the kilonova ejecta and calculate the non-therm…
The Wide-Field Infrared Transient Explorer (WINTER) is a new 1 $\text{deg}^2$ seeing-limited time-domain survey instrument designed for dedicated near-infrared follow-up of kilonovae from binary neutron star (BNS) and neutron star-black hole mergers. WINTER will observe in the near-infrared Y, J, and short-H bands (0.9-1.7 microns, to $\text{J}_{AB}=21$ magnitudes) on a dedicated 1-meter telescope at Palomar Observatory. To date, most prompt kilonova follow-up has been in optical wavelengths; however, near-infrared emission fades more slowly and depends less on geometry and viewing angle than …
We develop a method to compute synthetic kilonova light curves that combines numerical relativity simulations of neutron star mergers and the $\texttt{SNEC}$ radiation-hydrodynamics code. We describe our implementation of initial and boundary conditions, r-process heating, and opacities for kilonova simulations. We validate our approach by carefully checking that energy conservation is satisfied and by comparing the $\texttt{SNEC}$ results with those of two semi-analytic light curve models. We apply our code to the calculation of color light curves for three binaries having different mass rati…
A binary neutron star merger is expected to be associated by a kilonova, transient optical emission powered by radioactive decay of the neutron-rich ejecta. If the post-merger remnant is a long-lived neutron star, additional energy injection to the ejecta is possible. In this first paper of a series, we study the dynamical evolution of the engine-fed kilonova (mergernova) ejecta in detail. We perform a semi-analytical study of the problem by adopting a modified mechanical blastwave model that invokes interaction between a Poynting-flux-dominated flow and a non-magnetized massive ejecta. Shortl…
In this paper, we present a numerical method to study the predicted lightcurves as a function of viewing angle. We extrapolate the fitting formulae for the mass and velocity of tidal dynamical ejecta across a wide range of mass ratio validated with 66 simulations and use them in the calculations of the kilonova lightcurves. The calculated peak luminosity of a BH-NS merger kilonova is typically about a few times $10^{41}\ {\rm erg\ s^{-1}}$, which is always $\lesssim4.5\times10^{41}\ {\rm erg\ s^{-1}}$. This corresponds to the AB absolute magnitudes fainter than $\sim -15\ {\rm mag}$ in optical…
The kilonova associated with the neutron star merger GW170817 provides us with several hints to elucidate the nature of the r-process in the universe. In this article, we inspect the radioactive isotopes that powered the kilonova emission, provided that the merger ejecta consisted of material with a solar r-like abundance pattern. It is suggested that the early (1-10 days after merger) kilonova emission is mainly due to the beta-decay chain 66Ni -> 66Cu -> 66Zn, which can be the source of the steepening of the light curve at about 7 days. The late time (> 10 days) heaing is attributed to the a…
The binary neutron star merger gravitational-wave signal GW170817 was followed by three electromagnetic counterparts, including a kilonova arising from the radioactivity of freshly synthesized $r$-process elements in ejecta from the merger. Finding kilonovae after gravitational-wave triggers is crucial for (i) the search for further counterparts, such as the afterglow, (ii) probing the diversity of kilonovae and their dependence on the system's inclination angle, and (iii) building a sample for multi-messenger cosmology. During the third observing run of the gravitational-wave interferometer n…
We study the radioactively-powered transients produced by accretion disk winds following a compact object merger. Starting with the outflows generated in two-dimensional hydrodynamical disk models, we use wavelength-dependent radiative transfer calculations to generate synthetic light curves and spectra. We show that the brightness and color of the resulting kilonova transients carry information about the merger physics. In the regions of the wind where neutrino irradiation raises the electron fraction to Ye > 0.25, r-process nucleosynthesis halts before producing high-opacity, complex ions (t…
The recent discovery of the neutron star merger and multi-messenger event GW170817 (also known as kilonova GRB170817A) provides an unprecedented laboratory in which to study these mysterious objects, as well as an opportunity to test cutting-edge theories of gravity in the strong field regime. Before this event, such tests of our understanding of the nature of gravity were not possible. In this study, we analyze the X-ray observations of GW170817 obtained with NASA's Chandra X-ray Observatory following the 17 August 2017 detection of the event by the Laser Interferometer Gravitational Wave Obs…
We present detailed simulations of black hole-neutron star (BH-NS) mergers kilonova and gamma-ray burst (GRB) afterglow and kilonova luminosity function, and discuss the detectability of electromagnetic (EM) counterpart in connection with gravitational wave (GW) detections, GW-triggered target-of-opportunity observations, and time-domain blind searches. The predicted absolute magnitude of the BH-NS kilonovae at $0.5\,{\rm days}$ after the merger falls in $[-10,-15.5]$. The simulated luminosity function contains the potential viewing-angle distribution information of the anisotropic kilonova em…
Here, we report the discovery of a kilonova associated with the nearby (350 Mpc) minute-duration GRB 211211A. In tandem with deep optical limits that rule out the presence of an accompanying supernova to $M_I > -13$ mag at 17.7 days post-burst, the identification of a kilonova confirms that this burst's progenitor was a compact object merger. While the spectrally softer tail in GRB 211211A's gamma-ray light curve is reminiscent of previous extended emission short GRBs (EE-SGRBs), its prompt, bright spikes last $\gtrsim 12$ s, separating it from past EE-SGRBs. GRB 211211A's kilonova has a simil…
Black hole-neutron star mergers are among the promising gravitational-wave sources for ground-based detectors, and gravitational waves from black hole-neutron star mergers are expected to be detected in the next few years. Simultaneous detection of electromagnetic counterparts with gravitational-wave detection provides rich information about the merger events. Among the possible electromagnetic counterparts from the black hole-neutron star merger, the emission powered by the decay of radioactive r-process nuclei, so called kilonova/macronova, is one of the best targets for follow-up observatio…
Multimessenger observations of the neutron star merger GW170817 and its kilonova proved that neutron star mergers can synthesize large quantities of r-process elements. If neutron star mergers in fact dominate all r-process element production, then the distribution of kilonova ejecta compositions should match the distribution of r-process abundance patterns observed in stars. The lanthanide fraction (XLa) is a measurable quantity in both kilonovae and metal-poor stars, but it has not previously been explicitly calculated for stars. Here, we compute the lanthanide fraction distribution of metal…
Ejected material from neutron star mergers give rise to electromagnetic emission powered by radioactive decays of r-process nuclei, which is so called kilonova or macronova. While properties of the emission are largely affected by opacities in the ejected material, available atomic data for r-process elements are still limited. We perform atomic structure calculations for r-process elements: Se (Z=34), Ru (Z=44), Te (Z=52), Ba (Z=56), Nd (Z=60), and Er (Z=68). We confirm that the opacities from bound-bound transitions of open f-shell, Lanthanide elements (Nd and Er) are higher than those of th…
The merger of two neutron stars has been predicted to produce an optical-infrared transient (lasting a few days) known as a 'kilonova', powered by the radioactive decay of neutron-rich species synthesized in the merger. Evidence that short gamma-ray bursts also arise from neutron-star mergers has been accumulating. In models of such mergers a small amount of mass ($10^{-4}$-$10^{-2}$ solar masses) with a low electron fraction is ejected at high velocities (0.1-0.3 times light speed) and/or carried out by winds from an accretion disk formed around the newly merged object. This mass is expected …
We present a comprehensive analysis of 653 optical candidate counterparts reported during the third gravitational wave (GW) observing run. Our sample concentrates on candidates from the 15 events (published in GWTC-2, GWTC-3 or not retracted on GraceDB) that had a >1% chance of including a neutron star in order to assess their viability as true kilonovae. In particular, we leverage tools available in real time, including pre-merger detections and cross-matching with catalogs (i.e. point source, variable star, quasar and host galaxy redshift datasets), to eliminate 65% of candidates in our samp…
We investigate the ejecta from black hole - neutron star mergers by modeling the formation and interaction of mass ejected in a tidal tail and a disk wind. The outflows are neutron-rich, giving rise to optical/infrared emission powered by the radioactive decay of $r$-process elements (a kilonova). Here we perform an end-to-end study of this phenomenon, where we start from the output of a fully-relativistic merger simulation, calculate the post-merger hydrodynamical evolution of the ejecta and disk winds including neutrino physics, determine the final nucleosynthetic yields using post-processin…
The recent discovery of a faint gamma-ray burst (GRB) coincident with the gravitational wave (GW) event GW 170817 revealed the existence of a population of low-luminosity short duration gamma-ray transients produced by neutron star mergers in the nearby Universe. These events could be routinely detected by existing gamma-ray monitors, yet previous observations failed to identify them without the aid of GW triggers. Here we show that GRB150101B was an analogue of GRB170817A located at a cosmological distance. GRB 150101B was a faint short duration GRB characterized by a bright optical counterpa…
We investigate the influence of parametric magnetic field configurations of a hypermassive neutron star (HMNS) on electromagnetic (EM) observables, specifically the kilonova lightcurves and nucleosynthesis yields. We perform three-dimensional (3D) dynamical-spacetime general-relativistic magnetohydrodynamic (GRMHD) simulations, including a neutrino leakage scheme, microphysical finite-temperature equation of state (EOS), and an initial poloidal magnetic field. We find that varying the magnetic field strength and falloff impacts the formation of magnetized winds or mildy-relativistic jets, whic…
With the epochal first detection of gravitational waves from a binary neutron star (NS) merger with the GW170817 event, and its direct confirmation that NS-NS mergers are significant sources of the of the r-process nucleosynthesis of heavy elements, an immense new arena for prompt EM (X-rays through IR and radio) studies of fundamental physics has been opened. Over the next decade, GW observatories will expand in scale and sensitivity so the need for facilities that can provide prompt, high sensitivity, broad-band EM followup becomes more urgent. NS-NS or NS-black hole (BH) mergers will be ins…
The majority of existing results for the kilonova (or macronova) emission from material ejected during a neutron-star (NS) merger is based on (quasi-)one-zone models or manually constructed toy-model ejecta configurations. In this study we present a kilonova analysis of the material ejected during the first ~10ms of a NS merger, called dynamical ejecta, using directly the outflow trajectories from general relativistic smoothed-particle hydrodynamics simulations including a sophisticated neutrino treatment and the corresponding nucleosynthesis results, which have been presented in Part I of thi…
Recent detection of gravitational waves from a neutron star (NS) merger event GW170817 and identification of an electromagnetic counterpart provide a unique opportunity to study the physical processes in NS mergers. To derive properties of ejected material from the NS merger, we perform radiative transfer simulations of kilonova, optical and near-infrared emissions powered by radioactive decays of r-process nuclei synthesized in the merger. We find that the observed near-infrared emission lasting for > 10 days is explained by 0.03 Msun of ejecta containing lanthanide elements. However, the blu…
The mergers of binary neutron stars, as well as black hole-neutron star systems, are expected to produce an electromagnetic counterpart that can be analyzed to infer the element synthesis that occurred in these events. We investigate one source of uncertainties pertinent to lanthanide-rich outflows: the nuclear inputs to rapid neutron capture nucleosynthesis calculations. We begin by examining thirty-two different combinations of nuclear inputs: eight mass models, two types of spontaneous fission rates, and two types of fission daughter product distributions. We find that such nuclear physics …
Impact of in situ nuclear networks and atomic opacities on neutron star merger ejecta dynamics, nucleosynthesis, and kilonovae
Friedrich-Schiller-Universität Jena（耶拿弗里德里希·席勒大学）；Università di Trento（特伦托大学）；INFN-TIFPA, Trento Institute for Fundamental Physics and Applications（意大利国家核物理研究所-特伦托基础物理与应用研究所）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）
Kilonovae are cosmic optical flashes produced in the aftermath of the merger of two neutron stars. While the typical radiant flux of a kilonova can be as high as $10^{34}$\,W, they typically occur at cosmological distances, requiring meter-class or larger telescopes for their observation. Here we explore how a kilonova would look like from Earth if it occurred in the Solar System's backyard, 1000 light years from Earth. This is a small distance on cosmological scales, with only one neutron-star merger expected to occur this close every 100,000,000 years. While humans will likely only see kilon…
The mergers of neutron stars expel a heavy-element enriched fireball which can be observed as a kilonova. The kilonova's geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta. Previously, Sr$^+$ was identified in the spectrum of the the only well-studied kilonova AT2017gfo, associated with the gravitational wave event GW170817. Here we combine the strong Sr$^+$ P Cygni absorption-emission spectral feature and the blackbody nature…
Mergers of binary neutron stars (NSs) usually result in the formation of a hypermassive neutron star (HMNS). Whether- and when this remnant collapses to a black hole (BH) depends primarily on the equation of state and on angular momentum transport processes, both of which are uncertain. Here we show that the lifetime of the merger remnant may be directly imprinted in the radioactively powered kilonova emission following the merger. We employ axisymmetric, time-dependent hydrodynamic simulations of remnant accretion disks orbiting a HMNS of variable lifetime, and characterize the effect of this…
The mergers of double neutron star (NS-NS) and black hole (BH)-NS binaries are promising gravitational wave (GW) sources for Advanced LIGO and future GW detectors. The neutron-rich ejecta from such merger events undergoes rapid neutron capture (r-process) nucleosynthesis, enriching our Galaxy with rare heavy elements like gold and platinum. The radioactive decay of these unstable nuclei also powers a rapidly evolving, supernova-like transient known as a "kilonova". Kilonovae provide an approximately isotropic electromagnetic counterpart to the GW signal, which also provides a unique and direct…
We predict linear polarization for a radioactively-powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3-D Monte Carlo radiative transfer simulations for two different models, both featuring a lanthanide-rich dynamical ejecta component from numerical-relativity simulations while only one including an additional lanthanide-free disk wind component. We calculate polarization spectra for nine different orientations at 1.5, 2.5 and 3.5 d after the merger and in the $0.1-2\,μ$m wavelength range. We find that both models are polarized at a detectable le…
We present new numerical relativity results of neutron star mergers with chirp mass $1.188M_\odot$ and mass ratios $q=1.67$ and $q=1.8$ using finite-temperature equations of state (EOS), approximate neutrino transport and a subgrid model for magnetohydrodynamics-induced turbulent viscosity. The EOS are compatible with nuclear and astrophysical constraints and include a new microphysical model derived from ab-initio calculations based on the Brueckner-Hartree-Fock approach. We report for the first time evidence for accretion-induced prompt collapse in high-mass-ratio mergers, in which the tidal…
