Ejecta Of Neutron Star Mergers Research Articles

Non-thermal emission from mildly relativistic dynamical ejecta of neutron star mergers: spectrum and sky image

Abstract Binary neutron star mergers are expected to produce fast dynamical ejecta, with mildly relativistic velocities extending to β = v/c > 0.6. In a preceding paper, we derived an analytic description of the time-dependent radio to X-ray synchrotron flux produced by collisionless shocks driven by such fast ejecta into the interstellar medium, for spherical ejecta with broken power-law mass (or energy) distributions, M( > γβ)∝(γβ)−s with s = sKN at γβ < γ0β0 and s = sft at γβ > γ0β0 (where γ is the Lorentz factor). Here, we extend our analysis and provide analytic expressions for the self-absorption frequency, the cooling frequency, and the observed angular size of the emitting region (which appears as a ring in the sky). For parameter values characteristic of merger calculation results - a ”shallow” mass distribution, 1 < sKN < 3, for the bulk of the ejecta (at γβ ≈ 0.2), and a steep, sft > 5, ”fast tail” mass distribution – the analytic results reproduce well (to tens of percent accuracy) the results of detailed numeric calculations, a significant improvement over earlier order-of-magnitude estimates (based on extrapolations of results valid for γβ ≪ 1).

Enabling kilonova science with Nancy Grace Roman Space Telescope

Binary neutron star mergers and neutron star–black hole mergers are multi-messenger sources that can be detected in gravitational waves and in electromagnetic radiation. The low electron fraction of neutron star merger ejecta favors the production of heavy elements such as lanthanides and actinides via rapid neutron capture (r-process). The decay of these unstable nuclei powers an infrared-bright transient called a “kilonova”. The discovery of a population of kilonovae will allow us to determine if neutron star mergers are the dominant sites for r-process element nucleosynthesis, constrain the equation of state of nuclear matter, and make independent measurements of the Hubble constant. The Nancy Grace Roman Space Telescope (Roman) will have a unique combination of depth, near-infrared sensitivity, and wide field of view. These characteristics will enable Roman’s discovery of GW counterparts that will be missed by optical telescopes, such as kilonova that are associated with large distances, high lanthanide fractions, high binary mass-ratios, large dust extinction in the line of sight, or that are observed from equatorial viewing angles. In preparation for Roman’s launch and operations, our analysis suggests to (i) make available a rapid (∼1 week) Target of Opportunity mode for GW follow-up; (ii) include observations of the High Latitude Time-Domain survey footprint in at least two filters (preferably the F158 and F213 filters) with a cadence of ≲8 days; (iii) operate in synergy with Rubin Observatory. Following these recommendations, we expect that 1–6 kilonovae can be identified by Roman via target of opportunity observations of well localized (A<10deg2, 90% C.I.) neutron star mergers during 1.5 years of the LIGO-Virgo-KAGRA fifth (or ∼4–21 in during the sixth) observing run. A sample of 5–40 serendipitously discovered kilonovae can be collected in a 5-year high latitude survey.

The Nuclear Reaction Network WinNet

We present the state-of-the-art single-zone nuclear reaction network WinNet, which is capable of calculating the nucleosynthetic yields of a large variety of astrophysical environments and conditions. This ranges from the calculation of the primordial nucleosynthesis, where only a few nuclei are considered, to the ejecta of neutron star mergers with several thousands of involved nuclei. Here we describe the underlying physics and implementation details of the reaction network. We additionally present the numerical implementation of two different integration methods, the implicit Euler method and Gears method, along with their advantages and disadvantages. We furthermore describe basic example cases of thermodynamic conditions that we provide together with the network and demonstrate the reliability of the code by using simple test cases. With this publication, WinNet will be publicly available and open source at GitHub and Zenodo.

Particle-in-cell Simulations of Mildly Relativistic Outflows in Kilonova Emissions

The electromagnetic emission from neutron star mergers is comprised of multiple components. Synchrotron emission from the disk-powered jet and thermal emission from the merger ejecta (powered by a variety of sources) are among the most studied sources. The low masses and high velocities of the merger ejecta quickly develop conditions where emission from collisionless shocks becomes critical and synchrotron emission from the merger ejecta constitutes a third component to the observed signal. The aim of this project is to examine shock development, magnetic field generation, and particle acceleration in the case of mildly relativistic shocks, which are expected when the tidal ejecta of neutron star mergers drive a shock into the external medium. Using LANL’s vector particle-in-cell (VPIC) code, we have run simulations of such mildly relativistic, collisionless, weakly magnetized plasmas and computed the resultant magnetic fields and particle energy spectra. We show the effects of varying plasma conditions, as well as explore the validity of using different proton-to-electron mass ratios in VPIC. Our results have implications for observing late-time electromagnetic counterparts to gravitational wave detections of neutron star mergers.

Cocoon cooling emission in neutron star mergers

ABSTRACT In the gravitational wave event GW170817, there was a ∼10 h gap before electromagnetic (EM) observations, without detection of the cocoon. The cocoon is heated by a short gamma-ray burst (sGRB) jet propagating through the ejecta of a neutron star (NS) merger, and a part of the cocoon escapes the ejecta with an opening angle of 20°–30°. Here, we model the cocoon and calculate its EM emission. Our 2D hydrodynamic simulations suggest that the density and energy distributions, after entering homologous expansion, are well-fitted with power-law functions, in each of the relativistic and non-relativistic parts of the escaped cocoon. Modelling these features, we calculate the cooling emission analytically. We find that the cocoon outshines the r-process kilonova/macronova at early times (10–103 s), peaking at UV bands. The relativistic velocity of the cocoon’s photosphere is measurable with instruments such as Swift, ULTRASAT, and LSST. We also imply that energetic cocoons, including failed jets, might be detected as X-ray flashes. Our model clarifies the physics and parameter dependence, covering a wide variety of central engines and ejecta of NS mergers and sGRBs in the multimessenger era.

High-energy Neutrino Emission Associated with Gravitational-wave Signals: Effects of Cocoon Photons and Constraints on Late-time Emission

We investigate prospects for the detection of high-energy neutrinos produced in the prolonged jets of short gamma-ray bursts (sGRBs). The X-ray light curves of sGRBs show extended emission components lasting for 100–1000 s, which are considered to be produced by prolonged engine activity. Jets produced by such activity should interact with photons in the cocoon formed by the propagation of the jet inside the ejecta of neutron star mergers. We calculate neutrino emission from jets produced by prolonged engine activity, taking account of the interaction between photons provided from the cocoon and cosmic rays accelerated in the jets. We find that IceCube-Gen2, a future neutrino telescope, with second-generation gravitational-wave detectors will probably be able to observe neutrino signals associated with gravitational waves with around 10 years of operation, regardless of the assumed value of the Lorentz factor of the jets. Neutrino observations may enable us to constrain the dissipation region of the jets. We apply this model to GRB 211211A, a peculiar long GRB whose origin may be a binary neutron star merger. Our model predicts that IceCube is unlikely to detect any associated neutrinos, but a few similar events will be able to put a meaningful constraint on the physical quantities of the prolonged engine activities.

Cocoon breakout and escape from the ejecta of neutron star mergers

ABSTRACT The cocoon is an inevitable product of a jet propagating through ambient matter, and takes a fair fraction of the jet energy. In short gamma-ray bursts (sGRBs), the ambient matter is the ejecta from the merger of neutron stars, expanding with a high velocity ∼0.2c, in contrast to the static stellar envelope in collapsars. Using 2D relativistic hydrodynamic simulations with the ejecta density profile as ρ ∝ r−2, we find that the expansion makes a big difference; only 0.5–5 per cent of the cocoon mass escapes from (faster than) the ejecta, with an opening angle 20°–30°, while it is $\sim 100{{\ \rm per\ cent}}$ and spherical in collapsars. We also analytically obtain the shares of mass and energies for the escaped and trapped cocoons. Because the mass of the escaped cocoon is small and the trapped cocoon is concealed by the ejecta and the escaped cocoon, we suggest that it is unlikely that cooling emission from the sGRB-jet heated cocoon was observed as a counterpart to the gravitational wave event GW170817.

Non-thermal emission from mildly relativistic dynamical ejecta of neutron star mergers

ABSTRACT Binary neutron star mergers are expected to produce fast dynamical ejecta, with mildly relativistic velocities extending to β = v/c &amp;gt; 0.6. We consider the radio to X-ray synchrotron emission produced by collisionless shocks driven by such fast ejecta into the interstellar medium. Analytical expressions are given for spherical ejecta with broken power-law mass (or energy) distributions, M(&amp;gt; γβ) ∝ (γβ)−s with s = sKN at γβ &amp;lt; γ0β0 and s = sft at γβ &amp;gt; γ0β0 (where γ is the Lorentz factor). For parameter values characteristic of merger calculation results – a ‘shallow’ mass distribution, 1 &amp;lt; sKN &amp;lt; 3, for the bulk of the ejecta (at γβ ≈ 0.2), and a steep, sft &amp;gt; 5, ‘fast tail’ mass distribution – our model provides an accurate (to tens of per cent) description of the evolution of the flux, including at the phase of deceleration to subrelativistic expansion. This is a significant improvement over earlier results, based on extrapolations of results valid for γβ ≫ 1 or ≪1 to γβ ≈ 1, which overestimate the flux by an order of magnitude for typical parameter values. It will enable a more reliable inference of ejecta parameters from future measurements of the non-thermal emission. For the merger event GW170817, the existence of a ‘fast tail’ is expected to produce detectable radio and X-ray fluxes over a time-scale of ∼104 d.

Theoretical investigation of energy levels and transitions for Pr iv

ABSTRACT We present extensive energy levels (1110 levels) and transition data for the Pr iv. We also show the effect of such atomic data on opacity for neutron star (NS) merger. We performed energy spectra and transition data calculations using the GRASP2018 package, which is based on the multiconfiguration Dirac–Hartree–Fock and relativistic configuration interaction methods. Using the GRASP2018 data, the opacities in the NS merger ejecta containing pure Pr are calculated. Energy levels are compared with recommended values from the NIST Atomic Spectra Database and other available works. Furthermore, electric dipole (E1) transition data, line strengths, weighted oscillator strengths, and transition rates are computed between the above levels. We estimate the uncertainties of the computed line strengths giving the accuracy class. The computed line strengths are also compared with other theoretical computations. We proposed few ways to estimate the uncertainties of the line strengths of the E1 transitions. Our calculation for opacity for a typical condition of NS merger ejecta at t ∼ 1 d shows that the Planck mean opacity of Pr3 + reaches $\sim 1\, \rm cm^{2}\, g^{-1}$ at $T\, \sim \, 15000$ K. The opacity is higher than that by the previous studies, thanks to the higher completeness of the atomic data. We also highlight the importance of accurate atomic data for the opacity by comparing it with the opacity using the atomic calculation from HULLAC. Moreover, we identify the important configurations, especially for the transitions between the high-lying levels for the opacity of 3+ ionized lanthanides.

Correlations between nuclear landscape boundaries and neutron-rich r -process abundances

Motivated by the newly observed $^{39}\mathrm{Na}$ in experiments, systematic calculations of global nuclear binding energies with seven Skyrme forces are performed. We demonstrate the strong correlation between the two-neutron separation energies of $^{39}\mathrm{Na}$ and the total number of bound nuclei of the whole nuclear landscape. Furthermore, with calculated nuclear masses, we perform astrophysical rapid neutron capture process ($r$-process) simulations by using the nuclear reaction code talys and the nuclear reaction network code skynet. $r$-process abundances from ejecta of neutron star mergers (NSM) and core-collapse supernova are compared. Prominent covariance correlations between nuclear landscape boundaries and $r$-process abundances in the NSM scenario before the third peak are shown. We also see that statistical correlations disappear where shell effects dominated. This study highlights the need for further experimental studies of drip-line nuclei around $^{39}\mathrm{Na}$ for better constraints on nuclear landscape boundaries and the $r$-process in extremely neutron-rich environments.

Theoretical investigation of energy levels and transition for Ce IV

Aims. We present extensive energy level and transition data for the Ce IV spectrum. By providing accurate atomic data, we evaluate the impact of atomic data on the opacity in the neutron star merger ejecta. Methods. We performed energy spectra and transition data calculations using the GRASP2018 package, which is based on the multiconfiguration Dirac–Hartree–Fock and relativistic configuration interaction methods, and the HULLAC code, which is based on a parametric potential method. Results. We present energy spectra calculated for the 225 levels for the Ce3+ ion. Energy levels are compared with recommended values from the NIST Atomic Spectra Database and other available works. The root-mean-square (rms) deviations obtained for the GRASP2018 energy levels of the 5p6nl configurations from the NIST data are 1270 cm−1. The rms deviations for the HULLAC results from the NIST data are 5780 cm−1. Furthermore, electric dipole (E1) transition data, line strengths, weighted oscillator strengths, and transition rates are computed between the above levels. The computed transition rates are compared with other theoretical computations. We also evaluate the accuracy of the wave functions and transition parameters by analyzing the dependencies of the line strength S on the gauge parameter G. The gauge dependency method also allows us to determine the transitions for which the ratio between the Babushkin and Coulomb gauges shows real agreement between forms and the transitions for which the agreement between both gauges is random. Using the GRASP2018 and HULLAC data, the opacities in the neutron star merger ejecta are also calculated. We find that the opacity of Ce IV is higher than that presented by previous works, which is because of the higher completeness of our atomic data. Although the differences in the energy levels and transition probabilities cause different features in the opacity spectrum, the Planck mean opacities of both data sets agree within 20%.

Dynamical ejecta of neutron star mergers with nucleonic weak processes I: nucleosynthesis

ABSTRACT We present a coherent study of the impact of neutrino interactions on the r-process element nucleosynthesis and the heating rate produced by the radioactive elements synthesized in the dynamical ejecta of neutron star–neutron star (NS–NS) mergers. We have studied the material ejected from four NS–NS merger systems based on hydrodynamical simulations which handle neutrino effects in an elaborate way by including neutrino equilibration with matter in optically thick regions and re-absorption in optically thin regions. We find that the neutron richness of the dynamical ejecta is significantly affected by the neutrinos emitted by the post-merger remnant, in particular when compared to a case neglecting all neutrino interactions. Our nucleosynthesis results show that a solar-like distribution of r-process elements with mass numbers $A \gtrsim 90$ is produced, including a significant enrichment in Sr and a reduced production of actinides compared to simulations without inclusion of the nucleonic weak processes. The composition of the dynamically ejected matter as well as the corresponding rate of radioactive decay heating are found to be rather independent of the system mass asymmetry and the adopted equation of state. This approximate degeneracy in abundance pattern and heating rates can be favourable for extracting the ejecta properties from kilonova observations, at least if the dynamical component dominates the overall ejecta. Part II of this work will study the light curve produced by the dynamical ejecta of our four NS merger models.

Dynamical ejecta of neutron star mergers with nucleonic weak processes – II: kilonova emission

ABSTRACT 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 $\sim 10\,$ ms 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 this study. We employ a multidimensional two-moment radiation transport scheme with approximate M1 closure to evolve the photon field and use a heuristic prescription for the opacities found by calibration with atomic-physics-based reference results. We find that the photosphere is generically ellipsoidal but augmented with small-scale structure and produces emission that is about 1.5–3 times stronger towards the pole than the equator. The kilonova typically peaks after $0.7\!-\!1.5\,$ d in the near-infrared frequency regime with luminosities between $3\!-\!7\times 10^{40}\,$ erg s−1 and at photospheric temperatures of $2.2\!-\!2.8\times 10^3\,$ K. A softer equation of state or higher binary-mass asymmetry leads to a longer and brighter signal. Significant variations of the light curve are also obtained for models with artificially modified electron fractions, emphasizing the importance of a reliable neutrino-transport modelling. None of the models investigated here, which only consider dynamical ejecta, produces a transient as bright as AT2017gfo. The near-infrared peak of our models is incompatible with the early blue component of AT2017gfo.

Two Steps Forward and One Step Sideways: The Propagation of Relativistic Jets in Realistic Binary Neutron Star Merger Ejecta

Abstract The association of GRB170817A with GW170817 has confirmed the long-standing hypothesis that binary neutron star (BNS) mergers are the progenitors of at least some short gamma-ray bursts (SGRBs). This connection has ushered in an era in which broadband observations of SGRBs, together with measurements of the time delay between the gravitational waves and the electromagnetic radiation, allow for probing the properties of the emitting outflow and its engine to an unprecedented detail. Because the structure of the radiating outflow is molded by the interaction of a relativistic jet with the binary ejecta, it is of paramount importance to study the system in a realistic setting. Here we present a three-dimensional hydrodynamic simulation of a relativistic jet propagating in the ejecta of a BNS merger, which were computed with a general relativistic magnetohydrodynamic simulation. We find that the jet’s centroid oscillates around the axis of the system, due to inhomogeneities encountered in the propagation. These oscillations allow the jet to find the path of least resistance and travel faster than an identical jet in smooth ejecta. In our setup the breakout time is ∼0.6 s, which is comparable to the expected central engine duration in SGRBs and possibly a non-negligible fraction of the total delay between the gravitational and gamma-ray signals. Our simulation also shows that energy is carried in roughly equal amounts by the jet and by the cocoon, and that about 20% of the injected energy is transferred to the ejecta via mechanical work.

Signatures of r-process Elements in Kilonova Spectra

Abstract Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound–bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr ii triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca ii triplet lines. This is natural because Ca and Sr are coproduced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb, and Th. Our results suggest that the line-forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr ii and Ca ii lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca ii line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is &lt;0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥0.40 and entropy per nucleon (in units of Boltzmann constant) ≥25.

Fission fragment distributions and their impact on the r -process nucleosynthesis in neutron star mergers

Neutron star merger ejecta are currently the most viable astrophysical site for $r$-process nucleosynthesis. Fission plays a fundamental role. The manuscript presents an updated scission-point model for fission fragment distributions, which improves agreement with experimental fission yields. Two astrophysical scenarios, based on alternative weak-interaction hypotheses, are employed to reanalyze the role of fission. Regions of the nuclear chart where fission is important are identified, and the impact of fission yields on the final $r$-process abundance distribution is elaborated.

FIRST J1419+3940 as the First Observed Radio Flare from a Neutron Star Merger

Abstract During their violent merger, two neutron stars can shed a few percent of their mass. As this ejecta expands, it collides with the surrounding interstellar gas, producing a slowly fading radio flare that lasts for years. Radio flares uniquely probe the neutron star merger populations as many events from past decades could still be detectable. Nonetheless, no radio flare observation has been reported to date. Here we show that the radio transient FIRST J1419+3940, first observed in 1993 and still detectable, could have originated from a neutron star merger. We carry out numerical simulations of neutron star merger ejecta to demonstrate that the observed radio light curve is well reproduced by a merger model with astrophysically expected parameters. We examine the observed radio data, as well as the host galaxy, to find clues that could differentiate the transient’s neutron star merger origin from the alternative explanation—the afterglow of an off-axis long gamma-ray burst. Near-future observations could find further evidence for the FIRST J1419+3940 radio transient’s origin. We show that existing radio surveys likely already recorded multiple radio flares, informing us of the origin and properties of neutron star mergers and their role in the nucleosynthesis of the heaviest elements in the universe.

Oscillation of high-energy neutrinos from choked jets in stellar and merger ejecta

We present a comprehensive study on oscillation of high-energy neutrinos from two different environments: blue supergiant progenitors that may harbor low-power gamma-ray burst (GRB) jets and neutron star merger ejecta that would be associated with short gamma-ray bursts. We incorporate the radiation constraint that gives a necessary condition for nonthermal neutrino production, and account for the time evolution of the jet, which allows us to treat neutrino oscillation in matter more accurately. For massive star progenitors, neutrino injection inside the star can lead to nonadiabatic oscillation patterns in the 1 TeV - 10 TeV and is also visible in the flavor ratio. For neutron star merger ejecta, we find a similar behavior in the 100 GeV - 10 TeV region and the oscillation may result in a $\nu_e$ excess around 1 TeV. These features, which enable us to probe the progenitors of long and short GRBs, could be seen by future neutrino detectors with precise flavor ratio measurements. We also discuss potential contributions to the diffuse neutrino flux measured by IceCube, and find parameter sets allowing choked low-power GRB jets to account for the neutrino flux in the 10 TeV - 100 TeV range without violating the existing constraints.

Signatures of r-process elements in kilonova spectra

AbstractBinary neutron star (NS) mergers have been expected to synthesize r-process elements and cause electromagnetic radiation called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, individual elements have not been identified except for strontium. Toward identification of elements in kilonova spectra, we perform radiative transfer simulations in NS merger ejecta. We find that Sr II triplet lines appear in the spectrum, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca II triplet lines. Absence of the Ca II line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is &lt; 0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥ 0.40 and entropy per nucleon (in units of Boltzmann constant) ≥ 25. Identification of absorption lines in near-infrared wavelengths which have not yet been decoded may lead to clarify the abundances synthesized in NS merger ejecta.

Modeling kilonova emission from neutron star mergers

AbstractCoalescence of binary neutron stars gives rise to kilonova, thermal emission powered by radioactive decays of newly synthesized r-process nuclei. Observational properties of kilonova are largely affected by bound-bound opacities of r-process elements. It is, thus, important to understand atomic properties of heavy elements to link the observed signals with nucleosynthesis of neutron star mergers. In this paper, we introduce the latest status of kilonova modeling by focusing on the aspects of atomic physics. We perform systematic atomic structure calculations of r-process elements to understand element-to-element variation in the opacities. We demonstrate that the properties of the atomic structure of heavy elements are imprinted in the opacities of the neutron star merger ejecta and consequently in the kilonova light curves and spectra. Using this latest opacity dataset, we briefly discuss implications for GW170817, expected diversity of kilonova emission, and prospects for element identification in kilonova spectra.