Detecting the thermal emission from double neutron star merger events is a challenging task because of the quick fading of the observed flux. In order to create an efficient observing strategy for their observing method it is crucial to know their intrinsic rate. Unfortunately, the numerous models existing today predict this rate on a vary wide range. Hence, our goal in this paper is to investigate the effect of different level of approximations on the relative rate predictions. Also, we study the effect of distinct ejecta mass lay-outs on the light curve. We find that the ratio of the expected kilonova detections of the spherical to axisymmetrical models is 6:1 (or 2:1, depending on the input parameter set applied in our work). Nevertheless, the light curve shape is only slightly affected by the various ejecta alignments. This means that different ejecta layouts can produce light curves with similar shapes making it a challenging task to infer the structure of the matter outflow. Thus, we conclude that the uncertainty in the rate predictions arising from the various ejecta mass distribution models is negligible compared to the errors present in other input parameters (e.g. binary neutron star merger rate). In addition, we show that up to moderate redshifts (z < 0.2) the redshift distribution type (observed or uniform in volume) does not affect the expected relative rate estimations.
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…
The merger of two neutron stars (NSs) or a neutron star and a black hole (BH) produces a radioactively-powered transient known as a kilonova, first observed accompanying the gravitational wave event GW170817. While kilonovae are frequently modeled in spherical symmetry, the dynamical ejecta and disk outflows can be considerably asymmetric. We use Monte Carlo radiative transfer calculations to study the light curves of kilonovae with globally axisymmetric geometries (e.g. an ellipsoid and a torus). We find that the variation in luminosity in these models is most pronounced at early times, and d…
The joint detection of the gravitational wave GW170817, of the short $γ$-ray burst GRB170817A and of the kilonova AT2017gfo, generated by the the binary neutron star merger observed on August 17, 2017, is a milestone in multimessenger astronomy and provides new constraints on the neutron star equation of state. We perform Bayesian inference and model selection on AT2017gfo using semi-analytical, multi-components models that also account for non-spherical ejecta. Observational data favor anisotropic geometries to spherically symmetric profiles, with a log-Bayes' factor of ${\sim}10^{4}$, and fa…
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…
We explore the impact of pulsar electromagnetic dipole and fallback accretion emission on the luminosity of a suite of kilonova models. The pulsar models are varied over pulsar magnetic field strength, pulsar lifetime, ejecta mass, and elemental abundances; the fallback models are varied over fallback accretion rate and ejecta mass. For the abundances, we use Fe and Nd as representatives of the wind and dynamical ejecta, respectively. We simulate radiative transfer in the ejecta in either 1D spherical or 2D cylindrical spatial geometry. For the grid of 1D simulations, the mass fraction of Nd i…
The detection of GW170817 and the accompanying electromagnetic counterpart, AT2017gfo, have provided an important set of observational constraints for theoretical models of neutron star mergers, nucleosynthesis, and radiative transfer for kilonovae. We apply the 3D Monte Carlo radiative transfer code ARTIS to produce synthetic light curves of the dynamical ejecta from a neutron star merger, which has been modelled with 3D smooth-particle hydrodynamics (SPH) and included neutrino interactions. Nucleosynthesis calculations provide the energy released from radioactive decays of r-process nuclei, …
AAssociation of GW170817/GRB170817A/AT2017gfo provides the first direct evidence for neutron star mergers as significant sources of $r$-process nucleosynthesis. A gamma-ray transient (GRT) would be powered by the radioactive decay of the freshly-synthesized $r$-process elements. By analyzing the composition and gamma-ray opacity of the kilonova ejecta in details, we calculate the lightcurve and spectrum of the GRT for a range of spherically symmetric merger ejecta models with mass $M_{\rm ej}=0.001 \sim 0.05M_{\odot}$ and expansion velocity $v_{\rm ej}= 0.1\sim 0.4c$. It is found that the peak…
Depending upon the properties of their compact remnants and the physics included in the models, simulations of neutron star mergers can produce a broad range of ejecta properties. The characteristics of this ejecta, in turn, define the kilonova emission. To explore the effect of ejecta properties, we present a grid of 2-component 2D axisymmetric kilonova simulations that vary mass, velocity, morphology, and composition. The masses and velocities of each component vary, respectively, from 0.001 to 0.1 M$_{\odot}$ and 0.05 to 0.3$c$, covering much of the range of results from the neutron star me…
Late-time emission-line profiles from kilonova models
Queen’s University Belfast（贝尔法斯特女王大学）；GSI Helmholtzzentrum für Schwerionenforschung（GSI亥姆霍兹重离子研究中心）；RIKEN Cluster for Pioneering Research（理化学研究所前沿研究集群）；Helmholtz Forschungsakademie Hessen für FAIR, GSI Helmholtzzentrum für Schwerionenforschung（黑森FAIR亥姆霍兹研究院，GSI亥姆霍兹重离子研究中心）；Technische Universität Darmstadt（达姆施塔特工业大学）
We consider a simple model for the nonthermal emission from GW170817, in which a quasi-spherical ejecta is released in the merger event, with the kinetic energy distributed over the momentum as $E(>γβ)\propto(γβ)^{-k}$. The ejecta drives a shock into the medium and gives rise to synchrotron radiation. Using multi-band observations we constrain that $k\approx6.7$; (assuming medium density of $\sim10^{-2}\rm cm^{-3}$ and postshock magnetic field carries a fraction $10^{-5}-10^{-3}$ of the postshock internal energy) the total kinetic energy is $(0.3-5)\times10^{51}$erg; the slowest ejecta velocit…
Beta-Particle Transport and Thermalization in Kilonova Ejecta with Detailed Atomic Microphysics
Princeton University（普林斯顿大学）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；The George Washington University（乔治华盛顿大学）；Obsidian Research（黑曜石研究）；University of Notre Dame（圣母大学）
We present radiative transfer simulations for blue kilonovae hours after neutron star (NS) mergers by performing detailed opacity calculations for the first time. We calculate atomic structures and opacities of highly ionized elements (up to the tenth ionization) with atomic number Z = 20 - 56. We find that the bound-bound transitions of heavy elements are the dominant source of the opacities in the early phase (t < 1 day after the merger), and that the ions with a half-closed electron shell provide the highest contributions. The Planck mean opacity for lanthanide-free ejecta (with electron fr…
Deep Newtonian Afterglows: Theoretical Light Curves for Quasi-spherical Outflows
Instituto de Astronomía, Universidad Nacional Autónoma de México（墨西哥国立自治大学天文学研究所）；Center for Theoretical Physics of the Universe, Institute for Basic Science (IBS)（基础科学研究院宇宙理论物理中心）；Instituto de Física, Universidad Nacional Autónoma de México（墨西哥国立自治大学物理研究所）；National Astronomical Observatory of Japan（日本国家天文台）；Space Science Institute（空间科学研究所）
We build three simple bipolar ejecta models for core collapse supernovae (CCSNe), as expected when the explosion is driven by strong jets, and show that for an observer located in the equatorial plane of the ejecta, the light curve has a rapid luminosity decline, and even an abrupt drop. In calculating the geometrically modified photosphere we assume that the ejecta has an axisymmetrical structure composed of an equatorial ejecta and faster polar ejecta, and has a uniform effective temperature. At early times the photosphere in the polar ejecta grows faster than the equatorial one, leading to …
Self-consistent 3D radiative transfer for kilonovae: directional spectra from merger simulations
GSI Helmholtzzentrum für Schwerionenforschung（GSI亥尔姆霍兹重离子研究中心）；Department of Physics and Astronomy, Ruprecht-Karls-Universität Heidelberg（海德堡大学物理与天文系）；Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research（理化学研究所先锋研究集群大爆炸天体物理实验室）；Institut für Kernphysik (Theoriezentrum), Technische Universität Darmstadt（达姆施塔特工业大学核物理研究所（理论中心））
Black hole - neutron star (BH-NS) mergers are a major target for ground-based gravitational wave (GW) observatories. A merger can also produce an electromagnetic counterpart (a kilonova) if it ejects neutron-rich matter that assembles into heavy elements through r-process nucleosynthesis. We study the kilonova signatures of the unbound dynamical ejecta of a BH-NS merger. We take as our initial state the results from a numerical relativity simulation, and then use a general relativistic hydrodynamics code to study the evolution of the ejecta with parameterized r-process heating models. The unbo…
Merging binaries consisting of two neutron stars (NSs) or an NS and a stellar-mass black hole typically form a massive accretion torus around the remnant black hole or long-lived NS. Outflows from these neutrino-cooled accretion disks represent an important site for $r$-process nucleosynthesis and the generation of kilonovae. We present the first three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulations including weak interactions and a realistic equation of state of such accretion disks over viscous timescales ($380\,\mathrm{ms}$). We witness the emergence of steady-stat…
We present a systematic numerical relativity study of the impact of different treatment of microphysics and grid resolution in binary neutron star mergers. We consider series of simulations at multiple resolutions comparing hydrodynamics, neutrino leakage scheme, leakage augmented with the M0 scheme and the more consistent M1 transport scheme. Additionally, we consider the impact of a sub-grid scheme for turbulent viscosity. We find that viscosity helps to stabilise the remnant against gravitational collapse but grid resolution has a larger impact than microphysics on the remnant's stability. …
We examine the long-term evolution of accretion tori around black hole (BH) remnants of compact object mergers involving at least one neutron star, to better understand their contribution to kilonovae and the synthesis of r-process elements. To this end, we modify the unsplit magnetohydrodynamic (MHD) solver in FLASH4.5 to work in non-uniform three-dimensional spherical coordinates, enabling more efficient coverage of a large dynamic range in length scales while exploiting symmetries in the system. This modified code is used to perform BH accretion disk simulations that vary the initial magnet…
R-process heating implementation in hydrodynamic simulations with neural networks
GSI Helmholtzzentrum für Schwerionenforschung（GSI亥姆霍兹重离子研究中心）；RIKEN Cluster for Pioneering Research（理研前沿研究集群）；Technische Universität Darmstadt（达姆施塔特工业大学）
