Constraint on the ejecta mass for a black hole-neutron star merger event candidate S190814bv

We derive the upper limit to the ejecta mass of S190814bv, a black hole-neutron star merger candidate, through the radiative transfer simulations for kilonovae with the realistic ejecta density profile as well as the detailed opacity and heating rate models. The limits to the ejecta mass strongly depend on the viewing angle. For the face-on observations ($\le45^\circ$), the total ejecta mass should be smaller than $0.1\,M_\odot$ for the average distance of S190814bv ($D=267$ Mpc), while larger mass is allowed for the edge-on observations. We also derive the conservative upper limits of the dynamical ejecta mass to be $0.02\,M_\odot$, $0.03\,M_\odot$, and $0.05\,M_\odot$ for the viewing angle $\le 20^\circ$, $\le 50^\circ$, and for $\le 90^\circ$, respectively. We show that the {\it iz}-band observation deeper than $22$ mag within 2 d after the GW trigger is crucial to detect the kilonova with the total ejecta mass of $0.06\,M_\odot$ at the distance of $D=300$ Mpc. We also show that a strong constraint on the NS mass-radius relation can be obtained if the future observations put the upper limit of $0.03\,M_\odot$ to the dynamical ejecta mass for a BH-NS event with the chirp mass smaller than $\lesssim 3\,M_\odot$ and effective spin larger than $\gtrsim 0.5$.Comment: 16 pages, 15 figure

Extracting the cutoff frequency in the gravitational-wave spectrum of black hole-neutron star mergers

The location of the cutoff in the gravitational-wave spectrum of black hole-neutron star mergers is correlated strongly with the neutron-star radius for the case that the neutron star is disrupted by the black hole during the merger. However, the modulation which appears in the spectrum due to the mode mixing makes it difficult to measure the cutoff frequency if gravitational waves are observed from inclined direction or the binary is precessing. In this letter, we show that the cutoff frequency can be measured even in such situations with a method we have recently proposed to reconstruct the face-on waveforms only from the strain observed from a particular direction. We show that the systematic error in the measurement of the neutron-star radius can be reduced to $\lesssim 5\%$ for the case that tidal disruption of the neutron star occurs significantly.Comment: 6 pages, 2 figure

Extracting the orbital axis from gravitational waves of precessing binary systems

We present a new method for extracting the instantaneous orbital axis only from gravitational wave strains of precessing binary systems observed from a particular observer direction. This method enables us to reconstruct the co-precessing frame waveforms only from observed quantities for the ideal case that the signal-to-noise ratio is high enough to analyze the waveforms directly. Specifically, we do not assume knowledge of the time evolution of the instantaneous orbital axis and the co-precessing waveforms before analyzing the data in our method. We test and measure the accuracy of our method using the numerical relativity simulation data of precessing binary black holes taken from the SXS Catalog. We show that the direction of the orbital axis is extracted within $\approx0.02~{\rm rad}$ error from gravitational waves emitted during the inspiral phase. The co-precessing waveforms are also reconstructed with high accuracy; the mismatch (assuming white noise) between them and the original co-precessing waveforms is typically a few times $10^{-3}$ including the merger-ringdown phase, and can be improved by an order of magnitude focusing only on the inspiral waveform. In this method, the co-precessing frame waveforms are not only the purely technical tools for understanding the complex nature of precessing waveforms but also direct observables.Comment: 12 pages, 8 figures, 1 table, published in PR

A Monte-Carlo based relativistic radiation hydrodynamics code with a higher-order scheme

We develop a new relativistic radiation hydrodynamics code based on the Monte-Carlo algorithm. In this code, we implement a new scheme to achieve the second-order accuracy in time in the limit of a large packet number for solving the interaction between matter and radiation. This higher-order time integration scheme is implemented in the manner to guarantee the energy-momentum conservation to the precision of the geodesic integrator. The spatial dependence of radiative processes, such as the packet propagation, emission, absorption, and scattering, are also taken into account up to the second-order accuracy. We validate our code by solving various test-problems following the previous studies; one-zone thermalization, dynamical diffusion, radiation dragging, radiation mediated shock-tube, shock-tube in the optically thick limit, and Eddington limit problems. We show that our code reproduces physically appropriate results with reasonable accuracy and also demonstrate that the second-order accuracy in time and space is indeed achieved with our implementation for one-zone and one-dimensional problems.Comment: 25 pages, 10 figures, submitted to PR

Radiative transfer simulation for the optical and near-infrared electromagnetic counterparts to GW170817

Recent detection of gravitational waves from a binary-neutron star merger (GW170817) and the subsequent observations of electromagnetic counterparts provide a great opportunity to study the physics of compact binary mergers. The optical and near-infrared counterparts to GW170817 (SSS17a, also known as AT 2017gfo or DLT17ck) are found to be consistent with a kilonova/macronova scenario with red and blue components. However, in most of previous studies in which contribution from each ejecta component to the lightcurves is separately calculated and composited, the red component is too massive as dynamical ejecta and the blue component is too fast as post-merger ejecta. In this letter, we perform a 2-dimensional radiative transfer simulation for a kilonova/macronova consistently taking the interplay of multiple ejecta components into account. We show that the lightcurves and photospheric velocity of SSS17a can be reproduced naturally by a setup consistent with the prediction of the numerical-relativity simulations.Comment: 6 pages, 3 figure

Frequency-domain gravitational waveform models for inspiraling binary neutron stars

We develop a model for frequency-domain gravitational waveforms from inspiraling binary neutron stars. Our waveform model is calibrated by comparison with hybrid waveforms constructed from our latest high-precision numerical-relativity waveforms and the SEOBNRv2T waveforms in the frequency range of $10$--$1000\,{\rm Hz}$. We show that the phase difference between our waveform model and the hybrid waveforms is always smaller than $0.1\, {\rm rad}$ for the binary tidal deformability, ${\tilde \Lambda}$, in the range $300\lesssim{\tilde \Lambda}\lesssim1900$ and for the mass ratio between 0.73 and 1. We show that, for $10$--$1000\,{\rm Hz}$, the distinguishability for the signal-to-noise ratio $\lesssim50$ and the mismatch between our waveform model and the hybrid waveforms are always smaller than 0.25 and $1.1\times10^{-5}$, respectively. The systematic error of our waveform model in the measurement of ${\tilde \Lambda}$ is always smaller than $20$ with respect to the hybrid waveforms for $300\lesssim{\tilde \Lambda}\lesssim1900$. The statistical error in the measurement of binary parameters is computed employing our waveform model, and we obtain results consistent with the previous studies. We show that the systematic error of our waveform model is always smaller than $20\%$ (typically smaller than $10\%$) of the statistical error for events with the signal-to-noise ratio of $50$.Comment: 22 pages, 16 figures, accepted for publication in PR

Towards rapid transient identification and characterization of kilonovae

With the increasing sensitivity of advanced gravitational wave detectors, the first joint detection of an electromagnetic and gravitational wave signal from a compact binary merger will hopefully happen within this decade. However, current gravitational-wave likelihood sky areas span $\sim 100-1000\,\textrm{deg}^2$, and thus it is a challenging task to identify which, if any, transient corresponds to the gravitational-wave event. In this study, we make a comparison between recent kilonovae/macronovae lightcurve models for the purpose of assessing potential lightcurve templates for counterpart identification. We show that recent analytical and parametrized models for these counterparts result in qualitative agreement with more complicated radiative transfer simulations. Our analysis suggests that with improved lightcurve models with smaller uncertainties, it will become possible to extract information about ejecta properties and binary parameters directly from the lightcurve measurement. Even tighter constraints are obtained in cases for which gravitational-wave and kilonovae parameter estimation results are combined. However, to be prepared for upcoming detections, more realistic kilonovae models are needed. These will require numerical relativity with more detailed microphysics, better radiative transfer simulations, and a better understanding of the underlying nuclear physics

Sub-radian-accuracy gravitational waveforms of coalescing binary neutron stars in numerical relativity

Extending our previous studies, we perform high-resolution simulations of inspiraling binary neutron stars in numerical relativity. We thoroughly carry through a convergence study in our currently available computational resources with the smallest grid spacing of $\approx 63$--86~meter for the neutron-star radius 10.9--13.7\,km. The estimated total error in the gravitational-wave phase is of order 0.1~rad for the total phase of $\gtrsim 210$\,rad in the last $\sim 15$--16 inspiral orbits. We then compare the waveforms (without resolution extrapolation) with those calculated by the latest effective-one-body formalism (tidal SEOBv2 model referred to as TEOB model). We find that for any of our models of binary neutron stars, the waveforms calculated by the TEOB formalism agree with the numerical-relativity waveforms up to $\approx 3$\,ms before the peak of the gravitational-wave amplitude is reached: For this late inspiral stage, the total phase error is $\lesssim 0.1$\,rad. Although the gravitational waveforms have an inspiral-type feature for the last $\sim 3$\,ms, this stage cannot be well reproduced by the current TEOB formalism, in particular, for neutron stars with large tidal deformability (i.e., lager radius). The reason for this is described.Comment: 13 pages, 11 figures, submitted to PR