We present a systematic numerical relativity study of the mass ejection and
the associated electromagnetic transients and nucleosynthesis from binary
neutron star (NS) mergers. We find that a few $10^{-3}\, M_\odot$ of material
are ejected dynamically during the mergers. The amount and the properties of
these outflow depend on binary parameters and on the NS equation of state
(EOS). A small fraction of these ejecta, typically ${\sim}10^{-6}\, M_\odot$,
is accelerated by shocks formed shortly after merger to velocities larger than
$0.6\, {\rm c}$ and produces bright radio flares on timescales of weeks,
months, or years after merger. Their observation could constrain the strength
with which the NSs bounce after merger and, consequently, the EOS of matter at
extreme densities. The dynamical ejecta robustly produce second and third
$r$-process peak nuclei with relative isotopic abundances close to solar. The
production of light $r$-process elements is instead sensitive to the binary
mass ratio and the neutrino radiation treatment. Accretion disks of up to
${\sim}0.2\, M_\odot$ are formed after merger, depending on the lifetime of the
remnant. In most cases, neutrino- and viscously-driven winds from these disks
dominate the overall outflow. Finally, we generate synthetic kilonova light
curves and find that kilonovae depend on the merger outcome and could be used
to constrain the NS EOS.

通过数值相对论进行的系统研究表明，双中子星合并会产生质量喷射、电磁暴及核合成等现象，其中的强冲击波会在合并后的数周、数月或数年内产生亮闪电，进而限制极端密度下物质的状态方程。

双中子星合并：质量抛射、电磁反物质和核合成

Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts  and Nucleosynthesis

We investigate the long-term evolution of black hole accretion disks formed
in neutron star mergers. These disks expel matter that contributes to an
$r$-process kilonova, and can produce relativistic jets powering short
gamma-ray bursts. Here we report the results of a three-dimensional,
general-relativistic magnetohydrodynamic (GRMHD) simulation of such a disk
which is evolved for long enough ($\sim 9$s, or $\sim 6\times 10^5 r_{\rm
g}/c$) to achieve completion of mass ejection far from the disk. Our model
starts with a poloidal field, and fully resolves the most unstable mode of the
magnetorotational instability. We parameterize the dominant microphysics and
neutrino cooling effects, and compare with axisymmetric hydrodynamic models
with shear viscosity. The GRMHD model ejects mass in two ways: a prompt
MHD-mediated outflow and a late-time, thermally-driven wind once the disk
becomes advective. The total amount of unbound mass ejected ($0.013M_\odot$, or
$\simeq 40\%$ of the initial torus mass) is twice as much as in hydrodynamic
models, with higher average velocity ($0.1c$) and a broad electron fraction
distribution with a lower average value ($0.16$). Scaling the ejected fractions
to a disk mass of $\sim 0.1M_\odot$ can account for the red kilonova from
GW170817 but underpredicts the blue component. About $\sim 10^{-3}M_\odot$ of
material should undergo neutron freezout and could produce a bright kilonova
precursor in the first few hours after the merger. With our idealized initial
magnetic field configuration, we obtain a robust jet and sufficient ejecta with
Lorentz factor $\sim 1-10$ to (over)produce the non-thermal emission from
GW1708107.

本文研究了发生在中子星合并过程中的黑洞吸积盘的长期演化现象，证实了该吸积盘会通过喷射物质产生 r 过程的巨新星，并能生成推动短时伽马射线爆发的相对论性喷流。

中子星合并吸积盘的长期 GRMHD 模拟：对电磁对应物的影响

Long-term GRMHD Simulations of Neutron Star Merger Accretion Disks:  Implications for Electromagnetic Counterparts

The discovery of a kilonova (KN) associated with the Advanced LIGO
(aLIGO)/Virgo event GW170817 opens up new avenues of multi-messenger
astrophysics. Here, using realistic simulations, we provide estimates of the
number of KNe that could be found in data from past, present and future surveys
without a gravitational-wave trigger. For the simulation, we construct a
spectral time-series model based on the DES-GW multi-band light-curve from the
single known KN event, and we use an average of BNS rates from past studies of
$10^3 \rm{Gpc}^{-3}/\rm{year}$, consistent with the $1$ event found so far.
Examining past and current datasets from transient surveys, the number of KNe
we expect to find for ASAS-SN, SDSS, PS1, SNLS, DES, and SMT is between 0 and
$0.3$. We predict the number of detections per future survey to be: 8.3 from
ATLAS, 10.6 from ZTF, 5.5/69 from LSST (the Deep Drilling / Wide Fast Deep),
and 16.0 from WFIRST. The maximum redshift of KNe discovered for each survey is
z = 0.8 for WFIRST, z = 0.25 for LSST and z = 0.04 for ZTF and ATLAS. For the
LSST survey, we also provide contamination estimates from Type Ia and
Core-collapse supernovae: after light-curve and template-matching requirements,
we estimate a background of just 2 events. More broadly, we stress that future
transient surveys should consider how to optimize their search strategies to
improve their detection efficiency, and to consider similar analyses for GW
follow-up programs.

通过建立基于 DES-GW 的多波段光曲线的光谱时间序列模型，我们使用真实的模拟数据估计了过去、现在和未来调研中可能发现的 KNe 的数量，并预测了未来调研的每个调研的最大红移。

过去、现在和未来的调查数据中能发现多少个千新星？

How Many Kilonovae Can Be Found in Past, Present, and Future Survey  Datasets?

The source of the gravitational-wave signal GW170817, very likely a binary
neutron star merger, was also observed electromagnetically, providing the first
multi-messenger observations of this type. The two week long electromagnetic
counterpart had a signature indicative of an r-process-induced optical
transient known as a kilonova. This Letter examines how the mass of the
dynamical ejecta can be estimated without a direct electromagnetic observation
of the kilonova, using gravitational-wave measurements and a phenomenological
model calibrated to numerical simulations of mergers with dynamical ejecta.
Specifically, we apply the model to the binary masses inferred from the
gravitational-wave measurements, and use the resulting mass of the dynamical
ejecta to estimate its contribution (without the effects of wind ejecta) to the
corresponding kilonova light curves from various models. The distributions of
dynamical ejecta mass range between $M_{ej} = 10^{-3} - 10^{-2} M_{\odot}$ for
various equations of state, assuming the neutron stars are rotating slowly. In
addition, we use our estimates of the dynamical ejecta mass and the neutron
star merger rates inferred from GW170817 to constrain the contribution of
events like this to the r-process element abundance in the Galaxy when ejecta
mass from post-merger winds is neglected. We find that if $\gtrsim10\%$ of the
matter dynamically ejected from BNS mergers is converted to r-process elements,
GW170817-like BNS mergers could fully account for the amount of r-process
material observed in the Milky Way.

本文研究了如何在没有直接电磁观测到光新星的情况下估算动力喷射物的质量，使用引力波观测和与动力喷射物对应的光曲线。研究表明，二元中子星合并释放出的动力喷射物可能占到系内流星云物质丰度的至少 10%。

估计动力喷射物在与 GW170817 相关的千新星中的贡献

Estimating the Contribution of Dynamical Ejecta in the Kilonova  Associated with GW170817

The cosmic origin of the elements heavier than iron has long been uncertain.
Theoretical modelling shows that the matter that is expelled in the violent
merger of two neutron stars can assemble into heavy elements such as gold and
platinum in a process known as rapid neutron capture (r-process)
nucleosynthesis. The radioactive decay of isotopes of the heavy elements is
predicted to power a distinctive thermal glow (a 'kilonova'). The discovery of
an electromagnetic counterpart to the gravitational-wave source GW170817
represents the first opportunity to detect and scrutinize a sample of freshly
synthesized r-process elements. Here we report models that predict the detailed
electromagnetic emission of kilonovae and enable the mass, velocity and
composition of ejecta to be derived from the observations. We compare the
models to the optical and infrared radiation associated with GW170817 event to
argue that the observed source is a kilonova. We infer the presence of two
distinct components of ejecta, one composed primarily of light (atomic mass
number less than 140) and one of heavy (atomic mass number greater than 140)
r-process elements. Inferring the ejected mass and a merger rate from GW170817
implies that such mergers are a dominant mode of r-process production in the
Universe.

两个中子星合并释放的物质可以以快速中子俘获核合成的形式聚集成黄金铂等重元素，此过程被称为 r-process 核合成，而 r-process 核合成产生的重元素同位素的放射性衰变会产生特征性的热辐射，也称为 kilonova。