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

GW170817 is the first gravitational wave detection of a binary neutron star
merger. It was accompanied by radiation across the electromagnetic spectrum and
localized to the galaxy NGC 4993 at a distance of 40 Mpc. It has been proposed
that the observed gamma-ray, X-ray and radio emission is due to an
ultra-relativistic jet launched during the merger, directed away from our line
of sight. The presence of such a jet is predicted from models positing neutron
star mergers as the central engines driving short-hard gamma-ray bursts
(SGRBs). Here we show that the radio light curve of GW170817 has no direct
signature of an off-axis jet afterglow. While we cannot rule out the existence
of a jet pointing elsewhere, the observed gamma-rays could not have originated
from such a jet. Instead, the radio data requires a mildly relativistic
wide-angle outflow moving towards us. This outflow could be the high velocity
tail of the neutron-rich material dynamically ejected during the merger or a
cocoon of material that breaks out when a jet transfers its energy to the
dynamical ejecta. The cocoon scenario can explain the radio light curve of
GW170817 as well as the gamma-rays and X-rays (possibly also ultraviolet and
optical emission), and hence is the model most consistent with the
observational data. Cocoons may be a ubiquitous phenomenon produced in neutron
star mergers, giving rise to a heretofore unidentified population of radio,
ultraviolet, X-ray and gamma-ray transients in the local universe.

通过对引力波检测结果 GW170817 的分析，我们发现该结果的伽马射线、X 射线和射电辐射可能来自一种称为 “cocoon” 的材料包裹，这可能是由于中心引擎引起的中子星并合现象。

GW170817 中的轻相对论宽角度喷流

A mildly relativistic wide-angle outflow in the neutron star merger  GW170817

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

We present UV, optical, and NIR photometry of the first electromagnetic
counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary
neutron star merger GW170817. Our data set extends from the discovery of the
optical counterpart at $0.47$ days to $18.5$ days post-merger, and includes
observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2
(GS/F2), and the {\it Hubble Space Telescope} ({\it HST}). The spectral energy
distribution (SED) inferred from this photometry at $0.6$ days is well
described by a blackbody model with $T\approx 8300$ K, a radius of $R\approx
4.5\times 10^{14}$ cm (corresponding to an expansion velocity of $v\approx
0.3c$), and a bolometric luminosity of $L_{\rm bol}\approx 5\times10^{41}$ erg
s$^{-1}$. At $1.5$ days we find a multi-component SED across the optical and
NIR, and subsequently we observe rapid fading in the UV and blue optical bands
and significant reddening of the optical/NIR colors. Modeling the entire data
set we find that models with heating from radioactive decay of $^{56}$Ni, or
those with only a single component of opacity from $r$-process elements, fail
to capture the rapid optical decline and red optical/NIR colors. Instead,
models with two components consistent with lanthanide-poor and lanthanide-rich
ejecta provide a good fit to the data, the resulting "blue" component has
$M_\mathrm{ej}^\mathrm{blue}\approx 0.01$ M$_\odot$ and
$v_\mathrm{ej}^\mathrm{blue}\approx 0.3$c, and the "red" component has
$M_\mathrm{ej}^\mathrm{red}\approx 0.04$ M$_\odot$ and
$v_\mathrm{ej}^\mathrm{red}\approx 0.1$c. These ejecta masses are broadly
consistent with the estimated $r$-process production rate required to explain
the Milky Way $r$-process abundances, providing the first evidence that BNS
mergers can be a dominant site of $r$-process enrichment.

本文报道了 LIGO/Virgo 首个引力波源的电磁反应物，二进制中子星合并事件 GW170817 的紫外、光学和近红外光度测量结果，这些结果揭示了黑体模型在 0.6 天内的光谱能量分布等参数，并鉴定了来自 r - 过程元素的东西的贡献对多成分 SED 的影响。

LIGO/VIRGO GW170817 双中子星合并的电磁对应物 II: 紫外，光学和近红外光曲线及其与千新星模型的比较

The Electromagnetic Counterpart of the Binary Neutron Star Merger  LIGO/VIRGO GW170817. II. UV, Optical, and Near-IR Light Curves and Comparison  to Kilonova Models