GW170817之后的非热辐射与初始洛伦兹因子$\lesssim10$的锥形径向分层外流一致
The non-thermal emission following GW170817 is consistent with a conical radially-stratified outflow with initial Lorentz factor $\lesssim10$
AI总结：
本文提出GW170817的非热辐射可由初始洛伦兹因子不超过10的锥形径向分层外流解释，其流量上升由径向结构决定，区别于高相对论性结构化喷流模型，并预测了不同的观测角依赖关系。
摘要：
我们表明，中子星并合事件GW170817之后的非热射电至X射线辐射，与由锥形径向分层外流驱动的无碰撞激波进入星际介质（ISM）所产生的同步辐射一致。该外流以约0.25弧度离轴观测，其质量对动量的依赖关系为幂律形式$M(>γβ)\propto(γβ)^{-4}$，最大洛伦兹因子$γ=10$，张角（半角）约为0.15弧度，总能量约为$5\times10^{50}$尔格。流量上升阶段的时间依赖关系由径向分层结构决定，该结构决定了外流能量沉积到星际介质中的速率。这与高度相对论性的结构化喷流模型（$γ\approx100$）形成对比，在后一类模型中，角向喷流结构通过减速过程中逐渐“揭开”更大角向截面（这些截面最初因相对论性集束效应而“隐藏”）来决定时间依赖关系，通常预测峰值后的流量下降速度比观测到的更快。我们的模型预测了对观测角的依赖关系，这与高度相对论性喷流模型的预测不同。特别是，预测在更靠近对称轴方向观测到的类似并合事件会表现出类似延长的流量随时间上升的持续时间。我们的分析表明，数据并不要求存在高度相对论性的$γ\approx100$分量，但不能排除存在张角$\ll0.15$弧度、能量$\ll5\times10^{50}$尔格的此类分量。
英文摘要：
We show that the non-thermal radio to X-ray emission following the neutron star merger GW170817 is consistent with synchrotron emission from a collisionless shock driven into the interstellar medium (ISM) by a conical radially stratified outflow observed $\approx0.25$~rad off-axis, with a power-low mass dependence on momentum, $M(>γβ)\propto(γβ)^{-4}$, maximum Lorenz factor $γ=10$, opening (half-)angle $\approx0.15$~rad, and total energy of $\approx5\times10^{50}$erg. The temporal dependence of the flux during its rising phase is determined by the radial stratification structure, which determines the rate at which outflow energy is deposited in the ISM. This is in contrast with highly relativistic, $γ\approx100$, structured jet models, where the angular jet structure determines the time dependence through the gradual "unveiling" by deceleration of larger angular sections of the jet (which are initially "hidden" by relativistic beaming), typically leading to a predicted flux decline after the peak that is faster than observed. Our model predicts a dependence on the observing angle, which is different than that predicted by highly relativistic jet models. Particularly, similar merger events observed closer to the symmetry axis are predicted to show a similarly extended duration of flux increase with time. Our analysis demonstrates that the data do not require a highly relativistic $γ\approx100$ component, but the presence of such a component with opening angle $\ll0.15$~rad and energy $\ll5\times10^{50}$~erg cannot be excluded.
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