The observations of GW170817/GRB170817A have confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst. In the standard picture of a short gamma-ray burst, a collimated highly relativistic outflow is launched after merger and it successfully breaks out from the surrounding ejected matter. Using initial conditions inspired from numerical-relativity binary neutron-star merger simulations, we have performed general-relativistic hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations in which the jet is launched and propagates self-consistently. The complete set of simulations suggests that: (i) MHD jets have an intrinsic energy and velocity polar structure with a ``hollow core'' subtending an angle $\theta_{\rm core}\approx4^{\circ}-5^{\circ}$ and an opening angle of $\theta_{\rm jet}\gtrsim10^{\circ}$; (ii) MHD jets eject significant amounts of matter and two orders of magnitude more than HD jets; (iii) the energy stratification in MHD jets naturally yields the power-law energy scaling $E(>\Gamma\beta)\propto(\Gamma\beta)^{-4.5}$; (iv) MHD jets provide fits to the afterglow data from GRB170817A that are comparatively better than those of the HD jets and without free parameters; (v) finally, both of the best-fit HD/MHD models suggest an observation angle $\theta_{\rm obs} \simeq 21^{\circ}$ for GRB170817A.

中文翻译：

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中子星合并磁化射流的张角：以GRB170817A为例

GW170817/GRB170817A 的观测证实，中子星双星的聚结是短伽马射线爆发的前身。在短伽马射线暴的标准图片中，合并后发射出准直的高度相对论的外流，并成功地从周围喷出的物质中爆发出来。使用受数值相对论双中子星并合模拟启发的初始条件，我们进行了广义相对论流体动力学 (HD) 和磁流体动力学 (MHD) 模拟，其中射流被发射并自洽传播。完整的模拟表明：(i) MHD 射流具有内在能量和速度极性结构，带有“空心” 对角 $\theta_{\rm core}\approx4^{\circ}-5^{\circ}$ 和张角 $\theta_{\rm jet}\gtrsim10^{\circ}$；(ii) MHD 射流喷射大量物质，比 HD 射流多两个数量级；(iii) MHD 喷流中的能量分层自然产生幂律能量标度 $E(>\Gamma\beta)\propto(\Gamma\beta)^{-4.5}$；(iv) MHD 射流对来自 GRB170817A 的余辉数据的拟合优于 HD 射流并且没有自由参数；(v) 最后，两个最适合的 HD/MHD 模型都建议了 GRB170817A 的观察角 $\theta_{\rm obs} \simeq 21^{\circ}$。\Gamma\beta)\propto(\Gamma\beta)^{-4.5}$; (iv) MHD 射流对来自 GRB170817A 的余辉数据的拟合优于 HD 射流并且没有自由参数；(v) 最后，两个最适合的 HD/MHD 模型都建议了 GRB170817A 的观察角 $\theta_{\rm obs} \simeq 21^{\circ}$。\Gamma\beta)\propto(\Gamma\beta)^{-4.5}$; (iv) MHD 射流对来自 GRB170817A 的余辉数据的拟合优于 HD 射流并且没有自由参数；(v) 最后，两个最适合的 HD/MHD 模型都建议了 GRB170817A 的观察角 $\theta_{\rm obs} \simeq 21^{\circ}$。