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arXiv 2106.04788astro-ph.HE
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离轴观测的相对论电子-正电子对风的GRB 170817 A余辉

GRB 170817A Afterglow from a Relativistic Electron-Positron Pair Wind Observed Off-axis

Long Li, Zi-Gao Dai

摘要:

来自快速旋转、强磁化中子星(NS)的相对论性电子-正电子($e^{+}e^{-}$)对风将与伽马射线暴(GRB)外部冲击相互作用,并重塑余辉发射特征。假设GW 170817的合并残留物是一个长寿命的NS,我们表明,一个相对论$e^{+}e^{-}$对风模型与一个简单的礼帽喷流离轴观察可以再现多波长余辉光变曲线和超光速运动的GRB 170817 A。采用马尔可夫链蒙特卡罗(MCMC)方法得到最佳拟合参数,即射流半开角$θ_{j}\approx0.11$ rad和视角$θ_{v}\approx0.23$ rad。$θ_{v}$的最佳拟合值接近于根据重力波和电磁波观测值选择的先验的下限。此外,我们还导出了初始洛伦兹因子$Γ_{0}\approx47$和各向同性动能$E_{\rm K,iso}\approx2\times10^{52}\rm\ erg$。修正后的GRB 170817 A轴上辐射值与短暴典型辐射值的一致性表明,我们的模型也能再现GRB 170817 A的瞬发辐射。在我们的拟合中获得了具有磁场强度$B_{p}\approx1.6\times10^{13}\rm\ G$的NS,这表明需要相对较低的热化效率$η\lesssim10^{-3}$来满足对千诺瓦的观测约束。此外,我们的模型能够再现X射线光变曲线的晚期浅衰变,并预测X射线和射电通量将在未来几年继续下降。

英文摘要:

A relativistic electron-positron ($e^{+}e^{-}$) pair wind from a rapidly rotating, strongly magnetized neutron star (NS) would interact with a gamma-ray burst (GRB) external shock and reshapes afterglow emission signatures. Assuming that the merger remnant of GW170817 is a long-lived NS, we show that a relativistic $e^{+}e^{-}$ pair wind model with a simple top-hat jet viewed off-axis can reproduce multi-wavelength afterglow lightcurves and superluminal motion of GRB 170817A. The Markov chain Monte Carlo (MCMC) method is adopted to obtain the best-fitting parameters, which give the jet half-opening angle $θ_{j}\approx0.11$ rad, and the viewing angle $θ_{v}\approx0.23$ rad. The best-fitting value of $θ_{v}$ is close to the lower limit of the prior which is chosen based on the gravitational-wave and electromagnetic observations. In addition, we also derive the initial Lorentz factor $Γ_{0}\approx47$ and the isotropic kinetic energy $E_{\rm K,iso}\approx2\times10^{52}\rm\ erg$. A consistence between the corrected on-axis values for GRB 170817A and typical values observed for short GRBs indicates that our model can also reproduce the prompt emission of GRB 170817A. An NS with a magnetic field strength $B_{p}\approx1.6\times10^{13}\rm\ G$ is obtained in our fitting, indicating that a relatively low thermalization efficiency $η\lesssim10^{-3}$ is needed to satisfy observational constraints on the kilonova. Furthermore, our model is able to reproduce a late-time shallow decay in the X-ray lightcurve and predicts that the X-ray and radio flux will continue to decline in the coming years.

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