论文标题
来自磁性血浆湍流的硬同步子光谱
Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence
论文作者
论文摘要
天体物理非热源的同步加速器发射通常假设发射颗粒是各向同性的。通过大规模的二维和三维粒子中的模拟，我们证明了配对等离子中磁为主的（$σ_0\ gg1 $）的耗散，导致各向异性粒子分布强烈。在Lorentz因子$ \simσ_0γ_{Th0} $（在这里，$γ_{Th0} $是初始Lorentz因子），粒子速度优先与局部磁场对齐；取而代之的是，最高的能量颗粒是各向同性的。这种依赖能量的各向异性导致同步频谱通量$νf_ν\ propto c n s $比各向同性颗粒要困难得多。 Remarkably, for $σ_0\gg1$ we find that the solid-angle-averaged spectral slope in the slow cooling regime is $s\sim 0.5-0.7$ for a wide range of turbulence fluctuations, $0.25\lesssim δB_{\rm rms0}^2/B_0^2\lesssim 4$, despite significant variations in the power-law energy spectrum非热颗粒。这是因为较弱的湍流水平具有更强程度的各向异性，从而抵消了陡峭的颗粒光谱的影响。如果观察者位于平均磁场的平面中，则同步子光谱斜率可能更难，$ s \ gtrsim 0.7 $。我们的结果与域的大小和维度无关。我们的发现可能有助于解释天体物理非热源的硬同步子光谱的起源，最著名的是脉冲星风云的无线电光谱。
Synchrotron emission from astrophysical nonthermal sources usually assumes that the emitting particles are isotropic. By means of large-scale two- and three-dimensional particle-in-cell simulations, we demonstrate that the dissipation of magnetically-dominated ($σ_0\gg1$) turbulence in pair plasmas leads to strongly anisotropic particle distributions. At Lorentz factors $\sim σ_0 γ_{th0}$ (here, $γ_{th0}$ is the initial Lorentz factor), the particle velocity is preferentially aligned with the local magnetic field; instead, the highest energy particles are roughly isotropic. This energy-dependent anisotropy leads to a synchrotron spectral flux $νF_ν\propto ν^s$ that is much harder than for isotropic particles. Remarkably, for $σ_0\gg1$ we find that the solid-angle-averaged spectral slope in the slow cooling regime is $s\sim 0.5-0.7$ for a wide range of turbulence fluctuations, $0.25\lesssim δB_{\rm rms0}^2/B_0^2\lesssim 4$, despite significant variations in the power-law energy spectrum of nonthermal particles. This is because weaker turbulence levels imprint a stronger degree of anisotropy, thereby counteracting the effect of the steeper particle spectrum. The synchrotron spectral slope may be even harder, $s\gtrsim 0.7$, if the observer is in the plane perpendicular to the mean magnetic field. Our results are independent of domain size and dimensionality. Our findings may help explaining the origin of hard synchrotron spectra of astrophysical nonthermal sources, most notably the radio spectrum of Pulsar Wind Nebulae.
