Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence
Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence
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DOI：
10.3847/2041-8213/ab93dc
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发表时间：
2020-04
期刊：
影响因子：
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通讯作者：
L. Comisso;E. Sobacchi;L. Sironi
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文献类型：
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作者：
L. Comisso;E. Sobacchi;L. Sironi
来自天体物理非热源的同步加速器发射通常假设发射粒子是各向同性的。通过大尺度的二维和三维细胞内粒子模拟，我们证明了磁主导（）湍流在对等离子体中的耗散导致了强各向异性粒子分布。在洛伦兹因子下（此处为初始洛伦兹因子），粒子速度优先与局部磁场对齐；相反，最高能量的粒子优先定向在垂直于场的平面上。这种能量相关的各向异性导致同步加速器光谱通量比各向同性粒子更难。值得注意的是，我们发现，尽管非热粒子的幂律能谱发生了显著变化，但慢冷却状态下的角积分谱斜率适用于大范围的湍流波动。这是因为较弱的湍流水平印记了较强程度的各向异性，从而抵消了较陡的粒子谱的影响。如果观测者处于与平均磁场垂直的平面上，同步加速器的光谱斜率就更难计算了。我们的结果与域的大小和维数无关。我们的发现可能有助于解释天体物理非热源的硬同步加速器光谱的起源，最值得注意的是脉冲星风星云的无线电频谱。
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 ( ) turbulence in pair plasmas leads to strongly anisotropic particle distributions. At Lorentz factors (here, is the initial Lorentz factor), the particle velocity is preferentially aligned with the local magnetic field; instead, the highest energy particles are preferentially oriented in the plane perpendicular to the field. This energy-dependent anisotropy leads to a synchrotron spectral flux that is much harder than for isotropic particles. Remarkably, for we find that the angle-integrated spectral slope in the slow cooling regime is for a wide range of turbulence fluctuations, , 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, , 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 explain the origin of hard synchrotron spectra of astrophysical nonthermal sources, most notably the radio spectrum of pulsar wind nebulae.