标题： 脉冲星风星云中扩散冲击加速产生的最大难度光谱 显示英文标题

标题： Maximally-hard spectra from diffusive shock-acceleration in pulsar-wind nebulae

摘要： 导致脉冲星风星云（PWNe）异常坚硬的射电光谱的过程尚不清楚。 文献中 $29$ PWNe 的射电光子光谱指数显示出近似正态， $\alpha=0.2\pm0.2$ 分布。 我们提供了 $\sim 3\sigma$ 证据，表明存在一个独特的PWNe子群体，其光谱在终止激波附近较硬，而在其他地方明显较软，可能是由于最近激波周围区域的疏散所致。 此类光谱，尤其是在硬子群体中，暗示了极端情况下的费米过程，如扩散激波加速， $\alpha=0.01\pm0.06$ 极限。 我们表明，当小角度散射足够各向异性时，就会接近这个极限，对于接近激波前缘的粒子，在激波两侧都会增强。 在上游，光谱硬化主要与增强的能量增益有关，这可能由穿过激波的同一批束状粒子驱动。 $\alpha=0$ 下游，主要影响是逃逸概率降低，但这也降低了加速效率，分别为$\lesssim25\%$对$\alpha=0.3$和$\lesssim1\%$对$\alpha=0.03$。 显示英文摘要

摘要： The processes leading to the exceptionally hard radio spectra of pulsar-wind nebulae (PWNe) are not yet understood. Radio photon spectral indices among $29$ PWNe from the literature show an approximately normal, $\alpha=0.2\pm0.2$ distribution. We present $\sim 3\sigma$ evidence for a distinct sub-population of PWNe, with a hard spectrum $\alpha=0.01\pm0.06$ near the termination shock and significantly softer elsewhere, possibly due to a recent evacuation of the shock surroundings. Such spectra, especially in the hard sub-population, suggest a Fermi process, such as diffusive shock acceleration, at its extreme, $\alpha=0$ limit. We show that this limit is approached for sufficiently anisotropic small-angle scattering, enhanced on either side of the shock for particles approaching the shock front. In the upstream, the spectral hardening is mostly associated with an enhanced energy gain, possibly driven by the same beamed particles crossing the shock. Downstream, the main effect is a diminished escape probability, but this lowers the acceleration efficiency to $\lesssim25\%$ for $\alpha=0.3$ and $\lesssim1\%$ for $\alpha=0.03$.