化石AGN射流作为超高能粒子加速器
Fossil AGN jets as ultra high energy particle accelerators
摘要：
活动星系核喷流及其周围的茧的残余留下了巨大的磁流体动力学（MHD）化石结构，总能量约为10^{60}尔格。最初的活动星系核（AGN）可能已经死亡，但化石将保持其稳定的结构，类似于实验室MHD实验中遇到的反场箍缩（RFP）。在RFP中，纵向磁场在离轴的临界距离处改变方向，导致那里的磁重联，并导致大尺度RFP场的缓慢衰减。我们表明，这种场的衰减诱导大规模的电场，可以加速宇宙射线与E^{-2}幂律到超高能量与截止取决于化石参数。截止预计是刚性依赖，这意味着观察到的组成将改变从轻到重接近截止，如果一个或两个附近的活动星系核化石占主导地位。考虑到宇宙体积的百分之几可能容纳这种缓慢衰减的结构，这些化石甚至可能重新激发来自遥远/古老来源的超高能宇宙射线，抵消由于与宇宙微波背景辐射的光子相互作用而导致的“GZK损失”，并提供否则无法检测到化石的证据。在这种情况下，如果远距离来源或化石占主导地位，成分将保持轻到最高能量，但否则将混合。人们希望新一代的宇宙射线实验，如皮埃尔俄歇天文台和超高能中微子望远镜，如ANITA和月球切伦科夫实验将澄清这一点。
英文摘要：
Remnants of AGN jets and their surrounding cocoons leave colossal magnetohydrodynamic (MHD) fossil structures storing total energies ~10^{60} erg. The original active galacic nucleus (AGN) may be dead but the fossil will retain its stable configuration resembling the reversed-field pinch (RFP) encountered in laboratory MHD experiments. In an RFP the longitudinal magnetic field changes direction at a critical distance from the axis, leading to magnetic re-connection there, and to slow decay of the large-scale RFP field. We show that this field decay induces large-scale electric fields which can accelerate cosmic rays with an E^{-2} power-law up to ultra-high energies with a cut-off depending on the fossil parameters. The cut-off is expected to be rigidity dependent, implying the observed composition would change from light to heavy close to the cut-off if one or two nearby AGN fossils dominate. Given that several percent of the universe's volume may house such slowly decaying structures, these fossils may even re-energize ultra-high energy cosmic rays from distant/old sources, offsetting the ``GZK-losses'' due to interactions with photons of the cosmic microwave background radiation and giving evidence of otherwise undetectable fossils. In this case the composition would remain light to the highest energies if distant sources or fossils dominated, but otherwise would be mixed. It is hoped the new generation of cosmic ray experiments such as the Pierre Auger Observatory and ultra-high energy neutrino telescopes such as ANITA and lunar Cherenkov experiments will clarify this.
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