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arXiv 2008.08659astro-ph.HE
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Super-QED磁场中的成对等离子体和磁星的硬X射线/光发射

Pair Plasma in Super-QED Magnetic Fields and the Hard X-ray/Optical Emission of Magnetars

Christopher Thompson, Alexander Kostenko

摘要:

本文计算了在超强磁场存在下,跨相对论对等离子体发射的光子谱,发现它与静止磁星的硬X射线谱有着显著的相似之处。这种发射是由对湮灭,与弱磁化对等离子体相比,显示出类似于韧致辐射的扩展低频尾部。在$10α_{\rm em}^{-1}B_{\rm Q} \gg B \gg B_{\rm Q}$体系中,电子-正电子湮灭/散射截面、双光子对产生截面和光子-$e^\pm$散射截面均采用我们早期的从头算QED计算结果。仔细注意的$u$通道散射共振。磁层弧锚定在强烈的地壳剪切区,延伸到磁星半径的两倍左右,被确定为持久的硬X射线发射的网站。我们推导出一个新的和稳定的磁层电路的配置,与高等离子体密度维持欧姆加热和原位对创建。对来源于非本地的光子碰撞在弱电流区,如极冠。湮灭韧致辐射延伸到光学红外波段,等离子体截止的位置。磁星的上层大气经历了强电流驱动的离子声湍流的增长,这可能会限制正电子的扩散。相干光红外辐射的约束附近所观察到的通量诱导散射。这个模型适应快速X射线增亮的激活磁星,集中的热热点,和次优势的热X射线发射的一些活动磁星。电流注入归因于连续磁性编织,如Thompson,Yang和Ortiz的全局屈服计算中所见。

英文摘要:

The photon spectrum emitted by a transrelativistic pair plasma is calculated in the presence of an ultrastrong magnetic field, and is shown to bear a remarkable resemblance to the rising hard X-ray spectra of quiescent magnetars. This emission is powered by pair annihilation which, in contrast with a weakly magnetized pair plasma, shows an extended low-frequency tail similar to bremsstrahlung. Cross sections for electron-positron annihilation/scattering, two-photon pair creation, and photon-$e^\pm$ scattering are adopted from our earlier ab initio QED calculations in the regime $10α_{\rm em}^{-1}B_{\rm Q} \gg B \gg B_{\rm Q}$. Careful attention is given to the $u$-channel scattering resonance. Magnetospheric arcades anchored in zones of intense crustal shear and extending to about twice the magnetar radius are identified as the sites of the persistent hard X-ray emission. We deduce a novel and stable configuration for the magnetospheric circuit, with a high plasma density sustained by ohmic heating and in situ pair creation. Pairs are sourced non-locally by photon collisions in zones with weak currents, such as the polar cap. Annihilation bremsstrahlung extends to the optical-IR band, where the plasma cutoff is located. The upper magnetar atmosphere experiences strong current-driven growth of ion-acoustic turbulence, which may limit positron diffusion. Coherent optical-IR emission is bounded near the observed flux by induced scattering. This model accommodates the rapid X-ray brightening of an activating magnetar, concentrated thermal hotspots, and the subdominant thermal X-ray emission of some active magnetars. Current injection is ascribed to continuous magnetic braiding, as seen in the global yielding calculations of Thompson, Yang & Ortiz.

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