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arXiv 0910.5741astro-ph.HE
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Gamma-Ray脉冲星光变曲线的无力磁场模型

Modeling of Gamma-Ray Pulsar Light Curves with Force-Free Magnetic Field

Xue-Ning Bai, Anatoly Spitkovsky

摘要:

(节略)伽马射线辐射的恒星长期以来一直使用真空偶极场模拟。这种近似忽略了磁场结构的变化所造成的磁层等离子体和强大的等离子体电流。我们提出的第一个结果的伽玛射线脉冲星的光变曲线建模使用更现实的领域从三维无力磁层模拟。有了场的几何形状,我们对发射区的位置应用了几个处方,将光变曲线与观测结果进行比较。我们发现,传统的两极焦散模型不能产生双峰脉冲轮廓,主要是因为在无力磁层的极性帽的大小大于真空场极性帽。传统的外间隙模型在一般条件下只能产生一个峰,因为大部分的开场线不穿过零电荷表面。我们提出了一种新的“分界线层”模型,其中的高能量发射起源于一个薄层上的开放场线内的分界线,界定开放的通量管。由于我们称之为“天图停滞”(SMS)的效果,来自该层的发射在天图上产生两个强焦散。这与无力场渐近地接近旋转分裂磁力线的场,以及在外磁层中沿这种磁力线发射的光子同相到达观测者有关。双峰光变曲线是SMS的自然结果。我们表明,目前可用的伽玛射线脉冲星光变曲线的大部分功能可以合理地很好地再现和解释与sepatratrix模型使用的无力场。发射区与电流片的关联将指导今后更详细的磁层加速物理学研究。

英文摘要:

(Abridged) Gamma-ray emission from pulsars has long been modeled using a vacuum dipole field. This approximation ignores changes in the field structure caused by the magnetospheric plasma and strong plasma currents. We present the first results of gamma-ray pulsar light curve modeling using the more realistic field taken from 3D force-free magnetospheric simulations. Having the geometry of the field, we apply several prescriptions for the location of the emission zone, comparing the light curves to observations. We find that the conventional two-pole caustic model fails to produce double-peak pulse profiles, mainly because the size of the polar cap in force-free magnetosphere is larger than the vacuum field polar cap. The conventional outer-gap model is capable of producing only one peak under general conditions, because a large fraction of open field lines does not cross the null charge surface. We propose a novel "separatrix layer" model, where the high-energy emission originates from a thin layer on the open field lines just inside of the separatrix that bounds the open flux tube. The emission from this layer generates two strong caustics on the sky map due to the effect we term "Sky Map Stagnation" (SMS). It is related to the fact that force-free field asymptotically approaches the field of a rotating split monopole, and the photons emitted on such field lines in the outer magnetosphere arrive to the observer in phase. The double-peak light curve is a natural consequence of SMS. We show that most features of the currently available gamma-ray pulsar light curves can be reasonably well reproduced and explained with the sepatratrix model using the force-free field. Association of the emission region with the current sheet will guide more detailed future studies of the magnetospheric acceleration physics.

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