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arXiv 0910.5740astro-ph.HE
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用真空偶极磁场模拟Gamma-Ray脉冲星光变曲线

Uncertainties of Modeling Gamma-Ray Pulsar Light Curves with Vacuum Dipole Magnetic Field

Xue-Ning Bai, Anatoly Spitkovsky

摘要:

目前的脉冲星伽玛射线辐射模型使用真空中旋转偶极子的磁场作为充满等离子体的脉冲星磁层形状的第一近似。在本文中,我们重新审视的问题,伽玛射线光变曲线的形成,以确定的“两极焦散”(TPC)和“外间隙”(OG)模型的基础上的真空磁场的鲁棒性。我们指出了一个不一致的文献中使用的相对论性的像差公式,在几个作品中的真空场被视为已知的瞬时共转框架,而不是在实验室的框架。用修正后的公式,我们发现用真空场的TPC模型预测的光变曲线中的峰不那么尖锐。OG模型中峰的锐度受此变化的影响较小,但导致双峰光变曲线的磁倾角和观察几何形状的范围减小。在实际磁层中,由于等离子体效应引起的光柱附近场结构的改变可能会改变极帽的形状和发射区的位置。我们研究了光变曲线对静态和延迟真空偶极场的不同极帽形状的敏感性。特别是,我们认为极冠跟踪的最后一个开放的领域线,并比较它们的圆形极冠。我们发现TPC模型对极帽的形状非常敏感,圆形的极帽可以导致四个发射峰。OG模型受极帽形状的影响较小,但在光柱附近施加真空场时存在较大的不确定性。结果表明,偏离真空场会导致脉冲波形的较大不确定性,在脉冲星高能辐射研究中应采用更符合实际的无力场。

英文摘要:

Current models of pulsar gamma-ray emission use the magnetic field of a rotating dipole in vacuum as a first approximation to the shape of plasma-filled pulsar magnetosphere. In this paper we revisit the question of gamma-ray light-curve formation in pulsars in order to ascertain the robustness of the "two-pole caustic" (TPC) and "outer gap" (OG) models based on the vacuum magnetic field. We point out an inconsistency in the literature on the use of the relativistic aberration formula, where in several works the vacuum field was treated as known in the instantaneously corotating frame, rather than in the laboratory frame. With the corrected formula, we find that the peaks in the light curves predicted from the TPC model using the vacuum field are less sharp. The sharpness of the peaks in the OG model is less affected by this change, but the range of magnetic inclination angles and viewing geometries resulting in double-peaked light curves is reduced. In a realistic magnetosphere, the modification of field structure near the light cylinder due to plasma effects may change the shape of the polar cap and the location of the emission zones. We study the sensitivity of the light curves to different shapes of the polar cap for static and retarded vacuum dipole fields. In particular, we consider polar caps traced by the last open field lines and compare them to circular polar caps. We find that the TPC model is very sensitive to the shape of the polar cap, and a circular polar cap can lead to four peaks of emission. The OG model is less affected by polar cap shapes, but is subject to big uncertainties of applying the vacuum field near the light cylinder. We conclude that deviations from vacuum field can lead to large uncertainties in the pulse shapes, and a more realistic force-free field should be applied to the study of pulsar high energy emission.

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