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arXiv 2203.05396astro-ph.HE
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作为脉冲星射电辐射候选者的外部驱动等离子体模型

Externally-driven plasma models as candidates for pulsar radio emission

Sk. Minhajur Rahaman, Dipanjan Mitra, George I. Melikidze

摘要:

来自脉冲星的相干射电辐射起源于沿脉冲星开放磁场线流动的超相对论性和强磁化电子-正电子对等离子体中的激发等离子体波。传统的相干射电辐射模型依赖于这种对等离子体的不稳定性。最近有人提出了替代模型。这些模型呼吁直接耦合的外部电磁场的超光速O-模式($lt_2$模式)的时间依赖的对级联过程中的极隙。这项工作的目的是提供通用的约束等离子体模型的基础上$lt_2$模式使用现实的脉冲星参数。我们发现,非常短的时间尺度与对级联不允许$lt_{2}$模式被激发在射频和脉冲能量转移只能增加动力学传播(“温度”)的对等离子体粒子。此外,在均匀等离子体条件下,O模式的两个分支上的等离子体波(即,超光速$lt_2$和亚光速$lt_1$)不能逃离等离子体。在强磁化对等离子体中,只有非常模($t$模)能自由逃逸。我们表明,任何一般的虚拟机制不会导致波电场的$t$模式,以具有优势的取向平行或垂直于磁场平面观察。这种虚构的机制将不可避免地导致信号的去极化,并且不能解释在双极星中观察到的高度极化的单脉冲。我们建议相干曲率辐射作为一个有前途的候选脉冲星射电发射机制。

英文摘要:

Coherent radio emission from pulsars originates from excited plasma waves in an ultra-relativistic and strongly magnetized electron-positron pair plasma streaming along the open magnetic field lines of the pulsar. Traditional coherent radio emission models have relied on instabilities in this pair plasma. Recently alternative models have been suggested. These models appeal to direct coupling of the external electromagnetic field to the superluminal O-mode ($lt_2$ mode) during the time-dependent pair cascade process at the polar gap. The objective of this work is to provide generic constraints on plasma models based on $lt_2$ mode using realistic pulsar parameters. We find that the very short timescale associated with pair cascades does not allow $lt_{2}$ mode to be excited at radio frequencies and the impulsive energy transfer can only increase the kinetic spread ("temperature") of the pair plasma particles. Moreover, under homogeneous plasma conditions, plasma waves on both branches of O-mode (i.e. superluminal $lt_2$ and subluminal $lt_1$) cannot escape the plasma. In the strongly magnetized pair plasma, only the extraordinary mode ($t$ mode) can escape freely. We show that any generic fictitious mechanisms does not result in the wave electric field of $t$ mode to have predominant orientation either parallel or perpendicular to the magnetic field plane as observed. Such fictitious mechanisms will inevitably lead to depolarization of signals and cannot account for the highly polarized single pulses observed in pulsars. We suggest coherent curvature radiation as a promising candidate for pulsar radio emission mechanism.

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