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arXiv 2410.04065astro-ph.HE
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具有窄波束角的快速射电暴可以从磁星磁层中逃逸

Fast Radio Bursts with Narrow Beaming Angles Can Escape from Magnetar Magnetospheres

Yu-Chen Huang, Zi-Gao Dai

AI总结:

本研究证明窄波束角(约10^-2弧度或更小)的快速射电暴可显著减少磁层等离子体散射和双光子湮灭,从而成功逃逸磁星磁层,支持其磁层起源假说。

摘要:

快速射电暴(FRBs)是在射电波段观测到的毫秒级瞬变现象,其起源和辐射机制至今仍不清楚。越来越多的证据表明,至少部分FRBs起源于磁星,并可能在这些高磁化中子星的磁层内产生。然而,最近的一项研究表明,起源于磁星磁层的FRBs会被磁层中的电子-正电子对等离子体散射,使其无法成功逃逸。在本文中,我们首先证明,如果FRB传播方向与背景磁场之间的夹角约为$10^{-2}$弧度或更小,散射效应可以大大减弱。当夹角约为$10^{-1}$弧度时,FRB的集束效应在减少散射方面变得显著。这类FRB具有较小的横向空间尺寸,这有助于它们瞬间将前沿等离子体横向推出辐射区域。这显著减轻了FRB引起的双光子湮灭反应$γ+γ\to e^{-}+e^{+}$,该反应先前被认为是阻碍FRB传播的关键因素。对于各向同性光度$L_{\text{iso}}\sim 10^{42}\text{ erg s}^{-1}$且在磁星磁层中发射半径$r_{\text{em}}\lesssim 10^9\text{ cm}$的FRB,我们发现了一个临界辐射锥半开角,介于$10^{-3}-10^{-2}$弧度之间。较小的波束角和较大的发射半径更有利于FRB在磁层中的传播。我们的结果支持FRBs可能起源于磁星磁层的观点。

英文摘要:

Fast radio bursts (FRBs) are millisecond duration transients observed in the radio band, with their origin and radiation mechanism remaining unclear to date. Growing evidence indicates that at least some FRBs originate from magnetars and are likely generated within the magnetospheres of these highly magnetized neutron stars. However, a recent study suggested that FRBs originating from magnetar magnetospheres would be scattered by magnetospheric electron--positron pair plasma, making it impossible for them to escape successfully. In this paper, we first demonstrate that the scattering effect can be greatly attenuated if the angle between the FRB propagation direction and the background magnetic field is $\sim10^{-2} \text{ rad}$ or smaller. When the angle is around $10^{-1} \text{ rad}$, the beaming effect of FRBs becomes significant in reducing scattering. Such FRBs have small transverse spatial sizes, which can help them instantly push the front plasma laterally out of the radiation region. This significantly mitigates the FRB-induced two-photon annihilation reaction, $γ+γ\to e^{-}+e^{+}$, which was previously regarded as a key factor hindering the propagation of FRBs. A critical radiation cone half-opening angle between $10^{-3}-10^{-2}\text{ rad}$ is found for an FRB with isotropic luminosity $L_{\text{iso}}\sim 10^{42}\text{ erg s}^{-1}$ and emitted at a radius $r_{\text{em}}\lesssim 10^9\text{ cm}$ in the magnetosphere of a magnetar. Smaller beaming angles and larger emission radii can be more advantageous for the propagation of FRBs in magnetospheres. Our result supports the scenario that FRBs could originate from magnetar magnetospheres.

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