Transparency of Fast Radio Burst Waves in Magnetar Magnetospheres

Abstract

It is widely believed that at least some fast radio bursts (FRBs) are produced by magnetars. Even though mounting observational evidence points towards a magnetospheric origin of FRB emission, the question of the location for FRB generation continues to be debated. One argument recently suggested against the magnetospheric origin of bright FRBs is that the radio waves associated with an FRB may lose most of their energy before escaping the magnetosphere because the cross-section for 𝑒 ± to scatter large-amplitude EM waves in the presence of a strong magnetic field is much larger than the Thompson cross-section. We have investigated this suggestion in this work, and find that FRB radiation traveling through the open field line region of a magnetar's magnetosphere does not suffer much loss due to two previously ignored factors. First, the plasma in the outer magnetosphere (𝑟 > ∼ 10 9 cm), where the losses are potentially most severe, is likely to be flowing outward at a high Lorentz factor 𝛾 𝑝 ≥ 10 3 . Second, the angle between the wave vector and the magnetic field vector, 𝜃 𝐵 , in the outer magnetosphere is likely of the order of 0.1 radian or smaller due in part to the intense FRB pulse that tilts open magnetic field lines so that they get aligned with the pulse propagation direction. Both these effects reduce the interaction between the FRB pulse and the plasma substantially. We find that a bright FRB with an isotropic luminosity 𝐿 frb > ∼ 10 42 erg s −1 can escape the magnetosphere unscathed for a large section of the 𝛾 𝑝 − 𝜃 𝐵 parameter space, and therefore conclude that the generation of FRBs in magnetar magnetosphere passes this test.

FAST Observations of an Extremely Active Episode of FRB 20201124A. IV. Spin Period Search

“…It is worth mentioning that the above statements apply to pulsar-like emission models of FRBs that invoke emission of FRBs from the open field line regions of magnetars (e.g., Kumar et al 2017;Lu et al 2020;Wang et al 2019;Yang & Zhang 2018). However, since FRBs are much more energetic than radio pulsar emission, an FRB originating from the inner magnetosphere of a magnetar would likely greatly distort the field lines (Qu et al 2022), making the well-defined open field line region fuzzier and enlarging the observed duty cycles. Alternatively, if the FRB emission region is outside of the magnetopshere as some models suggest (e.g., Beloborodov 2020;Metzger et al 2019), then the emission phase could be more random.…”

Circular Polarization of Fast Radio Bursts in the Curvature Radiation Scenario

“…The radiation pressure of FRB emission may dominate over the gravitational pull and push the plasma ahead of it and result in a near vacuum condition (Section 2.2 in Wang et al 2022). Even considering possible plasma effects, the change may be mainly quantitative (Qu et al 2022).…”