Induced Polarization from Birefringent Pulse Splitting in Magneto-ionic Media

Abstract

Birefringence in ionized, magnetized media is usually measured as Faraday rotation of linearly polarized radiation. However, pulses propagating through regions with very large Faraday rotation measures (RMs) can split into circularly polarized components with measurable differences in arrival times ∝ ν −3 RM, where ν is the radio frequency. Differential refraction from gradients in DM (dispersion measure) and RM can contribute a splitting time ∝ |∇ ⊥ DM||∇ ⊥ RM| ν −5 . Regardless of whether the emitted pulse is unpolarized or linearly polarized, net circular polarization will be measured when splitting is a significant fraction of the pulse width. However, the initial polarization may be inferable from the noise statistics of the bursts. Extreme multipath scattering that broadens pulses can mask splitting effects. We discuss particular cases such as the Galactic center magnetar, J1745−2900, and the repeating fast radio burst source, FRB 121102. Both lines of sight have |RM| ∼ 10 5 rad m −2 that yields millisecond splittings at frequencies well below ∼ 1 GHz. We also consider the splitting of nanosecond shot pulses in giant pulses from the Crab pulsar and the minimal effects of birefringence on precision pulsar timing. Finally, we explore the utility of two-dimensional coherent dedispersion with DM and RM as parameters.

Fast Radio Bursts: An Extragalactic Enigma

“…For a magnetized plasma the leading terms in the frequency-dependent part of the arrival time are (e.g. Tanenbaum et al 1968;Tuntsov 2014;Suresh & Cordes 2019) where terms are included up to second order in (ωp e /ω) 2 and linear in ωB e /ω (where ωp e and ωB e are the electron plasma and cyclotron frequencies, respectively). Each term has an associated line-of-sight integral measure.…”

Sub-second periodicity in a fast radio burst

“…Conversely, we rule out the possibility of |RM| values larger than ∼ 200, 000 rad m −2 by detecting a lack of splitting in the burst morphology potentially caused by extreme RM values. 58 We note that ionospheric RM has not been corrected for in our analysis, but it does not exceed a few rad m −2 .…”

Magnetized filament models for diverging plasma lenses

“…as in eq. 2, except now we follow Suresh & Cordes (2019) and make use of the index of refraction that includes a contribution from the magnetic field…”

“…The + and − signs correspond to right and left-hand circular polarizations respectively. Following Suresh & Cordes (2019), we identify the right-handed circularly polarized wave as having positive helicity. The effective lens potential, in analogy with gravitational lensing, is…”

“…However, this effect is practical to measure only for the the large RMs observed along the line of sight to the Galactic centre magnetar, J1745-2900, and the repeating fast radio burst source FRB121102. Suresh & Cordes (2019) also used a filament lens with a constant magnetic field in the direction of the observer. Due to the small RM of the lenses discussed here, we believe this delay would be too small to measure for our model filaments.…”