Detection of Partial Coherence due to Multipath Propagation for FRB 20220413B with CHIME/FRB

TL;DR

This paper investigates whether the complex structure of FRB 20220413B is due to plasma lensing or scattering, finding evidence for scattering in the Milky Way but not for coherent plasma lensing effects.

Contribution

The study combines morphology fitting and correlation analyses to distinguish between plasma lensing and scattering, providing new insights into FRB propagation effects.

Findings

Burst components are consistent with scattering in the Milky Way.

No phase coherence signature supports scattering over plasma lensing.

Scintillation bandwidth matches Milky Way scattering expectations.

Abstract

Fast radio bursts (FRBs) are a $\sim$ millisecond-long transient phenomenon that propagate across extragalactic distances and are effectively a point source. Radio wave propagation through inhomogeneous distributions of plasma can act as a lens, generating multiple images of the emitted electric field. A lens can produce images of a point source where the phase of the electric field between images remains coherent when observed by a radio telescope. FRB 20220413B shows a complicated pulse structure with time separated components that may be image copies of the main components due to plasma lensing. We perform several analyses to determine if FRB 20220413B is consistent with expectations of a plasma lensed FRB. We analyze and fit the morphology of the burst to a plasma lens model and find consistency in the spectro-temporal profile but not the observed flux. Using the complex-valued channelized voltage data from the CHIME telescope, we perform a time-lag correlation analysis and report correlation signatures present in the electric field of FRB 20220413B. We find that there exists an excess correlation signature only in absolute power and not in phase. We perform a frequency-lag correlation analysis on the spectra of all subcomponents of the burst and find a consistent scintillation bandwidth across all components. We find the scintillation bandwidth is consistent with expectations of scattering due to the Milky Way. We interpret this as all burst components propagating through the same scintillation screen located in the Milky Way, which would generate the excess variance signature observed, even in the absence of phase coherence between burst components. We find that while the burst morphology can be modeled by a plasma lens, the coherent signature present in the time-lag correlation is consistent with the expectations of a common scattering screen, but not coherent plasma lensing.