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

TL;DR

FRB 20220413B is analyzed to test whether plasma lensing could produce partial coherence in its morphology. The study combines a cusp-caustic plasma-lens model fit to the frequency-time burst structure with time-lag analyses of complex voltage and frequency-lag analyses of spectra, comparing a coherent lensing scenario to a propagation through a common Galactic scattering screen. While the morphology can be reconciled with a plasma-lens model, the data show no localized phase-delayed DM signature, and scintillation bandwidths are consistent with Milky Way scattering across all components, favoring a common screen with incoherent intrinsic emission rather than fully coherent lensing. The work provides a framework to distinguish lensing from intrinsic emission in FRBs and underscores the importance of phase-coherence diagnostics and higher-frequency tests for detecting coherent lensing along FRB sightlines.

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.

Figures (12)

• Figure 1: The intensity profile of FRB 20220413B at 2.56 $\mu$s resolution dedispersed to the S/N maximizing DM is shown in the bottom panel, while the top panel contains the frequency-averaged intensity profile in units of S/N. The four burst components used in the presented analyses are labeled from left to right as A, B, C, and D. The burst contains many components featuring an upward drift structure and a possible bifurcation of the burst into components A and B around $\sim 550$ MHz. Masked frequency channels are marked with red.
• Figure 2: The fitted lensing model to FRB 20220413B. A 1D Gaussian plasma lens near a cusp caustic is able to replicate the observed frequency-time morphology. Key discrepancies are seen in the observed flux at frequencies $<$ 500 MHz and an apparent dual burst component at frequencies $>$ 600 MHz. This may be due to the emission process or a more complex plasma lens distribution. Masked channels are indicated in red.
• Figure 3: The power of the time-lag correlation for FRB 20220413B is shown in the left panels, while the intensity of the burst is shown in the right panels. The burst shows excess power in the time-lag around $\sim 1-3$ ms that matches the location of time-separated bursts. The frequency average power is shown in the top panels. The intensity profile additionally shows the matched filter (dashed red) while the time-lag power profile shows the measured response (black) and the phase scrambled mock response (blue). Masked frequency channels are marked with red.
• Figure 4: A DM search was applied to the intensity profile (left) and time-lag correlation (right) of FRB 20220413B over 400 - 500 MHz. A coherent DM phase delay will create a delta function response at a specific DM and time-lag if one exists. We find an increase in S/N corresponding to time-separated burst components similar to the DM search of the intensity profile. We do not find an exact DM phase difference between components in the DM search of the time-lag correlation. The zero-lag and $\pm2.56$$\mu$s, $\pm5.12$$\mu$s lags were masked in this search. These lags are known to correlate due to the CHIME channelization process.
• Figure 5: The upchannelized spectra of the four components that compose FRB 20220413B between 400 - 500 MHz are shown in blue, while the smoothed spectra are shown in black. There is a large variation in S/N and spectral structure between the burst components.