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Highly polarized microstructure from the repeating FRB 20180916B


Fast radio bursts (FRBs) are bright, coherent, short-duration radio transients of as-yet unknown extragalactic origin. FRBs exhibit a variety of spectral, temporal and polarimetric properties that can unveil clues into their emission physics and propagation effects in the local medium. Here, we present the high-time-resolution (down to 1 μs) polarimetric properties of four 1.7 GHz bursts from the repeating FRB 20180916B, which were detected in voltage data during observations with the European Very Long Baseline Interferometry Network. We observe a range of emission timescales that spans three orders of magnitude, with the shortest component width reaching 3–4 μs (below which we are limited by scattering). We demonstrate that all four bursts are highly linearly polarized (80%), show no evidence of significant circular polarization (15%), and exhibit a constant polarization position angle (PPA) during and between bursts. On short timescales (100 μs), however, there appear to be subtle PPA variations (of a few degrees) across the burst profiles. These observational results are most naturally explained in an FRB model in which the emission is magnetospheric in origin, in contrast to models in which the emission originates at larger distances in a relativistic shock.

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Fig. 1: Burst profiles at 16 μs and 1 μs time resolution for four 1.7 GHz bursts from FRB 20180916B.
Fig. 2: Microsecond structure in B4-sb2 consistent with amplitude-modulated noise.
Fig. 3: Polarimetric burst profiles and PPAs for four 1.7 GHz bursts from FRB 20180916B.
Fig. 4: Microsecond-resolution burst polarization profiles and PPAs for B3 and B4.

Data availability

The data that support the plots and results in this study are available at or from the corresponding author upon reasonable request.

Code availability

The code used to analyse the data and create the figures in this work can be found at


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We thank P. Kumar, B. Metzger, L. Sironi, M. Lyutikov, M. van der Klis and P. Uttley for helpful discussions. The EVN is a joint facility of independent European, African, Asian and North American radio astronomy institutes. Scientific results from data presented in this publication are derived from EVN project code EM135. This work was also based on simultaneous EVN and PSRIX data-recording observations with the 100 m telescope of the Max-Planck-Institut für Radioastronomie at Effelsberg, and we thank the local staff for this arrangement. J.W.T.H. acknowledges funding from the Netherlands Organisation for Scientific Research under a Vici grant (‘AstroFlash’, VI.C.192.045). F.K. acknowledges support from the Swedish Research Council. B.M. acknowledges support from the Spanish Ministerio de Economía y Competitividad (MINECO) under grant no. AYA2016-76012-C3-1-P and from the Spanish Ministerio de Ciencia e Innovación under grant nos. PID2019-105510GB-C31 and CEX2019-000918-M of ICCUB (Unidad de Excelencia ‘María de Maeztu’ 2020–2023).

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K.N. discovered the signals, led the data analysis, made the figures, and wrote most of the manuscript. J.W.T.H. guided the work and made important contributions to the writing and interpretation. A.K. performed pre-processing of the EVN voltage data. All other authors contributed significantly to aspects of the data acquisition, analysis strategy or interpretation.

Corresponding author

Correspondence to K. Nimmo.

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Extended data

Extended Data Fig. 1 Autocorrelation functions (ACFs) and power spectra for B4-sb1 and B3.

The temporal profiles of bursts B4-sb1 (top frame) and B3 (bottom frame) are shown in the top left of each frame (panels a and e). The corresponding ACF of these temporal profiles are shown in panels b and f. The corresponding power spectra of the temporal profiles are shown in panels c and g, and the orange line is the power spectrum downsampled in frequency by a factor of 3. Overplotted in pink on the power spectrum is the best fit power law plus white noise component. Panels d and h are the residuals (2 × power spectrum/best fit model).

Extended Data Fig. 2 Before and after calibrating the polarisation data of PSR B2111+46.

The average polarisation profiles (panels b and d) and polarisation position angle (panels a and c) of PSR B2111+46. Black represents the Stokes I profile, red is the unbiased linear polarisation profile (defined in Everett & Weisberg29, and rewritten here in Equation 1), and blue is the circular polarisation (Stokes V) profile. Panels a and b show the polarisation profile and position angle after Faraday-correcting to the true rotation measure79 of PSR B2111+46 (-218.7 rad m−2); here we are not correcting for the instrumental delay between polarisation hands. Panels c and d are Faraday-corrected with the rotation measure determined using the PSRCHIVE tool rmfit, which, in essence, accounts for the instrumental delay. For comparison, we plot the profile and position angle from the literature using more transparent colours80. This illustrates the calibration we applied to the bursts from FRB 20180916B.

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Nimmo, K., Hessels, J.W.T., Keimpema, A. et al. Highly polarized microstructure from the repeating FRB 20180916B. Nat Astron 5, 594–603 (2021).

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