A fast radio burst cyclone in technicolour: evidence of plasma lensing

Abstract

Fast radio bursts (FRBs) are bright, energetic, radio pulses of extragalactic origin. A dichotomy has emerged in the population: some produce repeat bursts, while the majority do not. Most repeating sources only show rare repetitions, and none have been studied extensively over the wide bandwidths necessary to disentangle the physical processes that produce emission from distortions to bursts caused by intervening ionised gas. Here we present radio observations of the most active repeating source, FRB 20240114A. Using an ultrawideband receiving system, we have detected 5526 repetitions, revealing an extreme spectral and temporal variability in the burst emission. The bursts exhibit longer-term broadband variations in central emission frequency over multiple months, and narrowband bursts that have correlations in central frequencies on time scales of milliseconds to minutes. The spectral and temporal properties are consistent with the source undergoing magnification by foreground plasma lenses, potentially embedded in a turbulent circumsource medium. This extreme example highlights the role of plasma lenses in the observed properties of burst emission and can explain the diversity in activity and energetics of the entire FRB population.

Figures (22)

• Figure 1: ESE Model for burst storms B4 and B5. Panel A shows the modelled plasma density variations. We use two Gaussian lenses to model burst storms B4 and B5. Panel B shows the ray trace diagram of the refracted waves due to the two lenses. Panel C shows the total effect of the lensing event, which causes an apparent change in the brightness of the source. The x-axis is the transverse scale.
• Figure 1: Subbanded burst rate for FRB 20240114A. The burst rate for four subbands is shown in separate panels. The centre frequencies are derived from the subbanded search pipeline. The purple-shaded region shows the on-source integration time. We show the Poissonian uncertainties in the burst rate for individual epochs.
• Figure 2: An example FRB20240114A burst from the MJD 60486 epoch. Panels A, B, C, and D show the dynamic spectra for Stokes-I, Q, U, and V, respectively, for the brightest burst from MJD 60486. The dynamic spectra have temporal and spectral resolutions of 64$\mu$sec and 0.5 MHz, respectively. Panels E and F show the PA angle and frequency-averaged pulse profile, respectively. The PA angles are shown for time bins with S/N $>$4. The error bars for PA angles are smaller than the individual markers. The frequency-averaged linear and circular polarisation profiles are shown in red and blue, respectively. Frequency channels that have been excised due to RFI contamination are denoted by a red bar.
• Figure 3: Faraday depth profiles of bursts from FRB 20240114A. We show the FDF profiles between -1000 and +1000 rad$\,$m$^{-2}\,$. The green colour vertical line shows the peak value of the clean FDF obtained from rmclean. The bursts \ref{['fig:FRB20240114A_Pol_1']}(a) and \ref{['fig:FRB20240114A_Pol_1']}(b) were detected at MJDs 60645 and 60420, respectively.
• Figure 4: The fluence pulse width relationship. The fluence estimate of the bursts from this study above an S/N of 20 and the estimated pulse width is shown for 20240114A in purple. We also show the pulse width fluence distribution for ASKAP FLYSEYE FRBs Shannon:2018, as well as for repeating FRBs 20121102A Hewitt:2022 and 20190520B Niu:2022. The pulse width for the bursts is estimated using a Bayesian fit to the frequency-averaged Stokes-I profile with a boxcar (see Section \ref{['sec:fluence_width_measurments']}). The histograms are shown in purple, and the purple line denotes the Log-Gaussian fit to the distribution.