A Nançay Radio Telescope study of the hyperactive repeating FRB 20220912A

TL;DR

This study analyzes 696 bursts from the hyperactive repeating FRB 20220912A using the Nançay Radio Telescope within the ÉCLAT monitoring program. By applying the CATCH classifier to an enhanced search pipeline, the authors derive population-level spectro-temporal properties, fluence and energetics, and dispersion-measure behavior, revealing a mean drift rate of $-8.8$ MHz ms$^{-1}$ and a largely constant DM when accounting for drift. The wait-time distribution is bimodal with peaks at $33.4$ ms and $67.0$ s, while the burst rate across the campaign follows a Weibull distribution with shape $k \approx 0.88$, indicating clustering. The energy distribution shows a high-energy excess relative to a simple power law, and bursts preferentially emit toward the lower end of the observing band, aligning with observations of other hyperactive repeaters and providing new population-level constraints for FRB progenitor theories.

Abstract

The repeating fast radio burst source FRB 20220912A was remarkably active in the weeks after its discovery. Here we report 696 bursts detected with the Nançay Radio Telescope (NRT) as part of the Extragalactic Coherent Light from Astrophysical Transients (ÉCLAT) monitoring campaign. We present 68 observations, conducted from October 2022 to April 2023, with a total duration of 61 hours and an event rate peaking at $75^{+10}_{-9}$ bursts per hour above a fluence threshold of 0.59 Jy ms in the $1.2-1.7$-GHz band. Most bursts in the sample occur towards the bottom of the observing band. They follow a bimodal wait-time distribution, with peaks at 33.4 ms and 67.0 s. We find a roughly constant dispersion measure (DM) over time ($δ$DM $\lesssim$ 2 pc cm$^{-3}$) when taking into account `sad-trombone' drift, with a mean drift rate of $-8.8 $MHz ms$^{-1}$. Nonetheless, we confirm small $\sim0.3$ pc cm$^{-3}$ DM variations using microshot structure, while finding that microstructure is rare in our sample -- despite the 16 $μ$s time resolution of the data. The cumulative spectral energy distribution shows more high-energy bursts ($E_ν\gtrsim 10^{31}$ erg/Hz) than would be expected from a simple power-law distribution. The burst rate per observation appears Poissonian, but the full set of observations is better modelled by a Weibull distribution, showing clustering. We discuss the various observational similarities that FRB 20220912A shares with other (hyper)active repeaters, which as a group are beginning to show a common set of phenomenological traits that provide multiple useful dimensions for their quantitative comparison and modelling.

Figures (19)

• Figure 1: A subset of the 696 NRT-detected bursts from FRB 20220912A. This sample consists of very bright bursts, some of which show multiple sub-bursts and/or microsecond-time-scale intensity fluctuations. Each thumbnail shows a dynamic spectrum (bottom-left panel; $\Delta t=16\,\upmu$s and $\Delta \nu=4\,$MHz), frequency-integrated lightcurve (top panel; integrated over the entire frequency band), and time-integrated spectrum (bottom right panel; integrated over the entire time range). All bursts have been coherently dedispersed within each channel to DM = 219.46 pc cm$^{-3}$ during recording. Additionally, each burst has been incoherently dedispersed between channels to the DM value (units pc cm$^{-3}$) reported in the top-right of every thumbnail. Also noted in the top-right of each thumbnail is the burst ID, which corresponds to the entries in Tables \ref{['tab:frbtable']} and \ref{['tab:dmtable']}. Horizontal white lines in the dynamic spectra represent channels removed due to the presence of RFI. For visual purposes, the colour maps of the dynamic spectrum range from the value 0.0 to the 99$^{\rm{th}}$ percentile. The plotted S/N values for these bursts are lower than those indicated in Table \ref{['tab:frbtable']} because this data has not been downsampled, and the S/N is calculated across the entire bandwidth. A detailed analysis of bursts B411, B491, and B582 was previously presented in hewitt2023dense.
• Figure 2: The temporal width distribution for our sample of 696 NRT-detected bursts from FRB 20220912A. The vertical dashed blue line indicates the median at 7.68 ms.
• Figure 3: The frequency extent, $\Delta \nu$, of each detected burst from FRB 20220912A is plotted against its central observed frequency $\nu_{\rm mid}$, and coloured by its fluence. The right and top panels show the distritbutions of $\Delta\nu$ and $\nu_{\rm mid}$, respectively, where the NRT bandpass has been added to the top histogram. The error bars represent the Poissonian uncertainty, with a 1$\sigma$ confidence interval. The dashed black triangular shape in the middle panel represents the limits of our observing band (see the text for more details). The colour of the bins of the side histograms represents the median fluence of the bursts within each bin.
• Figure 4: The burst fluence distribution for completeness thresholds 0.59, 0.69, 0.82 Jy ms, calculated for three temporal widths: 5.63, 7.68 and 10.79 ms. The grey areas indicate the incompleteness regions, whereas the purple areas indicate the 1$\sigma$ fit error of a log-normal function to the distributions. Fit residuals are shown in the smaller panels beneath each fluence distribution.
• Figure 5: Temporal evolution of the spectral energy distribution of FRB 20220912A bursts detected with the NRT. Slashes through Panels A and B indicate a break in the continuity of the plotted timeline. Panel A: The green bars show the duration of each observation, while the solid purple line indicates the cumulative burst count with time. Panel B: The green data points display the spectral energy of each detected burst, while the purple data points show the bursts' median spectral energy on each observing day. The vertical dashed black line indicates the time when CHIME/FRB reported the discovery of FRB 20220912A and when we started observing with NRT. Panel C: The spectral energy distribution, considering all the detected bursts, is shown in green with a log-normal fit and the 1$\sigma$ fit error indicted by the purple line and shaded region. The mean lies at $10^{29.48}$ erg/Hz, with a standard deviation of $10^{0.33}$ erg/Hz. Panel D: The cumulative spectral energy distribution plotted in green and fitted with a power law, show by the black dot-dashed line. The horizontal green lines indicate the theoretical completeness thresholds for various assumed burst widths, assuming a minimal S/N$=7$ detection, as in Figure \ref{['fig:fluences']}. The solid green line indicates the completeness threshold for assumed burst widths at 5.63 ms; the dash-dotted line at 7.68 ms; and the dashed line at 10.79 ms. The distribution is seen to flatten at higher energies, indicating that bursts with these energies happen more frequently than predicted by extrapolating a simple power law based on fitting lower-energy bursts. This effect is less obviously present in Panel C, as several one-burst bins are obscured by the fit line.