A HyperFlash and ÉCLAT view of the local environment and energetics of the repeating FRB 20240619D

TL;DR

The study conducts a high-cadence, multi-telescope monitoring campaign of FRB 20240619D with HyperFlash and ÉCLAT, capturing 217 bursts over >500 hours to probe the local magneto-ionic environment via DM and RM variability, scattering, and scintillation. The analysis reveals a dynamic, dense environment with a measured parallel magnetic field of $B_{\parallel} \approx 0.27\pm0.13$ mG, two distinct scintillation screens (Galactic and extragalactic/host), and significant intra-burst DM drift consistent with plasma lensing or height-dependent emission. The burst energy distribution shows a break near $\mathcal{F} \sim 25$ Jy ms, supporting previous evidence that hyperactive repeaters exhibit a break in their energy distributions, and providing redshift constraints $z<0.37$ from the Macquart relation and $z=0.042$–$0.240$ from the energy-break method. Together, these results place FRB 20240619D in a dense, turbulent magnetised environment with propagation effects that illuminate the circumsource medium and offer avenues for future host identification and modeling of FRB emission mechanisms.

Abstract

Time-variable propagation effects provide a window into the local plasma environments of repeating fast radio burst (FRB) sources. Here we report high-cadence observations of FRB 20240619D, as part of the HyperFlash and ÉCLAT programs. We observed for $500$h and detected $217$ bursts, including $10$ bursts with high fluence ($>25$ Jy ms) and implied energy. We track burst-to-burst variations in dispersion measure (DM) and rotation measure (RM), from which we constrain the parallel magnetic field strength in the source's local environment: $0.27\pm0.13$ mG. Apparent DM variations between sub-bursts in a single bright event are interpreted as coming from plasma lensing or variable emission height. We also identify two distinct scintillation screens along the line of sight, one associated with the Milky Way and the other likely located in the FRB's host galaxy or local environment. Together, these (time-variable) propagation effects reveal that FRB 20240619D is embedded in a dense, turbulent and highly magnetised plasma. The source's environment is more dynamic than that measured for many other (repeating) FRB sources, but less extreme compared to several repeaters that are associated with a compact, persistent radio source. FRB 20240619D's cumulative burst fluence distribution shows a power-law break, with a flat tail at high energies. Along with previous studies, this emphasises a common feature in the burst energy distribution of hyperactive repeaters. Using the break in the burst fluence distribution, we estimate a source redshift of $z=0.042$-$0.240$. We discuss FRB 20240619D's nature in the context of similar studies of other repeating FRBs.

Figures (14)

• Figure 1: Left: Evolution of the DM for five bursts observed with Westerbork, plotted over time. The baseband data and presence of microstructure enabled precise DM measurements; see Figure \ref{['fig:dyn_matrix']} for the corresponding dynamic spectra and S/N versus DM fitting curves. The DM varies by up to $0.41$ pc cm$^{-3}$ over a two-month span. Right: RM of ÉCLAT-detected bursts as a function of time. For each day with detections, the RM was measured for two bursts, to check for consistency. We calibrated the data using two different polarization calibration modelling (PCM) files taken on different dates. The four RM values per day are consistent with each other. We find that the absolute RM increases by $\sim$$80$ rad m$^{-2}$ over a period of 40 days.
• Figure 2: The dynamic spectra and time series for five bursts detected by Westerbork that show fine ($\lesssim 100\,\upmu \textrm{s}$) temporal structure. A single DM value does not appropriately correct for the dispersive delay. All bursts have been coherently dedispersed to the indicated DM and we zoom in on the parts of the bursts that show fine temporal structure. The second column displays the burst spectra and time series using the S/N-optimized DM. The time and frequency resolution used for each filterbank is noted in the top panels. For illustrative purposes, The first and third columns show the bursts dedispersed to the best DM minus and plus $0.1$ pc cm$^{-3}$, respectively. Finally, the fourth column shows the Gaussian fits to the S/N versus DM curves. The solid orange lines and the light orange regions indicate the best-fit DM and its uncertainty range. The dotted orange lines mark the $\pm\,0.1$ pc cm$^{-3}$ values used in the first and third columns.
• Figure 3: Burst (B06-dw) detected by Dwingeloo. Due to strong RFI in the data it was not possible to measure any burst properties accurately. The DM used is the same as for burst B06-wb as detected by Westerbork, shown in Figure \ref{['fig:dyn_matrix']}. This detection of this bright FRB demonstrates the observational capabilities of the Dwingeloo telescope.
• Figure 4: The bottom panel shows the dynamic spectrum of B26-NRT. The colour map is scaled logarithmically. This burst has been coherently dedispersed to a DM of $480$ pc cm$^{-3}$, and incoherently dedispersed to a DM of $464.86$ pc cm$^{-3}$ (see Section \ref{['sec:nrt-method']}). The horizontal white lines indicate channels that have been excised due to the presence of RFI. The middle panel shows the frequency-integrated burst profile in black, also on a logarithmic scale. The linear and circular polarization are shown in red and blue, respectively. The top panel shows the probability density function (for each time bin) of the PPA, which varies by less than a few degrees across the burst duration. The time resolution here has been downsampled by a factor of 16, to account for the DM smearing ($283$$\upmu$s in the lowest-frequency channel) due to the difference between the DM that was used for coherent dedispersion and the true DM of the burst.
• Figure 5: Time series (top panels) and dynamic spectra (bottom panels) for the brightest burst simultaneously detected by Nançay (B26-NRT) and Westerbork (B01-Wb). The orange patches indicate the $128$ MHz frequency range partially overlapping between the observing setups. The top-left panels show the plotted time and frequency resolutions as well as the applied DM. B26-NRT data was only incoherently dedispersed (between channels), whereas B01-Wb data was also coherently dedispersed (within channels). Although the applied DM values differ slightly between bursts, they remain consistent within the measurement uncertainties (Figures \ref{['fig:dyn_matrix']} and Appendix \ref{['fig:nrt_dm_opt']}). White vertical lines indicate channels zapped due to RFI or are channels at the subband edges.