Two new highly scattered fast radio bursts: evidence for scatter broadening by the circumsource medium

Two new highly scattered fast radio bursts: evidence for scatter broadening by the circumsource medium

Abstract

We found two highly scattered Fast Radio Bursts (FRBs) during commissioning of the Commensal Realtime ASKAP Fast Transient COherent (CRACO) backend. FRB 240210D and FRB 240312D have scattering times of 34 ± 6 and 300 ± 48 ms, respectively, when scaled to 1 GHz. FRB 240312D originates near a spiral arm of a face-on galaxy at a redshift of only 0.05. Scintillation from a Milky Way screen constrains the distance of the scattering screen to ∼ 10 pc from the source. FRB 240312D is therefore the first highly scattered FRB where scattering screens in the host galaxy centre, a background galaxy, or intervening structures can all be excluded, leaving only the circumsource medium. Integral field spectroscopy of the host reveals a Milky Way-like galaxy with a star-formation region at the FRB position. We find refractive scattering in a pulsar wind nebula as the most likely scattering origin. However, the explanation is not completely satisfactory as it requires a fine-tuned orientation. Hence, additional theoretical studies under different FRB progenitor models are needed. From the two FRBs, we calculate a total rate of Rtot = 210+460−180 events sky−1 day−1 with durations between 55.2 ms and 1 s and above a fluence of 9 Jy ms consistent with the rate of shorter FRBs. This elevated rate suggests that the strong scattering seen in other FRBs likewise does not arise from chance-aligned sightlines, but is instead causally linked to the FRB sources.

Key results and conclusions

• FRB 240312D shows scintillation consistent with a screen in the Milky Way and inconsistent with the scattering screen, implying a two-screen system. The resulting upper limit on the host screen distance of 8 pc (DMW/kpc)−1 and the offset location of the FRB from its host galaxy centre strongly suggest that the scattering originates in the circumburst medium.

• Three points argue in favour of refraction over diffraction as a cause of the scattering: (i) the extent of the screen; (ii) the very low required diffractive scale; and (iii) the implied density variations close to densities where the emission would be free-free absorbed.

• Through integral field spectroscopy, we identify the knot at the centre at the FRB localisation as a star-forming region. A supermassive black hole can be excluded, while other objects like an SNR or a PWN stay below our resolution and cannot be excluded.

• We find the origin of scattering with the most observational support to be the filamentary structure of a few hundred years old PWN, similar to what is seen in the Crab. The main difference from the Crab that causes the much larger scattering must mostly be a more inhomogeneous sightline e.g. through a filament, rather than age or mass in the SNR. Strong magnetar winds as in super luminous SNe where the wind breaks through the shell are less plausible in this scenario.

• Other models that have been proposed for PRSs of FRBs are possible as well within the observational constraints. A hypernebula does not seem implausible either, although scattering has not yet been a focus of the models.

• The finding that the high scattering of FRB 240312D can be traced back to the circumburst medium in this one FRB has important implications for the interpretation of other highly scattered FRBs. What used to be a very hypothetical possibility is now the most likely scenario for the scattering in e.g. FRB 240210D, FRB 200723B (Shin et al. 2025), FRB 221219A (Faber et al. 2024).

• Large scattering from the circumburst medium poses problems for methods using scattering to study the host or Milky Way ISM, or the CGM of intervening haloes. Furthermore, it questions the usability of scattering as an estimate for DMhost, the doubts are enforced by the relatively normal estimated DMhost seen in highly scattered FRBs.

• The rate calculated from the two reported FRBs is consistent with the rate seen at lower widths. This indicates the presence of a large population of highly scattered FRBs.

Improvements to AI Systems:

1. Scattering-Origin Classification Model

• Train a multi-class classifier that distinguishes between scattering screens in the Milky Way, host galaxy, intervening structures, and circumsource medium using multi-wavelength observables (scintillation, DM, redshift, host morphology, spectral index).

• The improved system can automatically flag FRBs where circumsource scattering is likely, reducing manual analysis time for transient surveys.

2. Bayesian Inference for Two-Screen Systems

• Implement a hierarchical Bayesian model that jointly fits scattering time, scintillation bandwidth, and host-galaxy offset to infer screen distances and densities, incorporating priors from pulsar wind nebula (PWN) and supernova remnant (SNR) models.

• The system can output posterior distributions for screen location and physical conditions, enabling rapid classification of new FRBs as circumburst-scattered.

3. Rate–Duration–Fluence Extrapolation Engine

• Build a generative model that predicts the all-sky rate of highly scattered FRBs as a function of scattering time, duration, and fluence, using the observed rate from this paper as a calibration anchor.

• The improved system can forecast detection yields for next-generation instruments (e.g., DSA-2000, CHIME outriggers) and optimize survey strategies for capturing scattered events.

4. PWN Filament Scattering Simulator

• Develop a radiative transfer + plasma lensing simulation that generates synthetic scattering profiles for FRBs passing through Crab-like PWN filaments, varying filament density, orientation, and age.

• The system can produce mock observables (scattering times, scintillation patterns) to test progenitor models and constrain fine-tuning requirements, as highlighted by the paper’s tension with orientation.

5. DMhost–Scattering Correlation Disentangler

• Create a supervised learning tool that separates the contribution of circumsource scattering from host-galaxy DM using a combination of scattering times, DM excess, and host-galaxy properties (star-formation rate, inclination).

• This improves DMhost estimates for highly scattered FRBs, directly addressing the paper’s warning that scattering-based DMhost estimates are unreliable.

6. Automated Host-Galaxy Knot Classifier

• Fine-tune a vision transformer on integral field spectroscopy (IFS) datacubes to classify central knots at FRB positions (star-forming region vs. AGN vs. SNR vs. PWN) with uncertainty quantification.

• The system can rapidly rule out supermassive black holes and flag candidate PWN/SNR origins for follow-up, as done manually for FRB 240312D.

7. Scattering-Linked Population Synthesis

• Integrate the finding that high scattering is causally linked to FRB sources (not chance alignments) into a population synthesis model that jointly predicts FRB rates, scattering distributions, and host-galaxy properties.

• The improved system can simulate full FRB populations to test whether the observed rate of highly scattered FRBs implies a distinct progenitor channel or a universal circumburst medium property.

8. Real-Time Scattering Screen Triangulation

• Implement an online inference pipeline that, upon detection of a new FRB with scintillation, immediately estimates the screen distance using Milky Way scintillation as a reference, and flags events where the screen is within 10 pc of the source.

• This enables rapid follow-up with VLBI or X-ray observations to directly image the scattering region, potentially resolving PWN filaments.

We found two highly scattered Fast Radio Bursts (FRBs) during commissioning of the Commensal Realtime ASKAP Fast Transient COherent (CRACO) backend. FRB 240210D and FRB 240312D have scattering times of 34 plus or minus6 and 300 plus or minus48 ms, respectively, when scaled to 1 GHz. FRB 240312D originates near a spiral arm of a face-on galaxy at a redshift of only 0.05. Scintillation from a Milky Way screen constrains the distance of the scattering screen to about 10 pc from the source. FRB 240312D is therefore the first highly scattered FRB where scattering screens in the host galaxy centre, a background galaxy, or intervening structures can all be excluded, leaving only the circumsource medium. Integral field spectroscopy of the host reveals a Milky Way-like galaxy with a star-formation region at the FRB position. We find refractive scattering in a pulsar wind nebula as the most likely scattering origin. However, the explanation is not completely satisfactory as it requires a fine-tuned orientation. Hence, additional theoretical studies under different FRB progenitor models are needed. From the two FRBs, we calculate a total rate of R tot=210+460-180, events, sky-1 day-1 with durations between 55.2 ms and 1 s and above a fluence of 9 Jy ms consistent with the rate of shorter FRBs. This elevated rate suggests that the strong scattering seen in other FRBs likewise does not arise from chance-aligned sightlines, but is instead causally linked to the FRB sources.