Estimation of intrinsic fast radio burst width and scattering distributions from CRAFT data

TL;DR

This work addresses how intrinsic FRB width $w_i$ and scattering $\tau$ distributions bias FRB detection and population inferences. By analyzing 29 localized FRBs with redshift from ASKAP/CRAFT and applying completeness-corrected modelling, the authors find no downturn in the intrinsic distributions within 0.01–40 ms: $\tau_{\rm 1\ GHz}$ is consistent with a log-uniform form above $\sim 0.04$ ms, while $w_i$ is Gaussian in log-space between $\sim 0.03$–$0.3$ ms before becoming log-uniform. These results challenge prior lognormal assumptions and are implemented in the zDM population code, which shows that adopting this updated model yields about 10–15% more FRBs at moderate redshifts (e.g., $z\sim1$) than alternative models, highlighting the importance of width/scattering biases in population and cosmological FRB studies. The findings imply strong observational limits on high-width and high-scattering FRBs and suggest that host-galaxy studies may be biased by current detection thresholds, though the exact impact remains uncertain. Overall, the work provides a more accurate framework for FRB population modelling and motivates searches extending to larger intrinsic widths and scattering times.

Abstract

The intrinsic width and scattering distributions of fast radio bursts (FRBs) inform on their emission mechanism and local environment, and act as a source of detection bias and, hence, an obfuscating factor when performing FRB population and cosmological studies. Here, we utilise a sample of 29 FRBs with measured high-time-resolution properties and known redshift, which were detected using the Australian Square Kilometre Array Pathfinder (ASKAP) by the Commensal Real-time ASKAP Fast Transients Survey (CRAFT), to model these distributions. Using this sample, we estimate the completeness bias of intrinsic width and scattering measurements, and fit the underlying, de-biased distributions in the host rest-frame. We find no evidence for a down-turn towards high values of the intrinsic distributions of either parameter in the 0.01-40 ms range probed by the data. Rather, the intrinsic scattering distribution at 1 GHz is consistent with a log-uniform distribution above 0.04 ms, while the intrinsic width distribution rises as a Gaussian in log-space in the 0.03-0.3 ms range, and is then log-uniform above that. This is inconsistent with previous works, which assumed that these parameters have lognormal distributions. This confirms that FRB observations are currently strongly width- and scattering-limited, and we encourage FRB searches to be extended to higher values of time-width. It also implies a bias in FRB host galaxy studies, although the form of that bias is uncertain. Finally, we find that our updated width and scattering model - when implemented in the zDM code - produces 10% more FRBs at redshift $z=1$ than at $z=0$ when compared to alternative width/scattering models, highlighting that these factors are important to understand when performing FRB population modelling.

Figures (6)

• Figure 1: Plot of scattering time at central frequency, $\tau_{\rm obs}$, against signal-to-noise maximising width, $w_{\rm snr}$, for the CRAFT HTR sample with known redshift. The 1-1 line is $w_{\rm snr} = 1.225 \tau_{\rm obs}$. The observed correlation is consistent with observational bias, as discussed in text, and by CHIME_baseband_morphology_2025.
• Figure 2: Illustration of the different fitting functions for the intrinsic distribution of $t=\tau_{\rm 1\,GHz},w_i$ considered in this work; 'sb' stands for 'smoothed boxcar'. These functions are defined in terms of the logarithm base 10 of $t$ in ms, and parameters $t_{\rm min}$, $t_{\rm max}$, $\mu_t$, and $\sigma_t$.
• Figure 3: FRB scattering distributions. (a) The observed distribution of rest-frame scattering normalised to $1$ GHz, $\tau_{\rm 1\,GHz}$, as well as fits to the intrinsic distribution adjusted for the completeness function; (b) intrinsic scattering distribution of FRBs, being the observed distribution adjusted for completeness, compared to intrinsic fitted functions.
• Figure 4: FRB width distributions. (a) The observed distribution of rest-frame intrinsic width, $w_i$, as well as fits to the intrinsic distribution adjusted for the completeness function; (b) intrinsic width distribution of FRBs, being the observed distribution adjusted for completeness, compared to intrinsic fitted functions.
• Figure 5: Relative FRB detection rate as a function of redshift for the ASKAP/CRAFT ICS survey at 1.3 GHz, relative to the best-fit distributions from this work, for different models of FRB scattering and width (see text).