Signatures of Two Distinct Epochs of FRB 20240114A from January to August 2024 Based on its Energy and Waiting Time Analysis

Abstract: A comprehensive analysis of the energy and waiting time distributions of the bursts from FRB 20240114A detected by the Five-hundred-meter Aperture Spherical Radio Telescope between 28 January and 29 August 2024 is presented. For the full sample, its energy distribution cannot be fitted with the simple power-law (SPL),bent power-law (BPL), thresholded power-law (TPL) or Band function models, and its waiting time distribution excluding intervals shorter than 0.5 s cannot be fitted with the Poisson or Weibull models. Nevertheless, for the subsamples with more than 50 bursts in single-day observations, their energy distributions can be fitted with the BPL or TPL models, and their waiting time distributions are better described by a Weibull model. It is noted that the best-fitting BPL parameter $β$ is approximately invariant within the epochs before and after 21 March 2024, with an average of $\bar βb = 1.006 \pm 0.074$ and $\bar β_a = 1.236 \pm 0.183$ (one standard deviation), respectively. Most subsamples from the later epoch have a smaller burst rate parameter $r$ in the Weibull model than those from the earlier epoch. The majority of bursts with $E>10^{39}$ erg occurred in the earlier epoch. The energy distributions in the high-energy range ($> 6\times10^{37}$ erg) differ significantly between the two epochs, and power-law fits to $dN/dE$ yield indices of $-1.97{-0.02}^{+0.02}$ and $-2.34_{-0.06}^{+0.06}$, respectively. The median of the waiting time distribution of the later epoch is larger than that in the earlier epoch. These results suggest that the two epochs may be dominated by different types of bursts, possibly attributed to changes in the physical properties of the emission region.

• The paper demonstrates statistical bimodality in FRB 20240114A's energy and waiting time distributions, indicating two distinct emission epochs.
• Methodologically, it employs unbinned maximum-likelihood estimation over 35 daily epochs to identify a sharp transition in burst energies and rates.
• Findings reveal that post-21 March 2024, a steep energy index and longer waiting times challenge homogeneous Poissonian emission models.

Energy and Waiting Time Bimodality in FRB 20240114A: Evidence for Two Distinct Emission Epochs

Context and Motivation

FRB 20240114A is an extreme repeater discovered by CHIME and subsequently monitored extensively, yielding unparalleled burst statistics via FAST in 2024. The motivation for this study is to resolve whether the source exhibits stable emission characteristics or if its burst energy and temporal statistics reveal temporal variability indicative of distinct physical episodes. Statistical parallels with other hyperactive repeaters such as FRB 20121102A and FRB 20201124A provide a comparative baseline for understanding burst-type variability and the implications for FRB progenitor models.

Data Overview and Methodology

The analysis utilizes 11,553 bursts detected by FAST over a seven-month campaign (January–August 2024) with a mean burst rate of 249 hr⁻¹ and a fluence threshold of 0.026 Jy ms. The burst energy and waiting time ($\Delta t$) distributions are modeled using several competing analytic forms:

• Energy CDF: SPL (Simple Power Law), BPL (Bent Power Law), TPL (Thresholded Power Law), Band function.
• Waiting Time CDF: Poisson process, Weibull process.

For the full dataset, no single model adequately describes the observed distributions. This is evident below.

Figure 1: Cumulative and differential distributions of burst energy and waiting time for the entire FAST FRB 20240114A dataset, including best-fit SPL, BPL, TPL, Band (energy), and Poisson/Weibull (waiting time) models.

Independent unbinned maximum-likelihood estimation further confirms that the full population is not consistent with a unimodal statistical process, suggesting multiple regimes or superposition of emission types.

Temporal Segmentation and Characterization

To address the statistical complexity, the data are segmented into single-day subsamples with at least 50 detected bursts, yielding 35 daily epochs. Within these, the BPL and TPL models describe energy CDFs robustly, except on 2024-03-12, which exhibits pathological deviation. The BPL index $\beta$ for each subsample clusters into two regimes:

• Before 21 March 2024: $\bar{\beta}_b = 1.006 \pm 0.074$
• After 21 March 2024: $\bar{\beta}_a = 1.236 \pm 0.183$

A concurrent shift is found in the Weibull waiting time parameter $r$ (and Poissonian $\lambda$), both decreasing in the post-21 March window, indicating suppressed burst rates in that epoch.

Figure 2: Left: Best-fitting BPL and TPL parameters for energy distributions, highlighting epochal invariance and transition; Right: Evolution of Weibull ($k$, $r$) and Poisson ($\lambda$) parameters, indicating reduced rates post-transition.

Statistical Contrasts: Energy and Waiting Time Distribution Evolution

Kolmogorov–Smirnov tests and probability histograms clarify the dichotomy. Most $E > 10^{39}$ erg bursts occur pre-21 March 2024, and KS statistics on large aggregated subsamples bracketed by this date show highly significant ($\beta$0) contrasts, especially at energies $\beta$1 erg.

Figure 3: Probability histograms of burst energy in six major time-aggregated subsamples. Pre- and post-transition epochs show distinct high-energy tails.

Differential energy distributions above $\beta$2 erg transition from index $\beta$3 (pre-transition) to $\beta$4 (post-transition), consistent with a softening or steepening break.

Figure 4: Differential burst energy distributions pre- and post-21 March 2024. Distinct power-law indices imply hard-to-soft evolution.

Waiting time analyses show the median $\beta$5 increases significantly from 5.87 s to 11.34 s across the two epochs—unlike the waiting time evolution in FRB 20121102A and FRB 20201124A, where the later active phase shortens, here the late phase is less active.

Figure 5: KDE of waiting times for FRB 20240114A, 20121102A, 20201124A across different epochs. Note the contrasting long-wait median in the late epoch for FRB 20240114A.

Interpretation and Theoretical Implications

The pronounced transition in both energy and waiting time statistics of FRB 20240114A, with nearly invariant BPL indices within each segment, is interpreted as evidence for two physically distinct emitting regimes or burst types. The statistical bimodality and epochal partitioning are incompatible with a homogeneous Poissonian emission process from a static underlying engine. Instead, they suggest episodic changes in the emission mechanism, magnetospheric state, or engine conditions, consistent with, but not limited to, magnetar models involving transitions between different magnetospheric or ejecta environments.

The observed transition in energy break, shift in power-law index, and waiting time elongation are not universal among hyperactive repeaters, underscoring the source-dependent nature of the variability. This challenges the general applicability of single-population FRB emission models and suggests an inherent diversity in repeating FRB engines or circumburst medium.

Relation to Broader FRB Population

Comparative analyses with FRB 20121102A and FRB 20201124A reinforce that episodic variability is an emergent property of well-monitored hyperactive repeaters, but FRB 20240114A's specific pattern (activity suppression and high-energy burst deficit in the later phase) is nonuniversal. This, coupled with the documented PMN/Weibull waiting time behavior, argues strongly for population and temporal diversity in the FRB phenomenon.

Future Directions

Further study should focus on:

• High-sensitivity, high-cadence monitoring to resolve intra-epoch substructure and potential further transitions
• Polarimetric and spectral evolution to constrain physical model degeneracies
• Cross-correlation with X-ray/gamma-ray observatories, especially in light of model analogies to magnetar flares

Real-time statistical monitoring is mandated to capture transition epochs and test stochastic versus deterministic duty cycle hypotheses.

Conclusion

This work provides a compelling statistical dissection of FRB 20240114A, establishing two distinct emission epochs with differing energy and temporal properties. The hard boundary at 21 March 2024, characterized by a steepening in energy index and elongation of waiting times, strongly supports a scenario wherein the emission region undergoes a discrete state change. These findings enhance constraints on FRB emission models and underscore the necessity of population-level, high-time-resolution observational campaigns for genuine physical insight.

(2607.01576)