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Discovery of Bimodal Drift Rate Structure in FRB 20240114A: Evidence for Dual Emission Regions

Santosh Arron

Abstract

We report the discovery of bimodal structure in the drift rate distribution of upward-drifting burst clusters from the hyperactive repeating fast radio burst FRB 20240114A. Using unsupervised machine learning (UMAP dimensionality reduction combined with HDBSCAN density-based clustering) applied to 233 upward-drifting burst clusters from the FAST telescope dataset, we identify a distinct subpopulation of 45 burst clusters (Cluster C1) with mean drift rates 2.5x higher than typical upward-drifting burst clusters (245.6 vs 98.1 MHz/ms). Gaussian mixture modeling reveals strong evidence for bimodality (delta-BIC = 296.6), with clearly separated modes (Ashman's D = 2.70 > 2) and a statistically significant gap in the distribution (11.3 sigma). Crucially, we demonstrate that this bimodality persists when restricting the analysis to single-component (U1) burst clusters only (delta-BIC = 19.9, Ashman's D = 2.71), confirming that the result is not an artifact of combining single- and multi-component burst clusters with different drift rate definitions. The extreme-drift subpopulation also exhibits systematically lower peak frequencies (-7%), shorter durations (-29%), and distinct clustering in multi-dimensional feature space. These findings are suggestive of two spatially separated emission regions in the magnetosphere, each producing upward-drifting burst clusters with distinct physical characteristics, although confirmation requires observations from additional epochs and sources.

Discovery of Bimodal Drift Rate Structure in FRB 20240114A: Evidence for Dual Emission Regions

Abstract

We report the discovery of bimodal structure in the drift rate distribution of upward-drifting burst clusters from the hyperactive repeating fast radio burst FRB 20240114A. Using unsupervised machine learning (UMAP dimensionality reduction combined with HDBSCAN density-based clustering) applied to 233 upward-drifting burst clusters from the FAST telescope dataset, we identify a distinct subpopulation of 45 burst clusters (Cluster C1) with mean drift rates 2.5x higher than typical upward-drifting burst clusters (245.6 vs 98.1 MHz/ms). Gaussian mixture modeling reveals strong evidence for bimodality (delta-BIC = 296.6), with clearly separated modes (Ashman's D = 2.70 > 2) and a statistically significant gap in the distribution (11.3 sigma). Crucially, we demonstrate that this bimodality persists when restricting the analysis to single-component (U1) burst clusters only (delta-BIC = 19.9, Ashman's D = 2.71), confirming that the result is not an artifact of combining single- and multi-component burst clusters with different drift rate definitions. The extreme-drift subpopulation also exhibits systematically lower peak frequencies (-7%), shorter durations (-29%), and distinct clustering in multi-dimensional feature space. These findings are suggestive of two spatially separated emission regions in the magnetosphere, each producing upward-drifting burst clusters with distinct physical characteristics, although confirmation requires observations from additional epochs and sources.
Paper Structure (12 sections, 4 figures)

This paper contains 12 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Data and Methods
  3. Dataset
  4. Unsupervised Clustering
  5. Bimodality Testing
  6. Results
  7. Discovery of the Extreme-Drift Subpopulation
  8. Bimodality Confirmation
  9. Single-Component Consistency Check
  10. Robustness
  11. Discussion
  12. Conclusions

Figures (4)

  • Figure 1: UMAP dimensionality reduction of 978 burst clusters colored by drift type (left: blue=Up, red=Down) and by HDBSCAN cluster assignment (right). Cluster C1 (45 burst clusters) forms a distinct island of exclusively upward-drifting burst clusters identified through multi-dimensional density-based clustering.
  • Figure 2: Characterization of Cluster C1 (green) compared to other upward-drifting burst clusters (blue) and downward-drifting burst clusters (red). C1 occupies a distinct region in bandwidth-energy and duration-drift parameter spaces.
  • Figure 3: Bimodality analysis of upward-drifting burst cluster drift rates. Top left: histogram with 2-component Gaussian mixture model fit. Top right: Q-Q plot showing deviation from unimodal normal distribution. Bottom panels: gap analysis and KDE comparison between upward- and downward-drifting populations.
  • Figure 4: Robustness validation of the extreme-drift cluster C1. The cluster is consistently identified across different UMAP parameters (top left), HDBSCAN parameters (top center), and bootstrap resamples (top right). Statistical tests confirm significant property differences (bottom).