FRB 20210601A: Extreme Temporal Clustering in a Repeating Fast Radio Burst Source as Evidence for a Hidden Class of Episodic Repeaters

Abstract

Population studies of fast radio burst (FRB) repetition typically model sources using a single distribution of steady, time-averaged burst rates, yet individual sources show significant departures from this assumption. Here we present FRB 20210601A, a source discovered by the Canadian Hydrogen Intensity Mapping Experiment FRB project (CHIME/FRB) that represents the most extreme such departure yet identified. The source produced at least nine bursts within a single 106.5 s interval – the highest instantaneous burst rate yet reported from a repeating FRB source at $\gtrsim 2$ Jy ms fluences – yet remained otherwise undetected despite $\sim 320$ hr of exposure spanning 7 yr. The active interval occurred while the source was in a side-lobe of the CHIME/FRB digitized beams, implying an instantaneous burst rate $\gtrsim2.3\times$ higher ($\gtrsim$ 21 bursts) at main-lobe sensitivity. The source's intense, brief burst storm flanked by apparent quiescence despite substantial exposure is unique among the CHIME/FRB repeater sample, although its burst morphology, excess dispersion measure ($\rm{DM_{excess}} \sim 360$ pc cm$^{-3}$), Faraday rotation measure ($|\rm{RM_{excess}}| \sim 150-170$ rad m$^{-2}$), and temporal scattering ($τ\lesssim 0.37$ ms at 600 MHz) are otherwise typical. Modeling a population of episodically active repeaters (i.e., short burst storms separated by long recurrence times) and accounting for declination-dependent exposure, we forecast $\sim1-4$ additional source discoveries similar to FRB 20210601A over the next decade of CHIME/FRB operations, corresponding to a CHIME/FRB-detectable population of $9^{+200}_{-8}$ sources (68% credible interval). These results demonstrate that highly episodic repeaters could constitute a selection-biased but detectable component of the FRB population, possibly reflecting distinct magnetospheric states or triggering mechanisms.