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

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 $\gtrsim 2.3\times$ 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 ($\mathrm{D}{\mathrm{M}}_{\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}}\sim 360$ pc cm${}^{-3}$), Faraday rotation measure ($\mathrm{\mid }\mathrm{R}{\mathrm{M}}_{\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}}\mathrm{\mid }\sim 150-170$ rad m${}^{-2}$), and temporal scattering ($\tau \lesssim 0.37$ 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 $\sim 1-4$ additional source discoveries similar to FRB 20210601A over the next decade of CHIME/FRB operations, corresponding to a CHIME/FRB-detectable population of $9_{-8}^{+200}$ 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.