Kavli Affiliate: Kiyoshi Masui
| Summary:
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 $gtrsim2.3times$ 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 ($rmDM_excess sim 360$ pc cm$^-3$), Faraday rotation measure ($|rmRM_excess| sim 150-170$ rad m$^-2$), and temporal scattering ($τlesssim 0.37$ 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.
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