Chromatic periodic activity down to 120 MHz in a Fast Radio Burst

TL;DR

This study demonstrates that the repeating FRB 20180916B emits detectable signals down to 120 MHz, challenging previous assumptions about low-frequency emission and revealing new insights into its activity window and environment.

Contribution

The paper provides the first evidence of FRB emission at frequencies as low as 120 MHz and shows that its activity window is narrower and earlier at higher frequencies, contrary to existing models.

Findings

FRB 20180916B emits down to 120 MHz.

Its activity window is narrower and earlier at higher frequencies.

Low-frequency emission can escape the local medium.

Abstract

Fast radio bursts (FRBs) are extragalactic astrophysical transients whose brightness requires emitters that are highly energetic, yet compact enough to produce the short, millisecond-duration bursts. FRBs have thus far been detected between 300 MHz and 8 GHz, but lower-frequency emission has remained elusive. A subset of FRBs is known to repeat, and one of those sources, FRB 20180916B, does so with a 16.3 day activity period. Using simultaneous Apertif and LOFAR data, we show that FRB 20180916B emits down to 120 MHz, and that its activity window is both narrower and earlier at higher frequencies. Binary wind interaction models predict a narrower periodic activity window at lower frequencies, which is the opposite of our observations. Our detections establish that low-frequency FRB emission can escape the local medium. For bursts of the same fluence, FRB 20180916B is more active below 200 MHz than at 1.4 GHz. Combining our results with previous upper-limits on the all-sky FRB rate at 150 MHz, we find that there are 3-450 FRBs/sky/day above 50 Jy ms at 90% confidence. We are able to rule out the scenario in which companion winds cause FRB periodicity. We also demonstrate that some FRBs live in clean environments that do not absorb or scatter low-frequency radiation.