Limits on Optical Counterparts to the Repeating FRB 20180916B from High-speed Imaging with Gemini-N/'Alopeke

TL;DR

This study used high-speed optical imaging with Gemini-N/'Alopeke to search for optical counterparts to repeating FRB 20180916B, setting stringent upper limits on optical emission and constraining progenitor models.

Contribution

First to place the most sensitive optical-to-radio fluence ratio limits on repeating FRBs at millisecond timescales, ruling out certain progenitor models.

Findings

No significant optical detections at the FRB position.

Set upper limits on optical fluence <8.3e-3 and <7.7e-3 Jy ms.

Optical-to-radio fluence ratio eta < 3e-3, constraining progenitor models.

Abstract

We report on contemporaneous optical observations at ~10 ms timescales from the fast radio burst (FRB) 20180916B of two repeat bursts (FRB 20201023, FRB 20220908) taken with the 'Alopeke camera on the Gemini North telescope. These repeats have radio fluences of 2.8 and 3.5 Jy ms, respectively, approximately in the lower 50th percentile for fluence from this repeating burst. The 'Alopeke data reveal no significant optical detections at the FRB position and we place 3-sigma upper limits to the optical fluences of <8.3e-3 and <7.7e-3 Jy ms after correcting for line-of-sight extinction. Together, these yield the most sensitive limits to the optical-to-radio fluence ratio of an FRB on these timescales with eta < 3e-3 by roughly an order of magnitude. These measurements rule out progenitor models where FRB 20180916B has a similar fluence ratio to optical pulsars similar to the Crab pulsar or optical emission is produced as inverse Compton radiation in a pulsar magnetosphere or young supernova remnant. Our ongoing program with 'Alopeke on Gemini-N will continue to monitor repeating FRBs, including FRB 20180916B, to search for optical counterparts on ms timescales.