Limits on Simultaneous and Delayed Optical Emission from Well-Localized Fast Radio Bursts

We present the largest compilation to date of optical observations during and following fast radio bursts (FRBs). The data set includes our dedicated simultaneous and follow-up observations, as well as serendipitous archival survey observations, for a sample of 15 well-localized FRBs, including 8 repeating and 7 one-off sources. Our simultaneous (and nearly simultaneous with a 0.4-sec delay) optical observations of 7 (1) bursts from the repeating FRB 20220912A provide the deepest such limits to date for any extragalactic FRB, reaching a luminosity limit of νL_ν ≤ 10⁴² erg s^-1 (≤ 2×10⁴¹ erg s^-1); these observations are also the deepest to date in terms of optical flux to radio fluence ratio of f_opt/F_radio ≤ 10^-7 ms^-1 (≤ 10^-8 ms^-1), and place a limit on the flux ratio of f_opt/f_radio ≤ 0.02 on a msec timescale or ≤ 2×10^-5 (≤ 10^-6) on a sec timescale. These simultaneous limits provide useful constraints in the context of FRB emission models, such as the pulsar magnetosphere and pulsar nebula models. Interpreting all available optical limits in the context of the FRB synchrotron maser model, we find that they constrain the flare energies to ≤10⁴³-10⁴⁷ erg (depending on the distances of the various repeating FRBs, with ≤10³⁹ erg for SGR 1935+2154). These limits are generally at least an order of magnitude larger than the energies inferred from the FRBs themselves, although in the case of FRB 20220912A our simultaneous and rapid follow-up observations severely restrict the model parameter space. We conclude by exploring the potential of future rapid response and simultaneous observations with large optical telescopes.