Unveiling the origin of fast radio bursts by optical follow-up observations

We discuss how we can detect and identify counterparts of fast radio bursts (FRBs) in future optical follow-up observations of FRBs if real-time alerts for FRBs become available. We consider kilonovae as candidates for FRB optical counterparts, as expected in the case that FRBs originate from mergers of double neutron star binaries. Although theoretical predictions on luminosities of kilonovae are still highly uncertain, recent models suggest that kilonovae can be detected at redshifts up to z ∼ 0.3 within the range of the uncertainties. We expect ∼ 1–5 unrelated supernovae (SNe) down to a similar variability magnitude in a five-day interval within the typical error radius of an FRB. We show, however, that a kilonova can be distinguished from these SNe by its rapid decay and/or color evolution, making it possible to verify the existence of a kilonova associated with an FRB. We also discuss the case that SNe Ia are FRB optical counterparts, as might be the case if FRBs originate from double white dwarf binaries. Verification of this scenario is also possible, since the chance probability of finding a SNe Ia having a consistent explosion time with that of an FRB within the FRB error region is small (typically ≲ 0.01).