2025ApJ...990...93G

Fast blue optical transients (FBOTs) are luminous, rapidly evolving events with blue spectra, possibly powered by newborn magnetars and linked to fast radio bursts (FRBs). Given this potential connection, we conducted deep radio observations of two nearby FBOTs (AT2018cow and CSS161010) using the Five-hundred-meter Aperture Spherical radio Telescope, but detected no FRB-like signals. Our observations establish the most stringent upper limits on millisecond radio transients from FBOTs, reaching ∼10 mJy flux density. Assuming a log-normal luminosity function analogous to the repeating FRB 121102, we constrain the burst rate from potential magnetars in FBOTs to < 0.01 hr^–1. The short ejecta escape timescale (∼2.6 yr) compared to our observation epochs (4-6 yr post-explosion) suggests that nondetection may not be attributed to FBOT's ejecta absorption. These findings impose useful constraints on the FRB activity emanating from newborn magnetars within FBOTs. They indicate that if there is a burst phase, it is either characterized by weaker bursts, occurs less frequently compared to those in known repeating FRB sources, or takes place beyond the time frame of our current observations. To gain deeper insights into the birth-related activity of magnetars, it is of importance to conduct timely and sustained FRB searches in FBOTs that emerge in the future.