  Radio observations of tidal disruption events (TDEs) - when a star is tidally
disrupted by a supermassive black hole (SMBH) - provide a unique laboratory for
studying outflows in the vicinity of SMBHs and their connection to accretion
onto the SMBH. Radio emission has been detected in only a handful of TDEs so
far. Here, we report the detection of delayed radio flares from an
optically-discovered TDE. Our prompt radio observations of the TDE ASASSN-15oi
showed no radio emission until the detection of a flare six months later,
followed by a second and brighter flare, years later. We find that the standard
scenario, in which an outflow is launched briefly after the stellar disruption,
is unable to explain the combined temporal and spectral properties of the
delayed flare. We suggest that the flare is due to the delayed ejection of an
outflow, perhaps following a transition in accretion states. Our discovery
motivates observations of TDEs at various timescales and highlights a need for
new models.
