Interaction of an Outflow with Surrounding Gaseous Clouds as the Origin of Late-time Radio Flares in Tidal Disruption Events

Close encounter between a star and a supermassive black hole (SMBH) results in the tidal disruption of the star, known as a tidal disruption event (TDE). Recently, a few TDEs, e.g., ASASSN-15oi and AT2018hyz, have shown late-time (hundreds of days after their UV/optical peaks) radio flares with radio luminosities of 10 erg s. The super-Eddington fallback or accretion in a TDE may generate a mass outflow. Here, we investigate a scenario that the late-time radio flares come from the interaction of the outflow with the circumnuclear gaseous clouds, in addition to the slow-evolving emission component due to the outflow-diffuse medium interaction. We calculate the associated radio temporal and spectral signatures and find that they reproduce well the observations. The outflows have the inferred velocity of 0.2c ∼ 0.6c, the total mass of 10 ∼ 10M and the ejection duration of a month to a year. The distances of the clouds to the SMBH are 0.1 ∼ 1 pc. This scenario has advantages in explaining the long delay, sharpness of the rise, and the multiplicity of the late radio flares. Future observations may build up a much larger sample of late-time radio flares and enable their use as a probe of the TDE physics and the host circumnuclear environment.