Interaction of an outflow with surrounding gaseous clouds as the origin of the late-time radio flares in TDEs

TL;DR

This paper proposes that late-time radio flares in TDEs result from outflows interacting with surrounding gaseous clouds, explaining observed features and providing insights into TDE physics and environments.

Contribution

It introduces a model where outflow-cloud interactions produce late radio flares, matching observations and offering a new perspective on TDE late-time emissions.

Findings

Outflows have velocities of 0.2c to 0.6c.

Radio signatures match observed late-time flares.

Clouds are located 0.1 to 1 pc from SMBH.

Abstract

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 $10^{38\sim39}$ 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 circum-nuclear 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 0.2$c\sim0.6$$c$, the total mass of $10^{-3}\sim10^{-1}$ $\mathrm{M_{\odot}}$ and the ejection duration of a month to a year. The distances of the clouds to the SMBH are $0.1\sim1$ 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.