The properties of tidal disruption event infrared counterparts produced by dust rings and inference of the observing angle

Abstract

A substantial fraction of tidal disruption events (TDEs), resulting from a black holes's disruption and accretion of a star, exhibit infrared (IR) counterparts thought to arise from spherical shells of dust reprocessing the TDE emission. Some modelling of TDEs also predicts an angular dependence in their observed properties with more X-ray emission on-axis and more optical emission at higher angles. However, there is growing evidence that X-ray rich TDEs are more likely to exhibit IR counterparts, contradicting the spherical shell model that predicts no significant variation in IR luminosity with angle. Here, I demonstrate that this result naturally follows for dust arranged in a ring instead of a spherical shell. I present a toy model of this scenario and show that on-axis angles result in a brighter counterpart. I also show that on-axis angles result in a delay in the initial rise and off-axis angles may display a double-peaked structure. Crucially, this model also allows the observing angle of the TDE to be constrained. Finally, I demonstrate that this model reproduces the properties of two IR counterparts, including constraining their observing angles and independently inferring an optical plateau, and briefly comment on its application to quasi-periodic eruption counterparts.