Infrared Echoes of Optical Tidal Disruption Events: ~1% Dust Covering Factor or Less at sub-parsec Scale

TL;DR

This study statistically analyzes infrared echoes of optical tidal disruption events, revealing that most have very low dust covering factors (~1%) at sub-parsec scales, indicating a scarcity of dust or flat disk geometries near SMBHs.

Contribution

It provides the first comprehensive statistical assessment of IR echoes in TDEs, showing most have low dust covering factors and highlighting observational biases.

Findings

Most TDEs show IR variability with low dust luminosity.

Dust covering factor is around 1%, much lower than in AGN tori.

One TDE, ASASSN-15lh, exhibits unusually high dust luminosity.

Abstract

The past decade has experienced an explosive increase of optically-discovered tidal disruption events (TDEs) with the advent of modern time-domain surveys. However, we still lack a comprehensive observational view of their infrared (IR) echoes in spite of individual detections. To this end, we have conducted a statistical study of IR variability of the 23 optical TDEs discovered between 2009 and 2018 utilizing the full public dataset of Wide-field Infrared Survey Explorer. The detection of variability is performed on the difference images, yielding out 11 objects with significant (>$3\sigma$) variability in at least one band while dust emission can be only fitted in 8 objects. Their peak dust luminosity is around $10^{41}$-$10^{42}$ erg/s, corresponding to a dust covering factor $f_c\sim0.01$ at scale of sub-parsec. The only exception is the disputed source ASASSN-15lh, which shows an ultra-high dust luminosity ($\sim10^{43.5}$ erg/s) and make its nature even elusive. Other non-detected objects show even lower $f_c$, which could be one more order of magnitude lower. The derived $f_c$ is generally much smaller than those of dusty tori in active galactic nuclei (AGNs), suggesting either a dearth of dust or a geometrically thin and flat disk in the vicinity of SMBHs. Our results also indicate that the optical TDE sample (post-starburst galaxies overrepresented) is seriously biased to events with little dust at sub-pc scale while TDEs in dusty star-forming systems could be more efficiently unveiled by IR echoes.