Infrared Echoes of Optical Tidal Disruption Events: ∼1% Dust-covering Factor or Less at Subparsec Scale

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 the IR variability of the 23 optical TDEs discovered between 2009 and 2018 using the full public data set of the Wide-field Infrared Survey Explorer. The detection of variability is performed on the difference images, yielding 11 objects with significant (>3 σ ) variability in at least one band, while dust emission can be only fit in 8 objects. Their peak dust luminosity is around 10 41 –10 42 erg s −1 , corresponding to a dust-covering factor f c ∼ 0.01 at a subparsec scale. The only exception is the disputed source ASASSN-15lh, which shows an ultra-high dust luminosity (∼10 43.5 erg s −1 ), and this makes its nature even more elusive. Other nondetected objects show even lower f c , which could be lower by one more order of magnitude. The derived f c is generally much lower than those of dusty tori in active galactic nuclei, suggesting either a dearth of dust or a geometrically thin and flat disk in the vicinity of supermassive black holes. Our results also indicate that the optical TDE sample (post-starburst galaxies overrepresented) is seriously biased to events with little dust at subparsec scale, while TDEs in dusty star-forming systems could be more efficiently unveiled by IR echoes.