Eight Years of Light from ASASSN-15oi: Towards Understanding the Late-time Evolution of TDEs

TL;DR

This study presents an 8-year multiwavelength follow-up of TDE ASASSN-15oi, revealing complex late-time evolution, including delayed radio flares, absence of thermal X-rays, and persistent UV emission, advancing understanding of TDE dynamics.

Contribution

It provides the first comprehensive long-term multiwavelength dataset for a TDE, analyzing late-time emission mechanisms and proposing models for observed phenomena.

Findings

Delayed radio flares at 180 and 1400 days.

Absence of thermal X-rays after 1400 days.

Persistent UV emission above pre-TDE levels.

Abstract

We present the results from an extensive follow-up campaign of the Tidal Disruption Event (TDE) ASASSN-15oi spanning $\delta t \sim 10 - 3000$ d, offering an unprecedented window into the multiwavelength properties of a TDE during its first $\approx 8$ years of evolution. ASASSN-15oi is one of the few TDEs with strong detections at X-ray, optical/UV, and radio wavelengths and featured two delayed radio flares at $\delta t \sim 180$ d and $\delta t \sim 1400$ d. Our observations at $> 1400$ d reveal an absence of thermal X-rays, a late-time variability in the non-thermal X-ray emission, and sharp declines in the non-thermal X-ray and radio emission at $\delta t \sim 2800$ d and $\sim 3000$ d, respectively. The UV emission shows no significant evolution at $>400$ d and remains above the pre-TDE level. We show that a cooling envelope model can explain the thermal emission consistently across all epochs. We also find that a scenario involving episodic ejection of material due to stream-stream collisions is conducive to explaining the first radio flare. Given the peculiar spectral and temporal evolution of the late-time emission, however, constraining the origins of the second radio flare and the non-thermal X-rays remains challenging. Our study underscores the critical role of long-term, multiwavelength follow-up.