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

We present results from an extensive follow-up campaign of the tidal disruption event (TDE) ASASSN-15oi spanning δt ∼ 10-3000 days, offering an unprecedented window into the multiwavelength properties of a TDE during its first ≈8 yr of evolution. ASASSN-15oi is one of the few TDEs with strong detections at X-ray, optical/UV, and radio wavelengths and it also featured two delayed radio flares at δt ∼ 180 days and δt ∼ 1400 days. Our observations at δt > 1400 days reveal an absence of thermal X-rays, a late-time variability in the nonthermal X-ray emission, and sharp declines in the nonthermal X-ray and radio emission at δt ∼ 2800 days and ∼3000 days, respectively. The UV emission shows no significant evolution at δt > 400 days 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 can possibly explain 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 nonthermal X-rays remains challenging. Our study underscores the critical role of long-term, multiwavelength follow-up to fully characterize the extended evolutionary phases of a TDE.