The unusual late-time evolution of the tidal disruption event ASASSN-15oi

We present late-time optical spectroscopy and X-ray, UV, and optical photometry of the nearby (d = 214 Mpc, z = 0.0479) tidal disruption event (TDE) ASASSN-15oi. The optical spectra span 450 days after discovery and show little remaining transient emission or evolution after roughly three months. In contrast, the Swift and XMM-Newton observations indicate the presence of evolving X-ray emission and lingering thermal UV emission that is still present 600 days after discovery. The thermal component of the X-ray emission shows a unique, slow brightening by roughly an order of magnitude to become the dominant source of emission from the TDE at later times, while the hard component of the X-ray emission remains weak and relatively constant throughout the flare. The TDE radiated (1.32 ± 0.06) × 10⁵¹ erg across all wavelengths, and the UV and optical emission, is consistent with a power-law decline and potentially indicative of a late-time shift in the power-law index that could be caused by a transition in the dominant emission mechanism.