The Unusual Late-Time Evolution of the Tidal Disruption Event ASASSN-15oi

TL;DR

This study provides a comprehensive multi-wavelength analysis of the late-time evolution of the TDE ASASSN-15oi, revealing unique X-ray brightening and persistent UV emission over 600 days, with implications for TDE emission mechanisms.

Contribution

It offers the first detailed late-time optical, UV, and X-ray observations of ASASSN-15oi, highlighting a slow brightening of thermal X-ray emission and potential changes in emission mechanisms.

Findings

X-ray emission brightened by an order of magnitude over time.

UV and optical emission declined following a power law.

Total radiated energy was approximately 1.32 x 10^{51} ergs.

Abstract

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 3 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 $(1.32\pm0.06)\times10^{51}$ ergs 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.