The Unexpected, Long-Lasting, UV Rebrightening of the Super-Luminous Supernova ASASSN-15lh

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Given its peak luminosity and early-time spectra, ASASSN-15lh was classified as the most luminous supernova (SN) ever discovered (Dong et al. 2016). Here we report a UV rebrightening of ASASSN-15lh observed with Swift during our follow-up campaign. The rebrightening began at t ≃ 90 days (observer frame) after the primary peak and was followed by a 120-day long plateau in the bolometric luminosity, before starting to fade again at t≃ 210 days. ASASSN-15lh rebrightened in the Swift UV bands by ΔmUVW2 ≃ -1.75 mag, ΔmUVM2 ≃ -1.25 mag, and ΔmUVW1 ≃ -0.8 mag, but did not rebrighten in the optical bands. Throughout its initial decline, subsequent rebrightening, and renewed decline, the spectra did not show evidence of interactions between the ejecta and circumstellar medium (CSM) such as narrow emission lines. There are hints of weak Hα emission at late-times, but Margutti et al. (2016) have shown that it is narrow line emission consistent with star formation in the host nucleus. By fitting a blackbody we find that during the rebrightening the effective photospheric temperature increased from TBB ≃ 11000 K to TBB ≃ 18000 K. Over the 550 days since its detection, ASASSN-15lh has radiated 1.7 - 1.9 × 10⁵² ergs. Although its physical nature remains uncertain, the evolution of ASASSN-15lh's photospheric radius, its radiated energy, and the implied event rate, are all more similar to those of H-poor superluminous supernovae (SLSNe-I) than to tidal disruption events (TDEs).