The unexpected, long-lasting, UV rebrightening of the superluminous supernova ASASSN-15lh

• Published in: Monthly notices of the Royal Astronomical Society Vol. 466; no. 2; p. 1428
• Main Authors: Godoy-Rivera, D, Stanek, K Z, Kochanek, C S, Chen, Ping, Dong, Subo, Prieto, J L, Shappee, B J, Jha, S W, Foley, R J, Pan, Y-C, Holoien, T W-S, Thompson, Todd A, Grupe, D, Beacom, J F
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01-04-2017
• Subjects: ASASSN-15lh; Astronomy; ASTRONOMY AND ASTROPHYSICS; Emission; Fittings; H alpha line; Luminosity; Nuclei; Photosphere; Star & galaxy formation; Supernovae; Symbols
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stw3237

Given its peak luminosity and early-time spectra, ASASSN-15lh was classified as the most luminous supernova ever discovered. Here, we report a UV rebrightening of ASASSN-15lh observed with Swift during our follow-up campaign. The rebrightening began at t ... 90 d (observer frame) after the primary peak and was followed by a ~120-d long plateau in the bolometric luminosity, before starting to fade again at t ... 210 d. ASASSN-15lh rebrightened in the Swift UV bands by ...mUVW2 ... -1.75 mag, ...mUVM2 ... -1.25 mag and ...mUVW1 ... -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 such as narrow emission lines. There are hints of weak Ha emission at late-times, but Margutti et al. 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 ... 11 000 K to TBB ... 18 000 K. Over the ~ 550 d since its detection, ASASSN-15lh has radiated ~1.7 -1.9 x 10 super( 52) erg. 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 than to tidal disruption events. (ProQuest: ... denotes formulae/symbols omitted.)