SN 2015da: late-time observations of a persistent superluminous Type IIn supernova with post-shock dust formation

We present photometry and spectroscopy of the slowly evolving superluminous Type IIn supernova (SN) 2015da. SN 2015da is extraordinary for its very high peak luminosity, and also for sustaining a high luminosity for several years. Even at 8 yr after explosion, SN 2015da remains as luminous as the peak of a normal SN II-P. The total radiated energy integrated over this time period (with no bolometric correction) is at least 1.6x10(51) erg (or 1.6 FOE). Including a mild bolometric correction, adding kinetic energy of the expanding cold dense shell of swept-up circumstellar material (CSM), and accounting for asymmetry, the total explosion kinetic energy was likely 5-10 FOE. Powering the light curve with CSM interaction requires an energetic explosion and 20 M-circle dot of H-rich CSM, which in turn implies a massive progenitor system >30 M-circle dot. Narrow P Cyg features show steady CSM expansion at 90 km s(-1), requiring a high average mass-loss rate of similar to 0.1 M-circle dot yr(-1) sustained for two centuries before explosion (although ramping up toward explosion time). No current theoretical model for single-star pre-SN mass-loss can account for this. The slow CSM, combined with broad wings of H alpha indicating H-rich material in the unshocked ejecta, disfavours a pulsational pair instability model for the pre-SN mass-loss. Instead, violent pre-SN binary interaction is a likely culprit. Finally, SN 2015da exhibits the characteristic asymmetric blueshift in its emission lines from shortly after peak until the present epoch, adding another well-studied superluminous SNe IIn with unambiguous evidence of post-shock dust formation.