SN 2016esw: A luminous Type II supernova observed within the first day after the explosion

We present photometry, spectroscopy, and host-galaxy integral-field spectroscopy of the Type II supernova (SN II) 2016esw in CGCG 229-009 from the first day after the explosion up to 120 d. Its light-curve shape is similar to that of a typical SN II; however, SN 2016esw is near the high-luminosity end of the SN II distribution, with a peak of M_V ^max = -18.36 mag. The V-band light curve exhibits a long recombination phase for a SN II (similar to the longlived plateau of SN 2004et). Considering the well-known relation between the luminosity and the plateau decline rate, SN 2016esw should have a V-band slope of ~2.10 mag (100 d)^-1; however, SN 2016esw has a substantially flatter plateau with a slope of 1.01 ± 0.26 mag (100 d)^-1, perhaps indicating that interacting Type II supernovae are not useful for cosmology. At 19.5 d post-explosion, the spectrum presents a boxy H α emission line with flat absorption profiles, suggesting interaction between the ejecta and circumstellar matter. Finally, based on the spectral properties, SN 2016esw shows similarities with the luminous and interacting SN 2007pk at early epochs, particularly in terms of observable line features and their evolution.