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Supernova (SN) 2015U (also known as PSN J07285387+3349106) was discovered inNGC 2388 on 2015 Feb. 11. A rapidly evolving and luminous event, it showedeffectively hydrogen-free spectra dominated by relatively narrow helium P-Cygnispectral features and it was classified as a SN Ibn. In this paper we presentphotometric, spectroscopic, and spectropolarimetric observations of SN 2015U,including a Keck/DEIMOS spectrum (resolution $\approx$ 5000) which fullyresolves the optical emission and absorption features. We find that SN 2015U isbest understood via models of shock breakout from extended and densecircumstellar material (CSM), likely created by a history of mass loss from theprogenitor with an extreme outburst within $\sim$1-2 yr of core collapse (butwe do not detect any outburst in our archival imaging of NGC 2388). We arguethat the high luminosity of SN 2015U was powered not through $^{56}$Ni decaybut via the deposition of kinetic energy into the ejecta/CSM shock interface.Though our analysis is hampered by strong host-galaxy dust obscuration (whichlikely exhibits multiple components), our dataset makes SN 2015U one of thebest-studied Type Ibn supernovae and provides a bridge of understanding toother rapidly fading transients, both luminous and relatively faint.

SN 2015U: A Rapidly Evolving and Luminous Type Ibn Supernova