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iPTF15dtg is a Type Ic supernova (SN) showing a broad light curve aroundmaximum light, consistent with massive ejecta if we assume aradioactive-powering scenario. We study the late-time light curve of iPTF15dtg,which turned out to be extraordinarily luminous for a stripped-envelope (SE)SN. We compare the observed light curves to those of other SE SNe and also withmodels for the $^{56}$Co decay. We analyze and compare the spectra to nebularspectra of other SE SNe. We build a bolometric light curve and fit it withdifferent models, including powering by radioactivity, magnetar powering, aswell as a combination of the two. Between 150 d and 750 d past explosion,iPTF15dtg's luminosity declined by merely two magnitudes instead of the sixmagnitudes expected from $^{56}$Co decay. This is the firstspectroscopically-regular SE SN showing this behavior. The model with bothradioactivity and magnetar powering provides the best fit to the light curveand appears to be the more realistic powering mechanism. An alternativemechanism might be CSM interaction. However, the spectra of iPTF15dtg are verysimilar to those of other SE SNe, and do not show signs of strong CSMinteraction. iPTF15dtg is the first spectroscopically-regular SE SN whose lightcurve displays such clear signs of a magnetar contributing to the powering ofthe late time light curve. Given this result, the mass of the ejecta needs tobe revised to a lower value, and therefore the progenitor mass could besignificantly lower than the previously estimated $>$35 $M_{\odot}$.

The luminous late-time emission of the type Ic supernova iPTF15dtg - evidence for powering from a magnetar?