2017ApJ...835..177M

Many Type Ic superluminous supernovae have light-curve decline rates after their luminosity peak, which are close to the nuclear decay rate of ⁵⁶Co, consistent with the interpretation that they are powered by ⁵⁶Ni and possibly pair-instability supernovae. However, their rise times are typically shorter than those expected from pair-instability supernovae, and Type Ic superluminous supernovae are often suggested to be powered by magnetar spin-down. If magnetar spin-down is actually a major mechanism to power Type Ic superluminous supernovae, it should be able to produce decline rates similar to the ⁵⁶Co decay rate rather easily. In this study, we investigate the conditions for magnetars under which their spin-down energy input can behave like the ⁵⁶Ni nuclear decay energy input. We find that an initial magnetic field strength within a certain range is sufficient to keep the magnetar energy deposition within a factor of a few of the ⁵⁶Co decay energy for several hundreds of days. Magnetar spin-down needs to be by almost pure dipole radiation with the braking index close to three to mimic ⁵⁶Ni in a wide parameter range. Not only late-phase ⁵⁶Co-decay-like light curves, but also rise time and peak luminosity of most ⁵⁶Ni-powered light curves can be reproduced by magnetars. Bolometric light curves for more than 700 days are required to distinguish the two energy sources solely by them. We expect that more slowly declining superluminous supernovae with short rise times should be found if they are mainly powered by magnetar spin-down.