Abstract
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 3 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.
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