Gamma-Ray Thermalization and Leakage from Millisecond Magnetar Nebulae: Toward a Self-consistent Model for Superluminous Supernovae

Superluminous supernovae (SLSNe) are massive star explosions that are too luminous to be powered by traditional energy sources, such as the radioactive decay of ⁵⁶Ni. Instead, they may be powered by a central engine, such as a millisecond pulsar or magnetar, whose relativistic wind inflates a nebula of high-energy particles and radiation behind the expanding supernova ejecta. We present three-dimensional Monte Carlo radiative transfer calculations of SLSNe that follow the production of high-energy emission in the nebula and its subsequent thermalization into optical radiation within the surrounding ejecta and, conversely, determine the gamma-ray emission that escapes the ejecta without thermalizing. We identify a novel mechanism by which γγ pair creation in the upstream pulsar wind regulates the mean energy of particles entering the nebula over the first several years after the explosion, rendering our results on this timescale insensitive to the (uncertain) intrinsic wind pair multiplicity. To explain the observed late-time steepening of SLSN optical light curves as being the result of gamma-ray leakage, we find that the nebular magnetization must be very low, ${\epsilon }_B\lesssim {10}^{-6}\text{--}{10}^{-4}$. For higher ${\epsilon }_B$, the more efficiently thermalized lower-energy synchrotron emission would overproduce the late-time (≳1 yr) optical radiation, inconsistent with observations. For magnetars to remain as viable contenders for powering SLSNe, we conclude that either magnetic dissipation in the wind/nebula is extremely efficient or the spin-down luminosity decays significantly faster than the canonical dipole rate ∝t ⁻² in a way that coincidentally mimics gamma-ray escape.