Gamma-ray Thermalization and Leakage from Millisecond Magnetar Nebulae: Towards a Self-Consistent Model for Superluminous Supernovae

Abstract

Superluminous supernovae (SLSNe) are massive star explosions that are too luminous to be powered by traditional energy sources, such as the radioactive decay of 56 Ni. These transients may instead 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 which follow the production and thermalization of high energy radiation from the nebula into optical radiation and, conversely, determine the gamma-ray emission that escapes the ejecta without thermalizing. Specifically, we track the evolution of photons and matter in a coupled two-zone ("wind/nebula" and "ejecta") model, accounting for the range of radiative processes including (typically multiple generations of) pair creation. We identify a novel mechanism by which γγ pair creation in the upstream pulsar wind regulates the mean energy of particles injected into 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 SLSNe optical light curves as being the result of gammaray leakage, we find that the nebular magnetization must be very low ε B ∼ < 10 −6 − 10 −4 . For higher ε B synchrotron emission quickly comes to dominate the thermalized nebula radiation, and being readily photoelectrically absorbed because of its lower photon energies, results in the supernova optical light curve tracking the spin-down power even to late times ∼ > 1 year, inconsistent with observations. For magnetars to remain as viable contenders for powering SLSNe, we thus conclude that either magnetic dissipation in the wind/nebula is extremely efficient, or that the spin-down luminosity decays significantly faster than the canonical magnetic dipole rate ∝ t −2 in a way that coincidentally mimicks gamma-ray escape. Our models predict a late-time flattening in the optical light curves of SLSNe, for which there may be evidence in SN 2015bn.

Late-Time Hubble Space Telescope Observations of a Hydrogen-Poor Superluminous Supernova Reveal the Power-Law Decline of a Magnetar Central Engine

“…Recently, Vurm & Metzger (2021) presented the first radiative transfer models of magnetar-powered SN light curves, including the numerous photon-matter interactions that are possible in the magnetar wind nebula and expanding ejecta. They find that the thermalization opacity varies with time, and that the nebula magnetization strongly affects its evolution.…”

“…This implies that substantial magnetic dissipation in the nebula is required to explain events like SN 2015bn and SN 2016inl. While a numerical model using the engine parameters relevant for SN 2016inl has not been calculated by Vurm & Metzger (2021), its shallower powerlaw decline likely indicates a higher degreee of magnetization than in SN 2015bn.…”

“…Another consequence of a varying γ-ray opacity predicted by Vurm & Metzger (2021) is that the power-law index may change with time. In particular, their model that best matches SN 2015bn predicts a transition to a shallower power law during a phase in which the opacity increases; this effect may be discernible in the final two epochs of SN 2015bn, both of which exceed the bestfit MOSFiT model (Nicholl et al 2018).…”

ALMA and NOEMA constraints on synchrotron nebular emission from embryonic superluminous supernova remnants and radio-gamma-ray connection

“…We treat γ b as a parameter, assuming γe ∈ [10 3 , 10 7 ], where γ b does not have to be the same as the bulk Lorentz factor of the wind. The pair multiplicity is model dependent, and cascades in the nebula and/or wind may contribute at early times (Murase et al 2015;Vurm & Metzger 2021). Also, it has been known that the radio data of the Crab nebula require large multiplicities (with ∼ 10 6 ) that are theoretically challenging (e.g., Arons 2012), which may originate from continuously-heated pairs that were injected in the past or from another component (Atoyan 1999;Tanaka & Asano 2017).…”

“…Murase et al (2015) showed that external inverse-Compton emission is important until SN photons mostly escape. Intriguingly, extremely small values of B are independently motivated by a possible solution to the missing energy problem for SN 2015bn and SN 2017egm (Vurm & Metzger 2021). As an example, we take B = 10 −6 , γ b = 10 3 , and q1 = 1 (in which the Compton parameter YIC 10).…”