Optical Transients Powered by Magnetars: Dynamics, Light Curves, and Transition to the Nebular Phase

TL;DR

This paper models the dynamics and light curves of optical transients powered by magnetars, exploring how different spin-down timescales influence energy distribution, evolution into nebular phases, and peak luminosity timing.

Contribution

It provides a detailed analysis of how magnetar spin-down timescales affect transient dynamics, light curves, and the transition to nebular phases, including modifications to the peak luminosity law.

Findings

Short spin-down magnetars convert rotational energy into kinetic energy.

Long spin-down magnetars primarily emit radiation.

Photospheric recession delays peak luminosity, relativistic motion can alter this effect.

Abstract

Millisecond magnetars can be formed via several channels: core-collapse of massive stars, accretion-induced collapse of white dwarfs (WDs), double WD mergers, double neutron star (NS) mergers, and WD-NS mergers. Because the mass of ejecta from these channels could be quite different, their light curves are also expected to be diverse. We evaluate the dynamic evolution of optical transients powered by millisecond magnetars. We find that the magnetar with short spin-down timescale converts its rotational energy mostly into the kinetic energy of the transient, while the energy of a magnetar with long spin-down timescale goes into radiation of the transient. This leads us to speculate that hypernovae could be powered by magnetars with short spin-down timescales. At late times the optical transients will gradually evolve into a nebular phase because of the photospheric recession. We treat the photosphere and nebula separately because their radiation mechanisms are different. In some cases the ejecta could be light enough that the magnetar can accelerate it to a relativistic speed. It is well known that the peak luminosity of a supernova (SN) occurs when the luminosity is equal to the instantaneous energy input rate, as shown by Arnett (1979). We show that photospheric recession and relativistic motion can modify this law. The photospheric recession always leads to a delay of the peak time $t_{\mathrm{pk}}$ relative to the time $t_{\times }$ at which the SN luminosity equals the instantaneous energy input rate. Relativistic motion, however, may change this result significantly.