2D End-to-End Modeling of Kilonovae from Binary Neutron Star Merger Remnants

Abstract

We investigate the kilonova emission resulting from outflows produced in a three-dimensional (3D) general-relativistic magnetohydrodynamic (GRMHD) simulation of a hypermassive neutron star (HMNS) remnant. We map the outflows into the FLASH hydrodynamics code to model their expansion in axisymmetry, and study the effects of employing different $r$-process heating rates. Except for the highest heating rate prescription, we find no significant differences with respect to overall ejecta dynamics and morphology compared to the simulation without heating. Once homologous expansion is attained, typically after $\sim$ 2s for these ejecta, we map the outflows to the Sedona radiative transfer code and compute the spectral evolution of the kilonova and broadband light curves in various Legacy Survey of Space and Time (LSST) bands. The kilonova properties depend on the remnant lifetime, with peak luminosities and peak timescales increasing for longer-lived remnants that produce more massive ejecta. For all models, there is a strong dependence of both the bolometric and broadband light curves on the viewing angle. While the short-lived (12ms) remnant produces higher luminosities when viewed from angles closer to the pole, longer-lived remnants (240ms and 2.5s) are more luminous when viewed from angles closer to the equator. Our results highlight the importance of self-consistent, long-term modeling of merger ejecta, and taking viewing-angle dependence into account when interpreting observed kilonova light curves. We find that magnetized outflows from a HMNS -- if it survives long enough -- could explain blue kilonovae, such as the blue emission seen in AT2017gfo.