The Effect of Jet-Ejecta Interaction on the Viewing Angle Dependence of Kilonova Light Curves

TL;DR

This study investigates how jet-ejecta interactions in neutron star mergers influence kilonova light curves, revealing that jet disruption causes viewing angle-dependent brightness and color variations, especially in ultraviolet wavelengths.

Contribution

The paper introduces detailed 2D relativistic hydrodynamic simulations combined with radiation transport to quantify how jet disruption alters kilonova emission based on viewing angle.

Findings

Jet shock heating does not affect early kilonova emission.

Jet disruption creates a channel, making the kilonova brighter and bluer near the jet axis.

Ultraviolet emission is more affected by jet disruption than optical.

Abstract

The merger of two neutron stars produces an outflow of radioactive heavy nuclei. Within a second of merger, the central remnant is expected to also launch a relativistic jet, which shock-heats and disrupts a portion of the radioactive ejecta. Within a few hours, emission from the radioactive material gives rise to an ultraviolet, optical, and infrared transient (a kilonova). We use the endstates of a suite of 2D relativistic hydrodynamic simulations of jet-ejecta interaction as initial conditions for multi-dimensional Monte Carlo radiation transport simulations of the resulting viewing angle-dependent light curves and spectra starting at 1.5h after merger. We find that on this timescale, jet shock heating does not affect the kilonova emission. However, the jet disruption to the density structure of the ejecta does change the light curves. The jet carves a channel into the otherwise spheroidal ejecta, revealing the hot, inner regions. As seen from near ($\lesssim 30 \deg$) the jet axis, the kilonova is brighter by a factor of a few and bluer. The strength of this effect depends on the jet parameters, since the light curves of more heavily disrupted ejecta are more strongly affected. The light curves and spectra are also more heavily modified in the ultraviolet than in the optical.