Engine-fed Kilonovae (Mergernovae) -- I. Dynamical Evolution and Energy Injection / Heating Efficiencies

TL;DR

This paper models the dynamical evolution and energy injection processes in engine-fed kilonovae resulting from neutron star mergers, highlighting the efficiency of energy transfer and shock breakout timing.

Contribution

It introduces a semi-analytical model of engine-fed kilonova ejecta interaction, emphasizing the energy injection efficiency and shock dynamics, which differ from previous assumptions.

Findings

Energy injection efficiency can be as high as 0.6 but drops rapidly after shock breakout.

Most energy is stored as magnetic and kinetic energy, with internal energy fraction below 0.3.

Efficient heating occurs mainly before the forward shock breaks out, with efficiency between 0.006 and 0.3.

Abstract

A binary neutron star merger is expected to be associated by a kilonova, transient optical emission powered by radioactive decay of the neutron-rich ejecta. If the post-merger remnant is a long-lived neutron star, additional energy injection to the ejecta is possible. In this first paper of a series, we study the dynamical evolution of the engine-fed kilonova (mergernova) ejecta in detail. We perform a semi-analytical study of the problem by adopting a modified mechanical blastwave model that invokes interaction between a Poynting-flux-dominated flow and a non-magnetized massive ejecta. Shortly after the engine is turned on, a pair of shocks would be excited. The reverse shock quickly reaches the wind-acceleration region and disappears (in a few seconds), whereas the forward shock soon breaks out from the ejecta (in $10^2$ - $10^3$ seconds) and continues to propagate in the surrounding interstellar medium. Most of the energy injected into the blastwave from the engine is stored as magnetic energy and kinetic energy. The internal energy fraction is $f_{\rm int} < 0.3$ for an ejecta mass equal to $10^{-3}M_{\odot}$. Overall, the energy injecting efficiency $\xi$ is at most $\sim 0.6$ and can be as small as $\sim 0.04$ at later times. Contrary to the previous assumption, efficient heating only happens before the forward shock breaks out of the ejecta with a heating efficiency $\xi_t \sim (0.006 - 0.3)$, which rapidly drops to $\sim 0$ afterwards. The engine-fed kilonova lightcurves will be carefully studied in Paper II.