Investigating the jet-ejecta interaction in neutron star mergers

GW170817 represented the first detection of a binary neutron star merger, and the coalescence of the two neutron stars was followed by both the ejection of neutron-rich material and the launch of a relativistic jet.
These two dynamical components, mildly-relativistic ejecta, and ultra-relativistic outflow, were detected independently: the first one powered a “macronova” (or “kilonova”), a thermal transient powered by the radioactive decay of the freshly synthesized r-process nuclei. The jet instead powered a short GRB consisting of an early gamma-ray signal followed by a multi-wavelength afterglow.
The presence of a jet successfully breaking out from the ejecta was additionally confirmed by the detection of superluminal motion.
These two components were detected and modelled independently, but they interact before becoming visible, and this might have left an imprint on the detected signals.
I will discuss the results of the 3D special-relativistic simulations used to investigate the consequences of a jet propagating through a realistic environment, created by a neutrino-driven wind around the central remnant.
I will show how a jet can “punch-away” a fraction of high-opacity material at early times, before the brightening of the macronova, and how this is going to impact the observed signal.
Then I will show what happens in the other direction i.e., how the ejecta impact the jet. I will show that the emerging jets are mostly shaped by their interaction with the surrounding environment and their eventual initial structures only plays a minor role in the final outcome. Moreover, turbulent motions arising during the interaction produce inhomogeneities in the jet profile when observed face-on.
These inhomogeneities impact the peak afterglow flux, and the inferred kinetic energy of a single jet from different observers can cover a fraction about 1/3 of the observed short GRB population.