Can jets make the radioactively powered emission from neutron star mergers bluer?

Abstract

Neutron star mergers eject neutron-rich matter in which heavy elements are synthesised. The decay of these freshly synthesised elements powers electromagnetic transients (“macronovae” or “kilonovae”) whose luminosity and colour strongly depend on their nuclear composition. If the ejecta are very neutron-rich (electron fraction Ye < 0.25), they contain fair amounts of lanthanides and actinides which have large opacities and therefore efficiently trap the radiation inside the ejecta so that the emission peaks in the red part of the spectrum. Even small amounts of this high-opacity material can obscure emission from lower lying material and therefore act as a “lanthanide curtain”. Here, we investigate how a relativistic jet that punches through the ejecta can potentially push away a significant fraction of the high opacity material before the macronova begins to shine. We use the results of detailed neutrino-driven wind studies as initial conditions and explore with 3D special relativistic hydrodynamic simulations how jets are propagating through these winds. Subsequently, we perform Monte Carlo radiative transfer calculations to explore the resulting macronova emission. We find that the hole punched by the jet makes the macronova brighter and bluer for on-axis observers during the first few days of emission, and that more powerful jets have larger impacts on the macronova.

The Challenges Ahead for Multimessenger Analyses of Gravitational Waves and Kilonova: A Case Study on GW190425

“…Currently, the relativistic jet that is launched in some mergers (see, e.g., Ruiz et al 2021) is not taken into account in the model presented here. Neither is the interaction between the jet and the dynamical and disk wind ejecta, which can alter the kilonova light curve (Klion et al 2021;Nativi et al 2021;Nicholl et al 2021).…”

“…In this work, we will present a new multimessenger framework to analyze gravitational waves and the associated UVOIR light curves jointly. We leave the inclusion of the GRB and radio counterpart to future work, and furthermore will not include any interaction between the kilonova material and the jet (see e.g., Kasliwal et al 2017;Piro & Kollmeier 2018;Klion et al 2021;Nativi et al 2021). We will use the analytical formulae, fitted to numerical simulation of BNS and BHNS, derived in Krüger & Foucart (2020), to connect outflow properties to binary progenitor properties.…”

The effect of jet–ejecta interaction on the viewing angle dependence of kilonova light curves

“…It is expected that some components of neutron star merger ejecta are non-spherical, and may show latitudinal variation in density and opacity. Recent work has studied how ejecta components with different geometry and opacity can affect kilonova light curves (Wollaeger et al 2018;Kawaguchi et al 2020;Darbha & Kasen 2020;Korobkin et al 2020;Nativi et al 2021). In this study, we focus on another aspect of the asymmetry: the consequences of a jet evacuating a narrow cavity within the bulk of the ejecta.…”

Short gamma-ray burst jet propagation in binary neutron star merger environments

“…An exception is represented byNativi et al (2021), where the environment is set by importing data from a newtonian simulation of neutrino-driven winds produced by a massive neutron star remnant(Perego et al 2014). …”