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

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 multidimensional Monte Carlo radiation transport simulations of the resulting viewing angle-dependent light curves and spectra starting at $1.5\, \mathrm{h}$ after merger. We find that on this time-scale, jet shock heating does not affect the kilonova emission for the jet parameters we survey. 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 (≲30°) 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.

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 impact of r-process heating on the dynamics of neutron star merger accretion disc winds and their electromagnetic radiation

“…The cavity left by a jet can lead to brighter emission. Due to the low density directly on the poles, polar observers are also able to see 'deeper' into the bulk of the wind, which can lead them to see a hotter and brighter photosphere on the poles, even if said photosphere is at a lower velocity (Klion et al 2021).…”

“…The majority of r-process products have particularly high opacities in the ultraviolet and blue portions of the spectrum, so our grey prescription may lead us to overestimate the UV and blue emission. However, at very early times (< 0.5 day), the opacity may be suppressed due to the high temperatures and ionization fractions (Banerjee et al 2020;Klion et al 2021).…”

Dynamical ejecta of neutron star mergers with nucleonic weak processes II: Kilonova emission

“…Keeping the aforementioned aspects in mind, we also mention some alternative scenarios that could explain the blue component of AT2017gfo. The partial disruption of the dynamical ejecta due to a relativistic jet and its corresponding energy injection have been considered as a possible way to make the ejecta shine brighter and at higher temperatures (Nativi et al 2020;Klion et al 2021b). Moreover, neutrino oscillations could change the nucleosynthesis signature of the ejecta and reduce their lanthanide content (e.g.…”