Simulations of Early Kilonova Emission from Neutron Star Mergers

Abstract

We present radiative transfer simulations for blue kilonovae hours after neutron star (NS) mergers by performing detailed opacity calculations for the first time. We calculate atomic structures and opacities of highly ionized elements (up to the 10th ionization) with atomic number Z = 20–56. We find that the bound–bound transitions of heavy elements are the dominant source of the opacities in the early phase (t < 1 day after the merger) and that the ions with a half-closed electron shell provide the highest contributions. The Planck mean opacity for lanthanide-free ejecta (with electron fraction of Y e = 0.30–0.40) can only reach around at t = 0.1 days, whereas that increases up to at t = 1 day. The spherical ejecta model with an ejecta mass of M ej = 0.05 M ⊙ gives the bolometric luminosity of at t ∼ 0.1 days. We confirm that the existing bolometric and multicolor data of GW170817 can be naturally explained by the purely radioactive model. The expected early UV signals reach 20.5 mag at t ∼ 4.3 hr for sources even at 200 Mpc, which is detectable by the facilities such as Swift and the Ultraviolet Transient Astronomy Satellite (ULTRASAT). The early-phase luminosity is sensitive to the structure of the outer ejecta, as also pointed out by Kasen et al. Therefore, the early UV observations give strong constraints on the structure of the outer ejecta and the presence of a heating source besides r-process nuclei.

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

“…These are not currently available. Opacities have been calculated out to temperatures of only ∼ 10 4 K for the Lanthanides (Kasen et al 2017;Fontes et al 2020;Tanaka et al 2020) and ∼ 10 5 K for d-block elements (Banerjee et al 2020).…”

Coalescence of black hole–neutron star binaries

“…To derive nucleosynthetic yields and characteristics of electromagnetic counterparts, we need to understand properties of the ejecta such as the mass, the velocity, and the electron fraction that characterizes the neutron richness. In particular, the electron fraction primarily determines the nucleosynthetic yield, which controls features of the kilonova/macronova via the opacity (Kasen et al 2013;Tanaka and Hotokezaka 2013;Tanaka et al 2018Tanaka et al , 2020Banerjee et al 2020) and the heating rate (Hotokezaka et al 2016b; Barnes et al 2016;Kasen and Barnes 2019;Waxman et al 2019;Hotokezaka and Nakar 2020). If a significant fraction of the ejecta keeps extreme neutron richness of the neutron star, ultraheavy elements may be produced in abundance, and the associated fission and/or a-decay will power the kilonova/macronova at late times (Wanajo et al 2014;Zhu et al 2018;Wu et al 2019).…”

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

“…This would make the UVOT W2 and -band light curves dimmer, while causing more re-processed emission in the red and infrared. However, opacities at early ( 1 d) phases of kilonova evolution may be lower than they are later (Banerjee et al 2020). The effects of realistic opacities on the colours of these models will be an interesting direction for future work.…”

“…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).…”