Abstract
Abstract Ejected material from neutron star mergers gives rise to electromagnetic emission powered by radioactive decays of r-process nuclei, the so-called kilonova or macronova. While properties of the emission are largely affected by opacities in the ejected material, available atomic data for r-process elements are still limited. We perform atomic structure calculations for r-process elements: Se (Z = 34), Ru (Z = 44), Te (Z = 52), Ba (Z = 56), Nd (Z = 60), and Er (Z = 68). We confirm that the opacities from bound–bound transitions of open f-shell, lanthanide elements (Nd and Er) are higher than those of the other elements over a wide wavelength range. The opacities of open s-shell (Ba), p-shell (Se and Te), and d-shell (Ru) elements are lower than those of open f-shell elements, and their transitions are concentrated in the ultraviolet and optical wavelengths. We show that the optical brightness can be different by mag depending on the element abundances in the ejecta such that post-merger, lanthanide-free ejecta produce brighter and bluer optical emission. Such blue emission from post-merger ejecta can be observed from the polar directions if the mass of the preceding dynamical ejecta in these regions is small. For the ejecta mass of 0.01 , observed magnitudes of the blue emission will reach 21.0 mag (100 Mpc) and 22.5 mag (200 Mpc) in the g and r bands within a few days after the merger, which are detectable with 1 m or 2 m class telescopes.
Highlights
Direct detection of gravitational waves (GW) opened the era of GW astronomy (Abbott et al 2016c,a, 2017).A important step will be an identification of their electromagnetic (EM) counterparts to further study the astrophysical nature of the GW sources, as sky localization by GW detectors is not accurate enough to pin down their positions (Abbott et al 2016d)
From compact binary mergers including at least one neutron star (NS), i.e., NS-NS mergers and black hole (BH)-NS mergers, various EM signals are expected over a wide wavelength range (e.g., Metzger & Berger 2012; Rosswog 2015)
GRASP2K is based on the multiconfiguration Dirac-HartreeFock (MCDHF) and RCI methods taking into account Breit and quantum electrodynamic (QED) corrections (Grant 2007; Froese Fischer et al 2016)
Summary
Direct detection of gravitational waves (GW) opened the era of GW astronomy (Abbott et al 2016c,a, 2017). 2013; Tanaka & Hotokezaka 2013) This is due to the high opacities of r-process elements in the ejecta, especially those of Lanthanide elements (Kasen et al 2013). If the ejecta are free from Lanthanide elements, the emission from post-merger ejecta can be brighter and bluer, which can be called “blue kilonova” (Metzger & Fernández 2014; Kasen et al 2015). Due to the lack of atomic data of r-process elements, previous studies assume opacities of Fe for Lanthanide-free ejecta. We newly perform atomic structure calculations for selected r-process elements Using these data, we perform radiative transfer simulations and study the impact of element abundances to kilonova emission.
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