Impact of ejecta morphology and composition on the electromagnetic signatures of neutron star mergers

Abstract

The electromagnetic transients accompanying compact binary mergers (γ-ray bursts, afterglows and 'macronovae') are crucial to pinpoint the sky location of gravitational wave sources. Macronovae are caused by the radioactivity from freshly synthesised heavy elements, e.g. from dynamic ejecta and various types of winds. We study macronova signatures by using multidimensional radiative transfer calculations. We employ the radiative transfer code SuperNu and state-of-the art LTE opacities for a few representative elements from the wind and dynamical ejecta (Cr, Pd, Se, Te, Br, Zr, Sm, Ce, Nd, U) to calculate synthetic light curves and spectra for a range of ejecta morphologies. The radioactive power of the resulting macronova is calculated with the detailed input of decay products. We assess the detection prospects for our most complex models, based on the portion of viewing angles that are sufficiently bright, at different cosmological redshifts (z). The brighter emission from the wind is unobscured by the lanthanides (or actinides) in some of the models, permitting non-zero detection probabilities for redshifts up to z = 0.07. We also find the nuclear mass model and the resulting radioactive heating rate are crucial for the detectability. While for the most pessimistic heating rate (from the FRDM model) no reasonable increase in the ejecta mass or velocity, or wind mass or velocity, can possibly make the light curves agree with the observed nIR excess after GRB130603B, a more optimistic heating rate (from the Duflo-Zuker model) leads to good agreement. We conclude that future reliable macronova observations would constrain nuclear heating rates, and consequently help constrain nuclear mass models.

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. II. UV, Optical, and Near-infrared Light Curves and Comparison to Kilonova Models

“…The presence of an initially blue SED, which then transitions to a multi-component SED and finally to a red SED, is strongly suggestive of both blue and red KN emission; this is consistent with lanthanide-poor and rich ejecta components, respectively (Metzger et al 2010;Tanaka & Hotokezaka 2013;Metzger & Fernández 2014;Kasen et al 2015;Wollaeger et al 2017). Furthermore, the deviations from a pure blackbody spectrum at late times are indicative of the strong UV line blanketing expected for lanthanide-rich material, lending further evidence to the existence of a red KN component.…”

Electromagnetic counterparts to structured jets from gravitational wave detected mergers

“…BH-NS mergers would result in brighter macronova, ∼ 23.8, 23.2, 22.8 respectively, although the ejecta in these cases is not isotropic. The macronova estimates should be considered as upper limits, for the adopted model, as the peak flux depends on the inclination where the brightest emission coincides with the polar axis (the jet axis) (Tanaka 2016;Wollaeger et al 2017); however, macronova may be brighter than the adopted model i.e. Jin et al (2016).…”

Merger and Mass Ejection of Neutron Star Binaries

“…Notably, the bound-bound absorption opacity of open f -shell elements (lanthanides and actinides) differs significantly from the opacity of others, because open f -shell elements have such a high number of excited levels with relatively low excitation energy that the number of transition lines in the optical and IR bands is greatly enhanced (125,17,18). Radiation transfer simulations of merger ejecta show that the mean opacity, κ, is ∼ > 10 cm 2 /g for lanthanide-rich ejecta while it is ∼ 0.1 cm 2 /g for lanthanide-free ejecta (125,17,18,116,126,127). This finding implies that the Ye distribution of ejecta, which primarily determines the abundance pattern of r-process elements, is the key for determining the features of kilonovae.…”