Towards inferring the geometry of kilonovae

TL;DR

This paper investigates whether asymmetric kilonova ejecta can produce spectra consistent with observed high sphericity, demonstrating that even asymmetric models can appear spherical and yield accurate distance measurements.

Contribution

It shows that asymmetric kilonova ejecta can produce spectra and photospheric velocities consistent with spherical models, challenging previous assumptions about ejecta symmetry.

Findings

Synthetic spectra can mimic spherical ejecta when resembling blackbody distribution.

Photospheric velocities from different methods can be consistent in asymmetric models.

Spherical appearance allows accurate luminosity distance estimation within 4-7%.

Abstract

Recent analysis of the kilonova, AT2017gfo, has indicated that this event was highly spherical. This may challenge hydrodynamics simulations of binary neutron star mergers, which usually predict a range of asymmetries, and radiative transfer simulations show a strong direction dependence. Here we investigate whether the synthetic spectra from a 3D kilonova simulation of asymmetric ejecta from a hydrodynamical merger simulation can be compatible with the observational constraints suggesting a high degree of sphericity in AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line profile model leads to a value for the photospheric velocity that is consistent with the value obtained from the expanding photosphere method. We would infer that our kilonova simulation is highly spherical at early times, when the spectra resemble a blackbody distribution. The two independently inferred photospheric velocities can be very similar, implying a high degree of sphericity, which can be as spherical as inferred for AT2017gfo, demonstrating that the photosphere can appear spherical even for asymmetrical ejecta. The last-interaction velocities of radiation escaping the simulation show a high degree of sphericity, supporting the inferred symmetry of the photosphere. We find that when the synthetic spectra resemble a blackbody the expanding photosphere method can be used to obtain an accurate luminosity distance (within 4-7 per cent).