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Recent analysis of the kilonova, AT2017gfo, has indicated that this event washighly spherical. This may challenge hydrodynamics simulations of binaryneutron star mergers, which usually predict a range of asymmetries, andradiative transfer simulations show a strong direction dependence. Here weinvestigate whether the synthetic spectra from a 3D kilonova simulation ofasymmetric ejecta from a hydrodynamical merger simulation can be compatiblewith the observational constraints suggesting a high degree of sphericity inAT2017gfo. Specifically, we determine whether fitting a simple P-Cygni lineprofile model leads to a value for the photospheric velocity that is consistentwith the value obtained from the expanding photosphere method. We would inferthat our kilonova simulation is highly spherical at early times, when thespectra resemble a blackbody distribution. The two independently inferredphotospheric velocities can be very similar, implying a high degree ofsphericity, which can be as spherical as inferred for AT2017gfo, demonstratingthat the photosphere can appear spherical even for asymmetrical ejecta. Thelast-interaction velocities of radiation escaping the simulation show a highdegree of sphericity, supporting the inferred symmetry of the photosphere. Wefind that when the synthetic spectra resemble a blackbody the expandingphotosphere method can be used to obtain an accurate luminosity distance(within 4-7 per cent).

Towards inferring the geometry of kilonovae