Modelling the spectra of the kilonova AT2017gfo -- I: The photospheric epochs

Understand

The kilonova (KN) associated with the binary neutron star (BNS) merger GW170817 is the only known electromagnetic counterpart to a gravitational wave source.

• Here we produce a sequence of radiative transfer models (using $\textsc{tardis}$) with updated atomic data, and compare them to accurately calibrated spectra.
• We use element compositions from nuclear network calculations based on a realistic hydrodynamical simulation of a BNS merger.
• We show that the blue spectrum at +1.4 days after merger requires a nucleosynthetic trajectory with a high electron fraction.