Constraints on the ejecta of the GW170817 neutron star merger from its electromagnetic emission

We present a simple analytic model that captures the key features of the emission of radiation from material ejected by the merger of neutron stars (NSs), and construct the multiband and bolometric luminosity light curves of the transient associated with GW170817, AT 2017gfo, using all available data. The ultraviolet to infrared (IR) emission is shown to be consistent with a single ≈0.05M _⊙ component ejecta, with a power-law velocity distribution between ≈0.1c and > 0.3c, a low opacity, k < 1 cm ² g ^-1, and a radioactive energy release rate consistent with an initial Y _e < 0.4. The late-time spectra require an opacity of k _v ≈ 0.1 cm ² g ^-1 at 1 to 2 μm. If this opacity is provided entirely by lanthanides, their implied mass fraction is X _Ln≈ 10 ^-3, approximately 30 times below the value required to account for the solar abundance. The inferred value of X _Ln is uncertain due to uncertainties in the estimates of IR opacities of heavy elements, which also do not allow the exclusion of a significant contribution to the opacity by other elements (the existence of a slower ejecta rich in lanthanides, which does not contribute significantly to the luminosity, can also not be ruled out). The existence of relatively massive, ≈0.05M _⊙, ejecta with high velocity and low opacity is in tension with the results of numerical simulations of NS mergers.