2020ApJ...888...67D

There is a strong degeneracy between the luminosity distance (D_L) and the observer viewing angle (θ_obs; hereafter viewing angle) of the gravitational wave (GW) source with an electromagnetic counterpart, GW170817. Here, for the first time, we present independent constraints on θ_obs=32.5_–9.7○+11.7^ from broadband photometry of the kilonova (kN) AT2017gfo associated with GW170817. These constraints are consistent with independent results presented in the literature using the associated gamma-ray burst GRB 170817A. Combining the constraints on θ_obs with the GW data, we find an improvement of 24% on H₀. The observer angle constraints are insensitive to other model parameters, e.g., the ejecta mass, the half-opening angle of the lanthanide-rich region and the temperature. A broad wavelength coverage extending to the near-infrared is helpful to robustly constrain θ_obs. While the improvement on H₀ presented here is smaller than the one from high angular resolution imaging of the radio counterpart of GW170817, kN observations are significantly more feasible at the typical distances of such events from current and future LIGO-Virgo collaboration observing runs (D_L ∼ 100 Mpc). Our results are insensitive to the assumption of the peculiar velocity of the kN host galaxy.