2017ApJ...851L..36G

The luminosity distance measurement of GW170817 derived from gravitational-wave analysis in Abbott et al. (2017a, hereafter A17:H0) is highly correlated with the measured inclination of the NS-NS system. To improve the precision of the distance measurement, we attempt to constrain the inclination by modeling the broadband X-ray-to-radio emission from GW170817, which is dominated by the interaction of the jet with the environment. We update our previous analysis and we consider the radio and X-ray data obtained at t < 40 days since merger. We find that the afterglow emission from GW170817 is consistent with an off-axis relativistic jet with energy E_k ∼ 10⁴⁸ -3 x 10⁵⁰ erg propagating into an environment with density n ∼ 10^–2-10^–4 cm^–3, with preference for wider jets (opening angle θ_j = 15°). For these jets, our modeling indicates an off-axis angle θ_obs ∼ 25°-50°. We combine our constraints on θ_obs with the joint distance-inclination constraint from LIGO. Using the same ∼170 km s^–1 peculiar velocity uncertainty assumed in A17:H0 but with an inclination constraint from the afterglow data, we get a value of H₀=74.0±11.57.5 km s^–1 Mpc^–1, which is higher than the value of H₀=70.0±12.08.0 km s^–1 Mpc^–1 found in A17:H0. Further, using a more realistic peculiar velocity uncertainty of 250 km s^–1 derived from previous work, we find H₀=75.5±11.69.6 km s^–1 Mpc^–1 for H₀ from this system. This is in modestly better agreement with the local distance ladder than the Planck cosmic microwave background, though such a significant discrimination will require ∼50 such events. Measurements at t > 100 days of the X-ray and radio emission will lead to tighter constraints.