Improved Constraints on H₀ from a Combined Analysis of Gravitational-wave and Electromagnetic Emission from GW170817

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 × 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 ± $\frac{11.5}{7.5}$ km s^-1 Mpc^-1, which is higher than the value of H₀ 70.0 ± $\frac{12.0}{8.0}$ 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 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. Finally, measurements at t > 100 days of the X-ray and radio emission will lead to tighter constraints.