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 similar to 10(48) -3 x 10(50) erg propagating into an environment with density n similar to 10(-2) - 10(-4). cm(-3), with preference for wider jets (opening angle theta(j) = 15 degrees). For these jets, our modeling indicates an off-axis angle theta(obs) similar to 25 degrees-50 degrees. We combine our constraints on theta(obs) with the joint distance-inclination constraint from LIGO. Using the same similar to 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 H-0 = 74.0 +/- 11.5/7.5 km s(-1) Mpc(-1), which is higher than the value of H-0 = 70.0 +/- 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 H-0 = 75.5 +/- 11.6/9.6 km s(-1) Mpc(-1) for H-0 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 similar to 50 such events. Measurements at t > 100 days of the X-ray and radio emission will lead to tighter constraints.