Improved Constraints on H_0 from a Combined Analysis of Gravitational-wave and Electromagnetic Emission from GW170817

摘要

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-rayto- 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 ～ 10~(48) -3 × 10~(50) 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 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 ～50 such events. Measurements at t > 100 days of the X-ray and radio emission will lead to tighter constraints.