Revisiting GW170817 at Milliarcsecond Scale: High-precision Constraints on Jet Geometry and <i>H</i> ₀

Abstract The historic detection of gravitational waves from the electromagnetically bright binary neutron star merger GW170817 enabled the first standard siren measurement of Hubble’s constant ( H 0 ). The accuracy and precision of this measurement depend crucially on how well the merger inclination angle is constrained, given its strong covariance with luminosity distance ( D L ). Modeling the light curve of the jet’s afterglow provides constraints on inclination, but is highly dependent on the similarly uncertain jet opening angle. Past studies have improved on this by invoking high-resolution radio observations, obtained through very long baseline interferometry (VLBI). We present a Bayesian visibility-plane model-fitting framework that provides a more informed and robust measurement of the viewing geometry of GW170817 and of H 0 , by including all relevant VLBI data, robustly handling systematic uncertainties, and rigorously sampling model parameter space. By fitting new hydrodynamical afterglow models with a continuum of jet geometries, we obtain a viewing angle of 18 . ° 3 − 20 . ° 3 (for a fixed cosmology with D L = 40.7 Mpc, as used in most previous analyses). We extend our framework to fit for D L and H 0 directly, and marginalize over an ensemble of plausible peculiar velocity corrections to obtain viewing angle 16 . ° 8−19 . ° 2, D L = 44.0 ± 1.6 Mpc, and H 0 = 65.5 ± 4.4 km s −1 Mpc −1 . Notably, the peak of our H 0 posterior is within 0.5 σ of the early-Universe Planck H 0 value, but 1.7 σ from the late-Universe SH0ES measurement. We discuss potential caveats and the implications of this result in the context of the current discrepancy between early- and late-Universe measurements of the Hubble constant.