Improved constraints on H0 from a combined analysis of gravitational-wave and electromagnetic emission from GW170817

Abstract

The luminosity distance measurement of GW170817 derived from GW analysis in Abbott et al. 2017 (here, 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 broad-band 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 $10^{48}\mathrm{erg}<{\mathrm{E}}_{\mathrm{k}}\le 3\times 10^{50}\mathrm{erg}$ propagating into an environment with density $n\sim 10^{-2}-10^{-4}\mathrm{c}{\mathrm{m}}^{-3}$, with preference for wider jets (opening angle $\hat{I}{¸}_j=15$ deg). For these jets, our modeling indicates an off-axis angle $\hat{I}{¸}_{\mathrm{obs}}\sim 25-50$ deg. We combine our constraints on $\hat{I}{¸}_{\mathrm{obs}}$ with the joint distance-inclination constraint from LIGO. Using the same $\sim 170$ km/sec peculiar velocity uncertainty assumed in A17:H0 but with an inclination constraint from the afterglow data, we get a value of $H_0=$$74.0\pm \frac{11.5}{7.5}$ $\mbox{km/s/Mpc}$, which is higher than the value of $H_0=$$70.0\pm \frac{12.0}{8.0}$ $\mbox{km/s/Mpc}$ found in A17:H0. Further, using a more realistic peculiar velocity uncertainty of 250 km/sec derived from previous work, we find $H_0=$$75.5\pm \frac{11.6}{9.6}$ km/s/Mpc for H0 from this system. We note that this is in modestly better agreement with the local distance ladder than the Planck CMB, though a significant such discrimination will require $\sim 50$ such events. Future measurements at $t>100$ days of the X-ray and radio emission will lead to tighter constraints.

Submitted to ApJL. Comments Welcome. Revised uncertainties in v2