Improved constraints on H0 from a combined analysis of
  gravitational-wave and electromagnetic emission from GW170817
  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 $10^{48}\,\rm{erg}<E_{k}\le 3\times 10^{50} \,\rm{erg}$
propagating into an environment with density $n\sim10^{-2}-10^{-4}
\,\rm{cm^{-3}}$, with preference for wider jets (opening angle $\theta_j=15$
deg). For these jets, our modeling indicates an off-axis angle $\theta_{\rm
obs}\sim25-50$ deg. We combine our constraints on $\theta_{\rm obs}$ 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.
