Multimessenger parameter estimation of GW170817: from jet structure to
  the Hubble constant
  The electromagnetic radiation that followed the neutron star merger event
GW170817 revealed that gamma-ray burst afterglows from jets misaligned with our
line of sight exhibit a light curve with slowly rising flux. The slope of the
rising light curve depends sensitively on the angle of the observer with
respect to the jet axis, which is likely to be perpendicular to the merger
plane of the neutron star binary. Therefore, the afterglow emission can be used
to constrain the inclination of the merging system. Here, we calculate the
gamma-ray burst afterglow emission based on the realistic jet structure derived
from general-relativistic magnetohydrodynamical simulations of a black hole
torus system for the central engine of the gamma-ray burst. Combined with
gravitational wave parameter estimation, we fit the multi-epoch afterglow
emission of GW170817. We show that with such a jet model, the observing angle
can be tightly constrained by multi messenger observations. The best fit
observing angle of GW170817 is $\theta_{\rm v} = 0.38\pm 0.02$ rad. With such a
constraint, we can break the degeneracy between inclination angle and
luminosity distance in gravitational wave parameter estimation, and
substantially increase the precision with which the Hubble constant is
constrained by the standard siren method. Our estimation of the distance is
$D_{\rm L}=43.4\pm 1\ \rm Mpc$ and the Hubble constant constraint is $69.5\pm
4\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$. As a result, multimessenger observations of
short-duration gamma-ray bursts, combined with a good theoretical understanding
of the jet structure, can be powerful probes of cosmological parameters.
