  We combine gravitational wave (GW) and electromagnetic (EM) data to perform a
Bayesian parameter estimation of the binary neutron star (NS) merger GW170817.
The EM likelihood is constructed from a fit to a large number of numerical
relativity simulations which we combine with a lower bound on the mass of the
remnant's accretion disk inferred from the modeling of the EM light curve. In
comparison with previous works, our analysis yields a more precise
determination of the tidal deformability of the binary, for which the EM data
provide a lower bound, and of the mass ratio of the binary, with the EM data
favoring a smaller mass asymmetry. The 90\% credible interval for the areal
radius of a $1.4\ M_\odot$ NS is found to be $12.2^{+1.0}_{-0.8} \pm 0.2\ {\rm
km}$ (statistical and systematic uncertainties).
