A Globular Cluster Luminosity Function Distance to NGC 4993 Hosting a
  Binary Neutron Star Merger GW170817/GRB 170817A
  NGC 4993 hosts a binary neutron star merger emitting gravitational waves and
electromagnetic waves, GW170817/GRB 170817A. The distance to this galaxy is not
well established. We select the globular cluster candidates from the Hubble
Space Telescope/ACS F606W images of NGC 4993 in the archive, using the
structural parameters of the detected sources. The radial number density
distribution of these candidates shows a significant central concentration
around the galaxy center at the galactocentric distance $r<50''$, showing that
they are mostly the members of NGC 4993. Also the luminosity function of these
candidates is fit well by a Gaussian function. Therefore the selected
candidates at $r<50''$ are mostly considered to be globular clusters in NGC
4993. We derive an extinction-corrected turnover Vega magnitude in the
luminosity function of the globular clusters at $20''<r<50''$, F606W (max)$_0=
25.36\pm0.08$ ($V_0 =25.52\pm0.11$)} mag. Adopting the calibration of the
turnover magnitudes of the globular clusters, $M_V({\rm max})=-7.58\pm0.11$, we
derive a distance to NGC 4993, $d=41.65\pm3.00$ Mpc ($(m-M)_0=33.10\pm0.16$).
The systematic error of this method can be as large as $\pm0.3$ mag. This value
is consistent with the previous distance estimates based on the fundamental
plane relation and the gravitational wave method in the literature. The
distance in this study can be used to constrain the values of the parameters
including the inclination angle of the binary system in the models of
gravitational wave analysis.
