  NGC 4993 is the shell galaxy host of the GRB170817A short gamma-ray burst and
the GW170817 gravitational-wave event produced during a binary-neutron-star
coalescence. The galaxy shows signs, including the stellar shells, that it has
recently accreted a smaller, late-type galaxy. The accreted galaxy might be the
original host of the binary neutron star. We measured the positions of the
stellar shells of NGC 4993 in an HST/ACS archival image and use the shell
positions to constrain the time of the galactic merger. According to the
analytical model of the evolution of the shell structure in the expected
gravitational potential of NGC 4993, the galactic merger happened at least 200
Myr ago, with a probable time roughly around 400 Myr and the estimates higher
than 600 Myr being improbable. This constitutes the lower limit on the age of
the binary neutron star, because the host galaxy was probably quenched even
before the galactic merger, and the merger has likely shut down the star
formation in the accreted galaxy. We roughly estimate the probability that the
binary neutron star originates in the accreted galaxy to be around 30%.
