The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source

We present an analysis of the host-galaxy environment of Swope Supernova Survey 2017a (SSS17a), the discovery of an electromagnetic counterpart to a gravitational-wave source, GW170817. SSS17a occurred 1.9. kpc (in projection; 10 ''.2) from the nucleus of NGC. 4993, an S0 galaxy at a distance of 40. Mpc. We present a Hubble Space Telescope (HST) pre-trigger image of NGC. 4993, Magellan optical spectroscopy of the nucleus of NGC. 4993 and the location of SSS17a, and broadband UV-through-IR photometry of NGC. 4993. The spectrum and broadband spectral-energy distribution indicate that NGC. 4993 has a stellar mass of log(M/M-circle dot) 10.49(-0.20) (+0.08) and star formation rate of 0.003 M-circle dot yr(-1), and the progenitor system of SSS17a likely had an age of >2.8. Gyr. There is no counterpart at the position of SSS17a in the HST pre-trigger image, indicating that the progenitor system had an absolute magnitude M-V > -5.8 mag. We detect dust lanes extending out to almost the position of SSS17a and >100 likely globular clusters associated with NGC. 4993. The offset of SSS17a is similar to many short gamma-ray-burst offsets, and its progenitor system was likely bound to NGC. 4993. The environment of SSS17a is consistent with an old progenitor system such as a binary neutron star system.