The discovery of the electromagnetic counterpart of GW170817: kilonova
  AT 2017gfo/DLT17ck
  During the second observing run of the Laser Interferometer gravitational-
wave Observatory (LIGO) and Virgo Interferometer, a gravitational-wave signal
consistent with a binary neutron star coalescence was detected on 2017 August
17th (GW170817), quickly followed by a coincident short gamma-ray burst trigger
by the Fermi satellite. The Distance Less Than 40 (DLT40) Mpc supernova search
performed pointed follow-up observations of a sample of galaxies regularly
monitored by the survey which fell within the combined LIGO+Virgo localization
region, and the larger Fermi gamma ray burst error box. Here we report the
discovery of a new optical transient (DLT17ck, also known as SSS17a; it has
also been registered as AT 2017gfo) spatially and temporally coincident with
GW170817. The photometric and spectroscopic evolution of DLT17ck are unique,
with an absolute peak magnitude of Mr = -15.8 \pm 0.1 and an r-band decline
rate of 1.1mag/d. This fast evolution is generically consistent with kilonova
models, which have been predicted as the optical counterpart to binary neutron
star coalescences. Analysis of archival DLT40 data do not show any sign of
transient activity at the location of DLT17ck down to r~19 mag in the time
period between 8 months and 21 days prior to GW170817. This discovery
represents the beginning of a new era for multi-messenger astronomy opening a
new path to study and understand binary neutron star coalescences, short
gamma-ray bursts and their optical counterparts.
