  The LIGO detection of gravitational waves (GW) from merging black holes in
2015 marked the beginning of a new era in observational astronomy. The
detection of an electromagnetic signal from a GW source is the critical next
step to explore in detail the physics involved. The Antarctic Survey Telescopes
(AST3), located at Dome A, Antarctica, is uniquely situated for rapid response
time-domain astronomy with its continuous night-time coverage during the
austral winter. We report optical observations of the GW source (GW~170817) in
the nearby galaxy NGC 4993 using AST3. The data show a rapidly fading transient
at around 1 day after the GW trigger, with the $i$-band magnitude declining
from $17.23\pm0.13$ magnitude to $17.72\pm0.09$ magnitude in $\sim 1.8$ hour.
The brightness and time evolution of the optical transient associated with
GW~170817 are broadly consistent with the predictions of models involving
merging binary neutron stars. We infer from our data that the merging process
ejected about $\sim 10^{-2}$ solar mass of radioactive material at a speed of
up to $30\%$ the speed of light.
