Observations of the First Electromagnetic Counterpart to a Gravitational-wave Source by the TOROS Collaboration

We present the results of prompt optical follow-up of the electromagnetic counterpart of the gravitational-wave event GW170817 by the Transient Optical Robotic Observatory of the South Collaboration. We detected highly significant dimming in the light curves of the counterpart (Delta g= 0.17 +/- 0.03 mag, Delta r = 0.14 +/- 0.02 mag, Delta i = 0.10 +/- 0.03 mag) over the course of only 80 minutes of observations obtained similar to 35 hr after the trigger with the T80- South telescope. A second epoch of observations, obtained similar to 59 hr after the event with the EABA 1.5 m telescope, confirms the fast fading nature of the transient. The observed colors of the counterpart suggest that this event was a "blue kilonova" relatively free of lanthanides.