Observations of the first electromagnetic counterpart to a gravitational wave source by the TOROS collaboration

TL;DR

This paper reports the first optical follow-up observations of the electromagnetic counterpart to a gravitational wave event GW170817, revealing a rapidly fading blue kilonova consistent with neutron star merger models.

Contribution

It provides the first detailed optical light curves and color analysis of a kilonova associated with a gravitational wave source, confirming its rapid fading and blue color.

Findings

Detected significant dimming over 80 minutes post-trigger

Confirmed fast fading with a second observation at 59 hours

Observed colors indicate a lanthanide-poor blue kilonova

Abstract

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 (TOROS). 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 ~35 hr after the trigger with the T80-South telescope. A second epoch of observations, obtained ~59 hr after the event with the EABA 1.5m 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.