# TOROS Optical follow-up of the Advanced LIGO-VIRGO O2 second observational campaign

## TL;DR

This paper reports on the optical follow-up observations of gravitational wave events during LIGO-Virgo's second observing run, using targeted galaxy observations and difference imaging to identify potential electromagnetic counterparts.

## Contribution

The study introduces a targeted galaxy observation strategy combined with machine learning discrimination techniques for optical follow-up of gravitational wave events.

## Key findings

- No optical counterparts found for GW170104 and other events.
- Confirmed detection of optical transient for GW170817, consistent with expectations.
- Results support the binary black hole merger hypothesis for GW170104.

## Abstract

We present the results of the optical follow-up, conducted by the TOROS collaboration, of gravitational wave events detected during the Advanced LIGO-Virgo second observing run (Nov 2016 -- Aug 2017). Given the limited field of view ($\sim100\arcmin$) of our observational instrumentation we targeted galaxies within the area of high localization probability that were observable from our sites. We analyzed the observations using difference imaging, followed by a Random Forest algorithm to discriminate between real and bogus transients. For all three events that we respond to, except GW170817, we did not find any bona fide optical transient that was plausibly linked with the observed gravitational wave event. Our observations were conducted using telescopes at Estaci\'{o}n Astrof\'{\i}sica de Bosque Alegre, Cerro Tololo Inter-American Observatory, and the Dr. Cristina V. Torres Memorial Astronomical Observatory. Our results are consistent with the LIGO-Virgo detections of a binary black hole merger (GW170104) for which no electromagnetic counterparts were expected, as well as a binary neutron star merger (GW170817) for which an optical transient was found as expected.