# Optical follow-up of gravitational wave events during the second advanced LIGO/VIRGO observing run with the DLT40 Survey

## TL;DR

The DLT40 survey conducted optical follow-up of gravitational wave events during LIGO/Virgo's second observing run, successfully identifying an associated kilonova with GW170817 and planning to expand its search in future runs.

## Contribution

This study presents a real-time galaxy-targeted follow-up strategy for GW events and reports the first optical kilonova detection associated with GW170817.

## Key findings

- Detected the kilonova AT 2017fgo associated with GW170817
- Followed ten GW triggers, observing 20-100 galaxies per event
- Identified unrelated supernova SN 2017cbv in the follow-up

## Abstract

We describe the GW follow-up strategy and subsequent results of the DLT40 during the second science run (O2) of the LVC. Depending on the information provided in the GW alert together with the localization map sent by the LVC, DLT40 would respond promptly to image the corresponding galaxies selected by our ranking algorithm in order to search for possible EM counterparts in real time. During the LVC O2 run, DLT40 followed ten GW triggers, observing between $\sim$20-100 galaxies within the GW localization area of each event. From this campaign, we identified two real transient sources within the GW localizations with an appropriate on-source time -- one was an unrelated type Ia supernova (SN~2017cbv), and the other was the optical kilonova, AT 2017fgo/SSS17a/DLT17ck, associated with the binary neutron star coalescence GW170817 (a.k.a gamma-ray burst GRB170817A). We conclude with a discussion of the DLT40 survey's plans for the upcoming LVC O3 run, which include expanding our galaxy search fields out to $D\approx$65 Mpc to match the LVC's planned three-detector sensitivity for binary neutron star mergers.