GW150914: First search for the electromagnetic counterpart of a gravitational-wave event by the TOROS collaboration

TL;DR

The TOROS collaboration conducted optical follow-up observations of GW150914, the first gravitational-wave event, but found no electromagnetic counterpart, consistent with expectations for a binary black hole merger.

Contribution

This study presents the first optical follow-up of GW150914 using difference imaging and machine learning algorithms to search for electromagnetic counterparts.

Findings

No transient events detected down to r=21.7 mag

Follow-up observations are consistent with no electromagnetic emission from GW150914

Method demonstrates effective transient discrimination in gravitational-wave follow-up

Abstract

We present the results of the optical follow-up conducted by the TOROS collaboration of the first gravitational-wave event GW150914. We conducted unfiltered CCD observations (0.35-1 micron) with the 1.5-m telescope at Bosque Alegre starting ~2.5 days after the alarm. Given our limited field of view (~100 square arcmin), we targeted 14 nearby galaxies that were observable from the site and were located within the area of higher localization probability. We analyzed the observations using two independent implementations of difference-imaging algorithms, followed by a Random-Forest-based algorithm to discriminate between real and bogus transients. We did not find any bona fide transient event in the surveyed area down to a 5-sigma limiting magnitude of r=21.7 mag (AB). Our result is consistent with the LIGO detection of a binary black hole merger, for which no electromagnetic counterparts are expected, and with the expected rates of other astrophysical transients.