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

We present the results of the optical follow-up conducted by the TOROS collaboration of the rst gravitational-wave event GW150914. We conducted unltered CCD observations (0 :35 1 m) with the 1.5-m telescope at Bosque Alegre starting 2:5 days after the alarm. Given our limited eld of view ( 100u 0 ), 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 dierence-imaging algorithms, followed by a Random-Forest-based algorithm to discriminate between real and bogus transients. We did not nd any bona de transient event in the surveyed area down to a 5 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. Subject headings: Gravitational Waves, General relativity, GW150914, techniques: image processing