GW optical counterpart search in the Multi-Messenger Astronomy Era

We report on the deep optical follow-up surveys of the four gravitational-wave events accomplished by the GRAvitational Wave Inaf TeAm Collaboration (GRAWITA) using the VLT Survey Telescope (VST). We responded promptly to the gravitational-wave alerts sent by the LIGO and Virgo Collaborations, covering a region of $90$ deg$^2$, $72$ deg$^2$ and $99$ deg$^2$ for GW150914, GW151226 and GW170814 respectively, and kept observing the three areas over nearly two months. The surveys average limiting magnitude reached about 22 mag AB in the $r-$band. For GW170817 event we secured deep observations of AT2017gfo's host galaxy NGC4993. The paper outlines the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. For BBH events, numerous transients have been discovered, mostly variable stars and eclipsing binaries, but no candidates are identified as related to the gravitational-wave events. For GW170817 event the VST observations allows to measure the optical transient magnitudes at the faint end of the light curve.