The mini-GWAC optical follow-up of gravitational wave alerts - results from the O2 campaign and prospects for the upcoming O3 run

The second (O2) observational campaign of gravitational waves (GWs) organized by the LIGO/Virgo Collaborations has led to several breakthroughs such as the detection of GW signals from merger systems involving black holes or neutrons stars. During O2, 14 GW alerts were sent to the astronomical community with sky regions mostly covering over hundreds of square degrees. Among them, six were finally confirmed as real astrophysical events. Since 2013, a new set of ground-based robotic telescopes called Ground-based Wide Angle Camera system (GWAC) project and its pathfinder mini-GWAC has been developed to contribute to the various challenges of multi-messenger and time domain astronomy. The GWAC system is built up in the framework of the ground-segment system of the SVOM mission that will be devoted to the study of the multi-wavelength transient sky in the next decade. During O2, only the mini-GWAC telescope network was fully operational. Due to the wide field of view and fast automatic follow-up capabilities of the mini-GWAC telescopes, they were adept to efficiently cover the sky localization areas of GW event candidates. In this paper, we present the mini-GWAC pipeline we have set up to respond to GW alerts and we report our optical follow-up observations of eight GW alerts detected during the O2 run. Our observations provided the largest coverage of the GW localization areas with a short latency made by any optical facility. We found tens of optical transient candidates in our images, but none of those could be securely associated with any confirmed black hole - black hole merger event. Based on this first experience and the near future technical improvements of our network system, we will be more competitive in detecting the optical counterparts from some GW events that will be identified during the upcoming O3 run, especially those emerging from binary neutron star mergers.