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

The second (O2) observational campaign of gravitational waves (GW) organized by the LIGO/Virgo Collaborations have lead to several breakthroughs as the detection of gravitational signals from mergers systems involving black holes or neutrons stars. During O2, 14 gravitational waves alerts were sent to the astronomical community with sky regions covering mostly over hundreds of squares degrees. Among them, 6 have been confirmed as real events. Since 2013, a new set of ground-based robotical telescopes called GWAC (and its pathfinder mini-GWAC) has emerged to contribute to the various challenges of the multi-messenger and time domain astronomy. The GWAC system is developed within the framework of the ground-segment system of the SVOM mission aiming to study the multi-wavelength transient sky in the next decade. During O2, the mini-GWAC telescope network was fully operational. Due to their wide field of view and fast automatic follow-up, the mini-GWAC telescopes were well adapted to efficiently cover the sky localization area of the gravitational wave events. In this paper, we present the mini-GWAC pipeline we set up to answer to the GWalerts and we report our optical observations of 8 gravitational wave events detected during the O2 run. So far, our observations provide the largest probability coverage of the GW sky localization area in a short latency for any optical facilities. We found tens of optical transient candidates in our images, but none of these could be securely correlated to any confirmed black hole - black hole merger events. Based on this first experience and the near future technical improvements of our network system, we will be more competitive to detect the optical counterparts from some gravitational wave events detected during the upcoming O3 run, especially those from binary neutron star mergers.