Preparing for Gaia searches for optical counterparts of gravitational wave events during O4.

The discovery of gravitational wave (GW) events and the detection of electromagnetic counterparts from GW170817 has started the era of multimessenger GW astronomy. The field has been developing rapidly, and in this paper, we discuss the preparation for detecting these events with ESA's Gaia satellite, during the 4th observing run of the LIGO–Virgo–KAGRA (LVK) collaboration that has started on 2023 May 24. Gaia is contributing to the search for GW counterparts by a new transient detection pipeline called GaiaX. In GaiaX , a new source appearing in the field of view of only one of the two telescopes on-board Gaia is sufficient to send out an alert on the possible detection of a new transient. Ahead of O4, an experiment was conducted over a period of about two months. During the two weeks around New Moon in this period of time, the MeerLICHT (ML) telescope located in South Africa tried (weather permitting) to observe the same region of the sky as Gaia within 10 min. Any GaiaX detected transient was published publicly. ML and Gaia have similar limiting magnitudes for typical seeing conditions at ML. At the end of the experiment, we had 11 861 GaiaX candidate transients and 15 806 ML candidate transients, which we further analysed and the results of which are presented in this paper. Finally, we discuss the possibility and capabilities of Gaia contributing to the search for electromagnetic counterparts of gravitational wave events during O4 through the GaiaX detection and alert procedure. [ABSTRACT FROM AUTHOR]