The gravitational wave sources
For our purpose, GW will be mainly produced in galaxies from the merger of compact objects in binary systems involving either two black holes (BBH), two neutron stars
(BNS) or a mix of them (NSBH). From the GW data we have a direct access to the distance (so the number of the galaxy candidates) of the event and the masses (so the nature) of the two compact objects involved in the merger.
The BNS mergers are the most promising source of electromagnetic counterparts as a fraction of the neutron stars matter can be expulsed and radiate through several mechanisms right after the coalescence.