What can electromagnetic signals help in the age of gravitational wave astronomy

On 2017 August 17, the LIGO-Virgo scientific collaboration for the first time detected a gravitational wave (GW) signal from a binary neutron star (NS) merger event (i.e., GW170817). Multi-wavelength electromagnetic (EM) counterparts to GW170817 have also been detected. In this talk, I will briefly review what EM observations can help in the age of GW-led multi-messenger astronomy, in three different situations: 1) joint detection case; 2) Only GW detection case; 3) Only EM detection case.