counterpart

A use of the term counterpart in astronomy is for an astronomical object found for a second time, using another kind of radiation or other type of observation such as a GW detection. For example, if a radio signal has been received from some coordinates of the celestial sphere, then an object is seen at the same coordinates by visible light which is presumed to be the same object, that can be referred to as the radio source's optical counterpart. For observations in the opposite order, the radio signal could be referred to as the radio counterpart, and similarly for other types of EMR. The term counterpart suggests they are from the same object and is generally used when that is considered probable, but leaves open the possibility that this is not the case, such as if the angular resolutions of the observations leave some doubt as to the coordinates or the signals are from objects at different distances. A significant event in astronomy was the identification of an optical counterpart for the GW detection, GW170817.