Does a merging massive binary black hole ‘emits’ more than one gravitational wave? If we want to use gravitational waves (GW) to determine the Hubble constant, we need to find the source in the electromagnetic spectrum (EMS). However, we need to be lucky to ‘see’ it simultaneously in EMS and GW. This is a problem, but do BBH not ‘emit’ more than one GW? E.g. every time they spiral? Because than we have time to find it in the EMS, no? What makes it so difficult? Thanks in advance!
We can currently only detect gravitational radiation when it is extremely intense: in the last fraction of a second. For example the first gravitational wave detection lasted less 0.15 seconds. The black holes are releasing gravitational radiation with every orbit, but that radiation is too weak for us to detect. It takes a colossal amount of energy being released for it to be detected by current technology.  The inspiral of GW150914 released 3 solar masses of energy, nearly all in the final 0.15 seconds before merger.  Even so this only distorted space by less than 1/10000 of the diameter of a proton.  It is this that makes it so difficult.