American B-52 bomber makes a run over Raqqa city
There is no evidence to suggest that American strategic bombers are carrying out large-area carpet bombings over Raqqa and so it seems highly unlikely due to the political repercussions of such extreme military action by the US.
In any case, it is possible that the bombers, loaded with precision munitions (i.e. JDAM and Paveway guided bombs), constantly loiter over Raqqa to be on call for strikes against specific ISIS targets (tactical positions, car bombs, troop movements, etc) as they are identified by forward air controllers on the ground or by reconnaissance drones.