What you're seeing is the shockwaves that form around the aircraft as it moves through the air faster than the speed of sound. Essentially, the aircraft is moving too fast for the air to get out of the way, so it is compressed.
A plane doesn't make a sonic boom as it breaks the sound barrier. Instead, we hear a sonic boom as those shockwaves pass over us. What that means is that the plane is making a sonic boom the entire time it is flying supersonic. This boom can be fairly loud, about as loud as a gunshot. This is actually one of the main barriers to supersonic commercial flight. It is currently illegal to make a sonic boom over land in the US and Europe and most of the rest of the world. We (meaning NASA and a bunch of aerospace companies and some universities) are working on designing an aircraft that makes a "low boom" or "shaped boom" that isn't so loud, with the goal of making commercial supersonic flight possible. This imaging technique is one of the tools we're using to do that.
