A binary black hole system in an active galaxy about 4 billion light-years away has been seen lighting up dramatically, when one of the black holes plowed through the accretion disk of the other, for a brief moment creating a double quasar.
A quasar is the extremely active core of a distant galaxy. This activity is the product of a supermassive black hole hungrily consuming matter, so much matter, in fact, that it can't handle all of it — instead, lots of material gets indiscriminately spat out in a magnetically collimated jet instead of falling beyond the black hole's event horizon like the rest of the matter. When we see such a jet of charged particles (which moves at almost the speed of light) head-on, the quasar looks especially bright. We call that a blazar.
