Event Horizon Telescope

But what would two black holes look like? Well, NANOGrav is seeking gravitational waves — subtle perturbations in space-time that appear when a very massive object, like a supermassive black hole, is accelerated. We don’t expect to see bright gravitational waves from a single supermassive black hole, but when galaxies merge, two supermassive black holes can meet and begin a roughly billion-year waltz as they orbit, inspiral and eventually coalesce with one another. During their inspiral, they are constantly accelerating and will produce some of the brightest expected gravitational waves in the Universe.

Seeing gravitational waves will be an amazing feat, and will be enhanced multifold by the detection of light from those self-same systems: a “multi-messenger detection”. M87 has long been suggested to be a binary (rather than single) supermassive black hole, although the EHT refutes that idea; if it were two black holes, we would have seen a distorted event horizon or even two rings, rather than the near-perfect annular structure seen in the EHT image. While most of NANOGrav’s expected targets will not be nearby enough to resolve the two event horizons in the way EHT did, we can and will be looking for the signatures of binary motion from large-scale tracers of outflows from the black holes: periodically varying light, wobbling jet axes, double jets, moving emission lines that track the black holes’ orbits, and many other exciting signatures!