The galaxy MCG+11-11-032 is relatively close to us: its light takes about 150 million years to reach us. At its center, like in most large galaxies, sits a supermassive black hole. But a few years ago, astronomers began to suspect: what if there are two?
The clue came from the X-ray spectrum. Iron heated near a black hole emits very characteristic X-ray light — the so-called iron line. In the spectrum of MCG+11-11-032, this line appeared doubled: two peaks instead of one. That could happen if each of two black holes has its own disk of gas, and each disk produces its own line, slightly shifted due to motion.
To test the idea, astronomers pointed the XMM-Newton X-ray telescope at the galaxy. It observed it twice, six months apart. If the black holes were a pair, they would orbit each other, and the doubled line should persist. But in the new data, the iron line looks single. Both observation sets are perfectly described by a model with one black hole.
So the binary black hole system in MCG+11-11-032 most likely does not exist. The galaxy turned out to be ordinary for its class — with a single active black hole at its center. This doesn't mean binary black holes don't exist at all: they are searched for in other galaxies, and sometimes found. But this is not such a case.
The iron line in the X-ray spectrum is like a fingerprint: it always appears at the same energy if the gas is not moving. By its shift, astronomers measure the speed of gas around a black hole to within thousands of kilometers per second.