In the act of feeding black holes release X-rays, and these can reflect off material around the hole, which astronomers call “echoes”. Initial observations came from enormous black holes in other galaxies, but the same thing has since been reported in two smaller and closer black holes. Now, scientists have converted these "echoes" into sounds for the first time, and yes, they're eerie.

Such a small sample meant these echoes might have been a rare phenomenon. New research reveals it's actually much more common, with eight other examples in our galaxy announced in the Astrophysical Journal. The discovery of a population showing the same effect is allowing MIT graduate student Jingyi Wang and co-authors to track the cycle black holes go through to make these echoes in a way that can't be done with only one or two examples.

In addition to the supermassive black hole at its core, the Milky Way is believed to contain tens of millions of stellar black holes – the remains of supernova explosions in stars at least 25-40 times the mass of the Sun. Their number is based on modeling because actually finding most of these objects is beyond us; they're undetectable unless their gravity affects a star, or they are feeding on material pulled off a neighbor.

The minority that are feeding reveal themselves through the X-ray radiation they emit, caused by the high temperatures the gas around the black hole is heated to prior to consumption

In a search of 26 black holes known to emit X-rays as they feed on material from a companion star, Wang and co-authors found echoes in 10 of them, only two of which were previously known, where the initial flash of X-rays is followed by a delayed reflection. By measuring the delay between the initial X-ray burst and the reflected echoes, astronomers can work out the distance to whatever is reflecting it.

By observing cycles of emissions and intervals the authors create a story of black holes generating a jet of particles pushed out at close to the speed of light, accompanied by a corona of high-energy photons. This is referred to as a “hard state” and usually lasts several weeks. Then the black hole transitions to a soft state lasting weeks to months, where the environment around it is much lower energy. The transition is marked by a final high-energy flash lasting a few days.

During the transition the delays grew longer in all systems, indicating the reflection comes from further away from the black hole. The authors propose what we see as a flash involves a brief expansion of the plasma around the black hole, ejecting high-energy particles in the process.

The authors have collaborated with music scholars to turn these electromagnetic waves into sound, and you can experience the somewhat creepy results below.

_{Credit: Sound computed by Kyle Keane and Erin Kara, MIT. Animation computed by Michal Dovciak, ASU CAS.}

“We see new signatures of reverberation in eight sources,” Wang said in a statement. “The black holes range in mass from five to 15 times the mass of the Sun, and they’re all in binary systems with normal, low-mass, sun-like stars.”

“We’re at the beginnings of being able to use these light echoes to reconstruct the environments closest to the black hole,” said co-author Dr Erin Kara. “Now we’ve shown these echoes are commonly observed, and we’re able to probe connections between a black hole’s disk, jet, and corona in a new way.”