Listen to the Eerie Sound of Black Hole 'Echoes' for the First Time

You may have heard that black holes, hyper-dense objects that lurk in outer space, are notoriously hard to spot—let alone hear. 

But scientists have found a way to peer into the environments around these extreme entities, and even convert some of their so-called “echoes” into eerie sounds for the first time, shared in the below video, using an algorithm named the “reverberation machine,” reports a new study. 

Black holes are famous for their ability to trap anything, even light, inside their intense boundaries, known as the event horizon, making it tricky to detect them with light-based astronomy. But though these extreme objects are invisible, their interactions with nearby objects, such as stars or gas clouds, can produce some of the most luminous light shows in the universe.

Now, scientists led by Jingyi Wang, a PhD student at MIT, have used their reverberation machine to scan observations captured by Neutron star Interior Composition Explorer (NICER), an instrument aboard the International Space Station, to flag certain X-ray binaries, which are systems that contain a black hole that is pulling material off of a companion star and consuming it, a process that generates blasts of brilliant X-ray light. 

These systems can produce subtle X-ray echoes, also known as reverberation lags, which are time delays between light emitted directly from the corona, a bright ring of superhot particles surrounding the black hole, and reflected corona light that bounces off the disk of stellar material that is being consumed by the black hole. 

Scientists have previously detected reverberation lags in three low-mass X-ray binaries, which are systems that contain a relatively small companion star, about the size of the Sun. Now, Wang and her colleagues have discovered eight additional echoes, an achievement that “has not only increased the sample size by a factor of 5, but also expanded our horizons” regarding the dynamics of these intense systems, according to the team’s study, which was published on Monday in The Astrophysical Journal. 

“We perform the first systematic search of all NICER archival observations of black hole (and candidate) low-mass X-ray binaries for signatures of reverberation,” the researchers said in the study. “Reverberation lags result from the light travel time difference between the direct coronal emission and the reflected disk component, and therefore their properties are a useful probe of the disk-corona geometry.” 

“We detect new signatures of reverberation lags in eight sources, increasing the total sample from three to 11, and study the evolution of reverberation lag properties as the sources evolve in outbursts,” they added.  

These echoes offer a rarely seen glimpse into the otherworldly surroundings of stellar-mass black holes, which are about five to 15 times the mass of the Sun. For instance, Wang’s team reports that the profile of the lags changed as black holes transitioned from high-energy to low-energy states while feeding on their companions, a shift that reveals the hidden contours and relative distances of their coronas and disks.

These echoes can easily be turned into sound by matching their frequencies with corresponding acoustic signals. Indeed, this technique has already been used to eavesdrop on black holes that emit gravitational waves, which are ripples in spacetime, making them perfect candidates for sonification, as demonstrated in the above video. 

In this way, scientists can make these bizarre objects, which are still packed with mysteries, somewhat more relatable to the public—though these ghostly b-sides only reinforce the reputation of black holes as frightening and awesome entities that are best seen and heard from afar.