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Millions of elusive black holes hide in plain sight across the Milky Way galaxy, only giving away their presence occasionally through bursts of X-ray light when they feed on stars.
Astronomers have been able to pin down the locations of eight rare pairings of black holes and the stars orbiting them, thanks to the X-ray echoes they release. Previously, there were only two known pairs emitting X-ray echoes in our galaxy.
Black hole binaries occur when these celestial phenomena are orbited by a star, which they sometimes use to siphon gas and dust as a snack.
The echoes have been converted into sound waves that just may keep you awake at night.
The findings from the study, published Monday in The Astrophysical Journal, could help scientists understand how black holes evolve.
Anatomy of a black hole outburst
The research team developed an automated tool dubbed the “Reverberation Machine” to search for echoes from black hole binaries in satellite data.
During their study, the researchers used the Reverberation Machine to look through data collected by NASA’s X-ray telescope called the Neutron star Interior Composition Explorer, or NICER, which is part of the International Space Station.
“We see new signatures of reverberation in eight sources,” first study author Jingyi Wang, a graduate student at the Massachusetts Institute of Technology, 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.”
After collecting the eight echoes, the researchers compared them to see how a black hole changes when it releases an outburst of X-rays. A similar portrait emerged for the eight binary systems.
When black holes pull material from an orbiting star, they can launch bright “jets” of particles that go streaming into space at near the speed of light. The research team noticed that during this process, the black hole will release a final, highly energetic flash before it transitions to a low-energy state.
When that last blast occurs, it could mean the black hole’s highly energetic plasma ring (or corona) is releasing energized particles before it disappears.
Astronomers can apply this finding to larger supermassive black holes, which function as “engines” at the center of galaxies and can shoot out particles that can shape galactic formation.
“The role of black holes in galaxy evolution is an outstanding question in modern astrophysics,” study author Erin Kara, assistant professor of physics at MIT, said in a statement.
“Interestingly, these black hole binaries appear to be ‘mini’ supermassive black holes, and so by understanding the outbursts in these small, nearby systems, we can understand how similar outbursts in supermassive black holes affect the galaxies in which they reside.”
Turning X-ray echoes to sound
The echoes of these X-ray emissions can help astronomers map where black holes are located. It’s not unlike echolocation used by bats for navigation. Bats release calls that bounce off obstacles and return as an echo, and the length of the echo’s return helps bats determine the distance of objects.
Black hole echoes are created by two types of X-ray light released from the corona, and astronomers can use the amount of time it takes for the telescope to detect the two types to track how a black hole changes as it gobbles up material from the star.
The black hole echoes aren’t actual sounds we can hear without some help, so Kara collaborated with Kyle Keane, lecturer in MIT’s department of materials science and engineering, and Ian Condry, professor in MIT’s department of anthropology, to turn them into sound waves.
The team tracked changes in the X-ray echoes, determined time lags during transition stages and traced commonalities in the evolution of each black hole outburst.
The result sounds like something from a 1950s sci-fi film.
“We’re at the beginnings of being able to use these light echoes to reconstruct the environments closest to the black hole,” Kara said. “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.”