Anyone who has approached a canyon – grand or just big -- or the opening of a large cave or tunnel generally can’t resist yelling “Helloooo!” and waiting for a responding echo. Would you do the same if the opening was for a black hole? One might think sound wouldn’t escape that light-sucking abyss, but a new study by MIT scientists with help from NASA found eight black holes in the Milky Way spewing out echoes of the screams of the binary star they’re busy eating … and they’ve even converted the X-ray waves to sound so you can hear it.
“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.”
Erin Kara, an assistant professor of physics at MIT and co-author of a new study on echoing black holes published in The Astrophysical Journal, explains how black hole binary systems are more interesting to astronomers than those supermassive black holes at the center of galaxies because the black hole is sometimes shredding its companion star and, contrary to popular belief, allowing chunks and sounds to escape from their gravitational pull. Kara and a team of MIT researchers knew of two binary systems in the Milky Way that emitted these X-ray echoes or reverberations and believed they could find more. To do this, they developed an automated search tool called the "Reverberation Machine" to analyze data collected by NASA's Neutron star Interior Composition Explorer (NICER) on board the ISS.
“We see new signatures of reverberation in eight sources. 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.”
Study first author and MIT graduate student Jingyi Wang explains in an MIT press release that the echoes are massive bursts of X-ray light that bounce and ‘echo’ off gas spiraling into the black hole as it shreds its companion star, briefly illuminating its immediate surroundings. By breaking down and studying the phases of these echoes, they were able to identify the states a black hole goes through as it spews out the particles that shapes its accretion disk or, in the case of a supermassive black hole, its galaxy.
According to the study, the states of a black hole go from “hard”, where it whips up a corona of high-energy photons and spews particles at lose to the speed of light, to a state-ending high-energy flash, to a “soft” low-energy state, to a final burst of high-energy particles before disappearing entirely. Between the states, the echo times change, signaling the transition -- an event that will help find more of these echoing black holes.
For those who think The Reverberation Machine would make a great band name, Erin Kara worked with MIT education and music scholars Kyle Keane and Ian Condry to convert the emission from a typical X-ray echo into audible sound waves, creating the first soundtrack for The Reverberation Machine's debut album – listen to it here.
It’s strange but it sure beats “Hellooooo! Is anybody theeeeere?”