Astronomers have discovered eight new “echoing black hole binaries” in our galaxy with a new automated search tool that they call “Reverberation Machine.” These are systems with a star orbiting, and sometimes being eaten away by a black hole. Previously, only two such systems in the Milky Way were known to emit “X-ray echoes” that could be detected.
There are millions of black holes scattered across our galaxy. They are extremely strong wells of gravity that bend space and time. Nothing that falls in, even light, can escape from it. This makes them dark by definition, and difficult to detect. But as a black hole pulls in gas and dust from an orbiting star, it can give off bursts of X-ray light that bounce and echo off the gas spiralling into it and briefly illuminating the black hole’s surroundings.
The astronomers have published their findings in a research article titled, “The NICER “Reverberation Machine”: A Systematic Study of Time Lags in Black Hole X-Ray Binaries,” published in The Astrophysical Journal.
By comparing these X-ray echoes across systems, the team has pieced together a general picture of how a black hole evolves during an outburst. They observed that a black hole first undergoes a “hard: state, where it whips up a corona of high-energy photons along with a jet of relativistic particles that is then launched away at close to the speed of light.
At one point, the black hole gives off one final high-energy flash before transitioning to a “soft,” low-energy state. This flash could be a sign that the black hole’s corona (region of high-energy plasma right outside a black hole’s boundary) briefly expands and ejects a burst of high-energy particles before disappearing altogether.
These new findings could help scientists explain how larger, supermassive black holes at the centre of a galaxy can eject particles across vastly cosmic scales to shape a galaxy’s formation.
“The role of black holes in galaxy evolution is an outstanding question in modern astrophysicsInterestingly, 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,” says Erin Kara, assistant professor of physics at MIT, in a press statement.
Kara and her colleagues use X-ray echoes to map a black hole’s surroundings, similar to the way in which bats use echolocation to navigate their vicinity. Bats emit sounds that bounce off obstacles and return to the bat as an echo. The nocturnal animal can then calculate the distance between it and the obstacle based on the time it takes for the echo to return to them, helping them map their surroundings.
Similarly, the research team is looking to map the immediate vicinity of a black hole using X-ray echoes. These echoes present time delays between two types of X-ray light: light emitted directly from the corona, and light from the corona that bounces off the gas and dust spiralling into the black hole.
The researchers can observe the time at which a telescope receives light from the corona and compare it to when it receives the X-ray echoes to calculate an estimate of the distance between the corona and the accretion disk (disk-like flow of plasma, gas, dust and other matter around a black hole).
Observing how these time delays change over time will reveal how a black hole’s corona and disk evolve as the black hole consumes stellar material.
As a side project, Kara is working with MIT education and music scholars to convert these X-ray echo emissions into sound waves that can be heard by humans. You can listen to these waves in the video below.
The team identified a common theme of evolution in all systems. In the initial hard state, when the corona and high-energy particles dominate the black hole’s energy, they detected time lags that were short and fast, in the order of milliseconds. This state lasts for several weeks. After that, a transition occurs over several days, in which the corona and jet sputter and die out.
Then, the soft state dominated by lower-energy X-rays from the black hole’s accretion disk takes over. During this transition period, the astronomers discovered that time lags became longer for a short period in all ten systems, implying that the distance between the corona may briefly expand outward and upward in a high-energy burst before the black hole finishes consuming a bulk of the stellar material and goes quiet.