Eight new echoing black hole binaries have been discovered in our galaxy, allowing astronomers to piece together a general picture of how a black hole evolves during an outburst. The findings will aid scientists in tracing the evolution of a black hole as it feeds on stellar material.
Tens of millions of black holes strewn across our Milky Way galaxy — enormously strong gravitational wells of spacetime from which infalling matter, and even light, can never escape. Except on rare occasions when they feed, black holes are inherently dark. As a black hole absorbs gas and dust from an orbiting star, it can produce spectacular bursts of X-ray light that bounce and echo off the inspiraling gas, briefly illuminating the extreme surroundings.
MIT astronomers are now looking for flashes and echoes from nearby black hole X-ray binaries – systems in which a star orbits and is occasionally devoured by a black hole. They’re using echoes from such systems to reconstruct a black hole’s immediate, extreme surroundings.
The researchers report using a new automated search tool, dubbed the “Reverberation Machine,” to comb through satellite data for signs of black hole echoes in a study published today in the Astrophysical Journal. They discovered eight new echoing black hole binaries in our galaxy during their search. Only two such systems in the Milky Way were previously known to emit X-ray echoes.
The team has pieced together a general picture of how a black hole evolves during an outburst by comparing echoes across systems. Across all systems, they discovered that a black hole first enters a “hard” state, producing a corona of high-energy photons as well as a jet of relativistic particles that travels at close to the speed of light. The researchers discovered that at some point, the black hole emits one final, high-energy flash before transitioning to a “soft,” low-energy state.
We see new reverberation signatures in eight sources. The black holes have masses ranging from five to fifteen times that of the sun, and they’re all in binary systems with normal, low-mass, sun-like stars.
Jingyi Wang
This final flash could indicate that a black hole’s corona, the region of high-energy plasma just outside the black hole’s boundary, briefly expands, ejecting a final burst of high-energy particles before disappearing entirely. These findings could help to explain how larger, supermassive black holes at the center 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 astrophysics,” says Erin Kara, assistant professor of physics at MIT. “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.”
The study’s first author is MIT graduate student Jingyi Wang; other co-authors include Matteo Lucchini and Ron Remillard at MIT, along with collaborators from Caltech and other institutions.
X-ray delays
Kara and her colleagues are using X-ray echoes to map the vicinity of a black hole, similar to how bats use sound echoes to navigate their surroundings. When a bat calls, the sound may bounce off an object and return to the bat as an echo. The amount of time it takes for the echo to return is proportional to the distance between the bat and the obstacle, providing the animal with a mental map of its surroundings.
Similarly, the MIT team intends to use X-ray echoes to map the immediate vicinity of a black hole. The echoes represent the difference in time between two types of X-ray light: light emitted directly from the corona, and light from the corona that bounces off the accretion disk of inspiraling gas and dust.
The time when a telescope receives light from the corona, compared to when it receives the X-ray echoes, gives an estimate of the distance between the corona and the accretion disk. Watching how these time delays change can reveal how a black hole’s corona and disk evolve as the black hole consumes stellar material.
Echo evolution
In their new study, the team developed search algorithm to comb through data taken by NASA’s Neutron star Interior Composition Explorer, or NICER, a high-time-resolution X-ray telescope aboard the International Space Station. The algorithm picked out 26 black hole X-ray binary systems that were previously known to emit X-ray outbursts. Of these 26, the team found that 10 systems were close and bright enough that they could discern X-ray echoes amid the outbursts. Eight of the 10 were previously not known to emit echoes.
“We see new reverberation signatures in eight sources,” Wang says. “The black holes have masses ranging from five to fifteen times that of the sun, and they’re all in binary systems with normal, low-mass, sun-like stars.”
Kara is collaborating with MIT education and music scholars Kyle Keane and Ian Condry on a side project to convert the emission from a typical X-ray echo into audible sound waves. The algorithm was then applied to the 10 black hole binaries, and the data was divided into groups with similar “spectral timing features,” or delays between high-energy X-rays and reprocessed echoes. This aided in tracking the change in X-ray echoes at each stage of a black hole’s outburst.
The team identified a common evolution across all systems. In the initial “hard” state, in which a corona and jet of high-energy particles dominates the black hole’s energy, they detected time lags that were short and fast, on the order of milliseconds. This hard state lasts for several weeks. Then, a transition occurs over several days, in which the corona and jet sputter and die out, and a soft state takes over, dominated by lower-energy X-rays from the black hole’s accretion disk.
The team discovered that time lags grew momentarily longer in all ten systems during this hard-to-soft transition state, implying that the distance between the corona and disk grew as well. One theory is that the corona may briefly expand outward and upward in a final burst of high energy before the black hole finishes the majority of its stellar meal and goes quiet.
“We’re just getting started with using light echoes to reconstruct the environments closest to the black hole,” Kara says. “Now that we’ve demonstrated that these echoes are commonly observed, we can probe connections between a black hole’s disk, jet, and corona in a novel way.”
