Scattered across our galaxy, the Milky Way, are tens of millions of black holes – immensely powerful gravitational wells of space-time, from which matter, and even light, can never escape. Black holes are dark by definition, except on the rare occasions when they feed. When a black hole sucks in gas and dust from an orbiting star, it can emit spectacular bursts of X-rays that bounce off and echo the inspired gas, briefly illuminating a black hole’s extreme surroundings.
Now astronomers at MIT are looking for flashes and echoes from X-ray binaries of nearby black holes – systems with an orbiting star, and sometimes eaten away by a black hole. They analyze the echoes of such systems to reconstruct the immediate and extreme neighborhood of a black hole.
In a study published today in the Astrophysical Journal, the researchers report using a new automated search tool, which they dubbed the “Reverberation Machine,” to scour satellite data for signs of black hole echoes. In their search, they discovered eight new black hole binaries echoing in our galaxy. Previously, only two such systems in the Milky Way were known to emit X-ray echoes.
By comparing echoes between systems, the team pieced together a general picture of how a black hole moves during an explosion. In all systems, they observed that a black hole first experiences a “hard” state, whipping up a corona of high-energy photons with a jet of relativistic particles that is launched at near the speed of light. . The researchers found that at a certain point, the black hole emits a final high-energy flash, before transitioning into a low-energy “soft” state.
This final flash may be a sign that a black hole’s corona, the region of high-energy plasma just outside a black hole’s boundary, is briefly expanding, ejecting a final burst of high-energy particles. before disappearing completely. These findings could help explain how larger supermassive black holes at the center of a galaxy can eject particles across extremely cosmic scales to shape galaxy formation.
“The role of black holes in the evolution of galaxies is an open 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 explosions in these nearby small systems, we can understand how similar explosions in supermassive black holes affect galaxies in which they reside.”
The first author of the study is MIT graduate student Jingyi Wang; other co-authors include Matteo Lucchini and Ron Remillard of MIT, as well as collaborators from Caltech and other institutions.
Kara and her colleagues use X-ray echoes to map the vicinity of a black hole, much like bats use sound echoes to navigate their surroundings. When a bat calls, the sound can bounce off an obstacle and echo back to the bat. The time it takes for the echo to return is relative to the distance between the bat and the obstacle, giving the animal a mental map of its surroundings.
Similarly, the MIT team seeks to map the immediate vicinity of a black hole using X-ray echoes. Echoes represent delays between two types of X-ray light: light emitted directly by the corona and the light of the corona bouncing off the accretion disk of inspired gas and dust.
The time 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 timelines change can reveal how a black hole’s corona and disk evolve as the black hole consumes stellar matter.
In their new study, the team developed a search algorithm to sift through data taken by NASA’s Neutron Star Interior Composition Explorer, or NICER, a high temporal resolution X-ray telescope aboard the International Space Station. The algorithm selected 26 binary black hole X-ray systems that were previously known to emit X-ray bursts. Of these 26, the team found that 10 systems were close enough and bright enough to be able to discern the ray echoes. X in the middle of the explosions. Eight of the 10 were previously not known to emit echoes.
“We see new reverb signatures in eight sources,” says Wang. “Black holes have masses five to 15 times the mass of the sun, and they are all in binary systems with normal, low-mass, sun-like stars.”
In a side project, Kara is working with MIT education and music specialists Kyle Keane and Ian Condry to convert the emission of a typical X-ray echo into audible sound waves. Listen to the sound of a black hole echo here:
Credit: Sound calculated by Kyle Keane and Erin Kara, MIT. Animation calculated by Michal Dovciak, ASU CAS.
The researchers then ran the algorithm on the 10 black hole binaries and split the data into groups with similar “spectral timing characteristics”, i.e. similar delays between the high-energy X-rays and the echoes retired. This made it possible to quickly follow the evolution of X-ray echoes at each stage of a black hole explosion.
The team identified a common evolution across all systems. In the initial “hard” state, in which a corona and jet of high-energy particles dominate the black hole’s energy, they detected short and rapid time shifts, on the order of milliseconds. This hard state lasts for several weeks. Then a transition occurs over several days, during which the corona and jet sputter and die out, and a soft state takes over, dominated by lower energy X-rays from the hole’s accretion disk. black.
During this hard-to-soft transition state, the team found that time lags increased momentarily in all 10 systems, implying that the distance between the corona and disc also increased. One explanation is that the corona may briefly expand outward and upward, in a final burst of high energy before the black hole completes most of its stellar meal and quiets down.
“We are on the verge of being able to use these light echoes to reconstruct the environments closest to the black hole,” says Kara. “Now we’ve shown that these echoes are commonly observed, and we’re able to probe the connections between a black hole’s disk, jet, and crown in a new way.”
This research was supported, in part, by NASA.