As a stellar-mass black hole pulls in gas and dust from an orbiting star, it can give off spectacular bursts of X-rays that bounce and echo off the inspiraling gas, briefly illuminating a black hole’s extreme surroundings. In new research, astrophysicists from the MIT Kavli Institute for Astrophysics and Space Research and elsewhere have used an automated search tool — which they’ve coined the ‘Reverberation Machine’ — to comb through the data from NASA’s Neutron star Interior Composition Explorer (NICER), a high-time-resolution X-ray telescope aboard the International Space Station, for signs of black hole echoes. As a result, they’ve discovered eight new echoing black hole binaries in our Milky Way Galaxy. Previously, only two such systems in the Milky Way were known to emit X-ray echoes.
Black holes are the most extreme laboratories to study accretion and ejection physics, and to eventually test theories of gravity.
In addition to the supermassive black holes at the centers of galaxies, there are also stellar-mass black holes, which are mostly discovered when they are X-ray bright in systems called X-ray binaries.
These are binary systems comprised of a compact object accreting material from a companion star.
If the mass of the companion star is lower than roughly one solar mass, it is called a low-mass X-ray binaries. Depending on the nature of the central compact object, the source is categorized as either a black hole X-ray binary or a neutron star X-ray binary.
“We are using X-ray echoes to map a black hole’s vicinity, much the way that bats use sound echoes to navigate their surroundings,” said MIT astrophysicist Erin Kara and her colleagues.
“When a bat emits a call, the sound can bounce off an obstacle and return to the bat as an echo.”
“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.”
“In similar fashion, we are looking to map the immediate vicinity of a black hole using X-ray echoes.”
“The echoes represent time delays 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.”
In the study, the researchers developed a new search algorithm to comb through NICER data.
The algorithm picked out 26 black hole X-ray binary systems that were previously known to emit X-ray outbursts.
Of these 26, the scientists 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 signatures of reverberation in eight sources,” said MIT graduate student Jingyi Wang.
“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.”
The astrophysicists then ran the algorithm on the 10 black hole binaries and divided the data into groups with similar spectral timing features, that is, similar delays between high-energy X-rays and reprocessed echoes.
This helped to quickly track the change in X-ray echoes at every stage during a black hole’s outburst.
They 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.
During this hard-to-soft transition state, they discovered that time lags grew momentarily longer in all 10 systems, implying the distance between the corona and disk also grew larger.
One explanation is that the corona may briefly expand outward and upward, in a last high-energy burst before the black hole finishes the bulk of its stellar meal and goes quiet.
“We’re at the beginnings of being able to use these light echoes to reconstruct the environments closest to the black hole,” Dr. 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.”
The results appear in the Astrophysical Journal.
Jingyi Wang et al. 2022. The NICER ‘Reverberation Machine:’ A Systematic Study of Time Lags in Black Hole X-Ray Binaries. ApJ 930, 18; doi: 10.3847/1538-4357/ac6262
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