Search reveals eight new sources of black hole echoes

Scattered across our Milky Way galaxy are tens of millions of black holes -- immensely strong gravitational wells of spacetime, from which infalling matter, and even light, can never escape. Black holes are dark by definition, except on the rare occasions when they feed. As a black hole pulls in gas and dust from an orbiting star, it can give off spectacular bursts of X-ray light that bounce and echo off the inspiraling gas, briefly illuminating a black hole's extreme surroundings.

Now MIT astronomers are looking for flashes and echoes from nearby black hole X-ray binaries -- systems with a star orbiting, and occasionally being eaten away by, a black hole. They are analyzing the echoes from such systems to reconstruct a black hole's immediate, extreme vicinity.

In a study appearing today in the Astrophysical Journal, the researchers report using a new automated search tool, which they've coined the "Reverberation Machine," to comb through satellite data for signs of black hole echoes. In their search, they have discovered eight new echoing black hole binaries in our galaxy. Previously, only two such systems in the Milky Way were known to emit X-ray echoes.

In comparing the echoes across systems, the team has pieced together a general picture of how a black hole evolves during an outburst. Across all systems, they observed that a black hole first undergoes a "hard" state, whipping up a corona of high-energy photons along with a jet of relativistic particles that is launched away at close to the speed of light. The researchers discovered that at a certain point, the black hole gives off one final, high-energy flash, before transitioning to a "soft," low-energy 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, 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 a black hole's vicinity, much the way that bats use sound echoes to navigate their surroundings. 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, the MIT team is 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.

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 signatures of reverberation in eight sources," Wang says. "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."

As a side project, Kara is working 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.

Video Echos of a Black Hole: https://youtu.be/iIeIag2Ji8k

The researchers 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.

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.

During this hard-to-soft transition state, the team 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.

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Bats are kings of small talk in the air

Bat conversations might be light on substance, according to researchers from the University of Cincinnati.

Echoes from bats are so simple that a sound file of their calls can be compressed 90% without losing much information, according to a study published in the journal PLOS Computational Biology.

The study demonstrates how bats have evolved to rely on redundancy in their navigational "language" to help them stay oriented in their complex three-dimensional world.

"If you can make decisions with little information, everything becomes simpler. That's nice because you don't need a lot of complex neural machinery to process and store that information," study co-author Dieter Vanderelst said.

UC researchers suspected that the calls of bats contain redundant information and that bats might use efficient encoding strategies to extract the most relevant information from their echoes. Many natural stimuli encountered by animals have a lot of redundancy. Efficient neural encoding retains essential information while reducing this redundancy.

To test their hypothesis, they built their own "bat on a stick," a tripod-mounted device that emits a pulse of sound sweeping from 30 to 70 kilohertz, a frequency range used by many bats. By comparison, human speech typically ranges from 125 to 300 hertz (or 0.125 to 0.3 kHz).

More than 1,000 echoes were captured in distinct indoor and outdoor environments such as in a barn, in different-sized rooms, among bushes and tree branches and in a garden.

Researchers converted the recorded echoes to a graph of the sound, called a cochleogram. Then they subjected these graphs to 25 filters -- essentially compressing the data. They trained a neural network, a computer system modeled on the human brain, to determine if the filtered graphs still contained enough information to complete a number of sonar-based tasks known to be performed by bats.

They found that the neural network correctly identified the location of the echoes even when the cochleogram was stripped of as much as 90% of its data.

"What that tells us is you can compress that data and still do what you need to do. It also means if you're a bat, you can do this efficiently," said Vanderelst, an assistant professor in UC's College of Arts and Sciences and in the College of Engineering and Applied Science.

Vanderelst said researchers often can infer what bats are doing just by listening to their calls.

"Even if you don't see the bat, you can tell with a high degree of certainty what a bat is doing," he said. "If it calls more frequently, it's looking for something. If the calls are spread out, it's cruising or studying something far away."

Bats produce their ultrasonic calls with a larynx much like ours. But what a voice box. It can contract 200 times a second, making it the fastest known muscle in all mammals.

The nighttime forest can be deafening to people because of its chorus of frogs and drone of insects. But Vanderelst said the ultrasonic frequency by comparison is pretty quiet, allowing bats to hear their own chittering calls that bounce off tree branches and other obstacles during echolocation.

While bats use different chirps for navigating than for communicating with each other, Vanderelst said they're all pretty simple. But human language has lots of built-in redundancy as well, Vanderelst said.

Fr xmpl, cn y rd ths sntnc wth mssng vwls?

"Take out a lot of letters in a sentence and it's still readable," Vanderelst said.

UC graduate Adarsh Chitradurga Achutha, Vanderelst's student, was the study's lead author. Co-authors include Vanderelst's mentor Herbert Peremans at the University of Antwerp, Belgium, and bat expert Uwe Firzlaff with the University of Munich, Germany.

The way bats perceive the world is fascinating both from biological and engineering perspectives, Vanderelst said.

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