Research reveals eight new sources of black hole echoes

Scattered across our galaxy, the Milky Way, are tens of thousands of black holes – immensely powerful gravitational wells in space-time from which matter and even light can never escape. Black holes are dark by definition, except in the rare cases where they give birth. When a black hole sucks gas and dust in from an orbiting star, it can emit spectacular bursts of X-rays that bounce off and echo the inspired gas, briefly illuminating the extreme surroundings of a black hole.

Now astronomers at MIT are looking for glimpses 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 area of ​​a black hole.

In a study published today in Astrophysical journal, researchers report using a new automated search tool, dubbed the “Reverberation Machine”, to search satellite data for signs of black hole echoes. In their search, they discovered eight new black hole binaries that echo 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 put 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 stream of relativistic particles launched at almost the speed of light. The researchers found that the black hole at some point emits one last high-energy flash before turning into a low-energy “soft” state.

This last glimpse may be a sign that a black-hole corona, the area of ​​high-energy plasma just outside a black-hole boundary, is expanding briefly and emitting one last eruption of high-energy particles before it disappears completely. These results could help explain how larger supermassive black holes in the center of a galaxy can expel 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,” said 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 where they are located. sig. »

The first author of the study is MIT graduate student Jingyi Wang; other co-authors include Matteo Lucchini and Ron Remillard from MIT, as well as collaborators from Caltech and other institutions.

X-ray delays

Kara and her colleagues use X-ray echoes to map the vicinity of a black hole, just as 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 from 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 growth disk. Seeing how these timelines change can reveal how a black hole’s corona and slice evolve as the black hole consumes stellar matter.

Ekko Evolution

In their new study, the team developed a search algorithm to search data taken by NASA’s Neutron Star Interior Composition Explorer or NICER, a high-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 dense enough and bright enough to be able to distinguish the beam echoes. X in the middle of the explosions. Eight of the 10 were previously not known for emitting echoes.

“We’re seeing new reverb signatures in eight sources,” Wang said. “Black holes have a mass of 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.

Video echoes 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 characteristics”, i.e. similar delays between the high energy X-rays and the retracted echoes. 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 development across all systems. In the initial “hard” state, where a corona and a beam of high-energy particles dominate the energy of the black hole, they detected short and fast time shifts of the order of milliseconds. This severe condition lasts for several weeks. Then there is a transition over several days where the corona and jet splash and die out and a soft state takes over, dominated by lower energy x-rays from the accretion disk of the hole.

During this difficult-to-soft transition state, the team found that time delays increased momentarily in all 10 systems, suggesting that the distance between the corona and the disk also increased. One explanation is that the corona can briefly expand outward and upward, in a final eruption of high energy, before the black hole completes most of its star meal and calms 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 have shown that these echoes are commonly observed and we are able to investigate the connections between a black hole disk, jet and corona in a new way.”

This research was partially supported by NASA.

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