Eight new echoes from black holes discovered in our galaxy: MIT researchers

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Black holes are fascinating and mysterious objects. They are also feared because their gravitational pull is so strong that they will not let anything through, not even light, except on the rare occasions when they are feeding. When a black hole sucks gas and dust from an orbiting star, it emits spectacular X-rays that bounce and reverberate off the gas spiraling inward. During this phase, the rear hole illuminates its extreme surroundings. MIT researchers have now found eight new binaries with black hole echoes in our galaxy, the Milky Way – systems with a star orbiting a black hole and occasionally being eaten by a black hole. So far only two were known.

Researchers searched for flashes and echoes from nearby X-ray binaries of black holes using a new automated search tool called the Reverberation Machine. This research was partially supported by NASA.

By comparing the echoes, they created a general picture of how a black hole evolves during an eruption. They found that a black hole first goes through a ‘hard’ state, kicking up a corona of high-energy photons along with a jet of relativistic particles that are ejected at nearly the speed of light. A final, high-energy flash is emitted from the black hole at a specific point. The system then enters a low-energy (soft) state.

This latest flash could indicate that a black hole’s corona briefly expands before disappearing entirely. These findings, published in the Astrophysical Journal, could help explain how larger, supermassive black holes at the center of a galaxy shape their formation.

“The role of black holes in galaxy evolution is a prominent question in modern astrophysics,” Erin Kara, an assistant professor of physics at MIT, said in a statement. Kara said that by understanding the outburst in these small black hole binaries, they hope to understand how similar outbursts in supermassive black holes affect their native galaxies.

For their study, the team imaged 26 binary X-ray systems of black holes known to emit X-ray bursts. Of these, the team found that 10 systems were close enough and bright enough to see X-ray echoes amid the outbursts. Eight of the ten have never been known to produce echoes.


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