Eight New Echoing Black Hole Binaries Discovered in Our Galaxy: MIT Researchers

Black holes are intriguing and mysterious objects. They are dreaded, too, because their gravitational pull is so strong that they do not allow anything to pass through them, not even light, except on the rare occasions when they feed. When a black hole pulls in gas and dust from an orbiting star, it sends out spectacular bursts of X-ray that bounce and echo off the gas spiralling inwards. During this phase, the back hole illuminates its extreme surroundings. Researchers from MIT have now found eight new echoing black hole binaries — systems with a star orbiting, and occasionally being eaten away by, a black hole — in our galaxy Milky Way. Previously, only two were known.

The researchers looked for flashes and echoes from nearby black hole X-ray binaries, using a new automated search tool, called the “Reverberation Machine”. This research was supported, in part, by NASA.

By comparing the echoes, they created a general picture of how a black hole evolves during an outburst. They found 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. A final, high-energy flash is emitted by the black hole at a given point. The system then enters a low-energy (soft) condition.

This final flash could indicate that a black hole's corona extends briefly before disappearing completely. These findings, published in the Astrophysical Journal, could help explain how larger, supermassive black holes at the center of a galaxy shape its formation.

“The role of black holes in galaxy evolution is an outstanding question in modern astrophysics,” said Erin Kara, assistant professor of physics at MIT, in a statement. Kara said 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 picked up 26 black hole X-ray binary systems known to emit X-ray outbursts. Of these, 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 had never been known to produce echoes before.