The sound of the echoes of a binary black hole, courtesy of Erin Kara and Kyle Keane of MIT. Animation calculated by Michal Dovciak.
Black holes that feed on companion stars can cycle through high-energy bursts. MIT astronomers use X-ray echoes of those cycles to map the environment around these exotic objects, similar to how bats map their environment via echolocation. The astronomers hope to use this new data to learn more about the evolution of these types of black hole systems, and by extension, the formation of galaxies, according to a new paper published in the Astrophysical Journal.
“The role of black holes in the evolution of galaxies is an excellent question in modern astrophysics,” said study co-author Erin Kara of MIT. “These black hole binary stars 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.”
As we’ve reported before, it’s a popular misconception that black holes act like cosmic vacuum cleaners, sucking up all the matter in their environment like a madman. In reality, only things beyond the event horizon — including light — are gobbled up and can’t escape, though black holes are messy eaters too. That means some of an object’s matter is ejected in a powerful beam.
If that object is a star, such as the companion star of a double galaxy with a black hole, the process of fragmentation (or “spaghettification”) by a black hole’s powerful gravitational pull takes place outside the event horizon, and some of the original mass is is forcibly thrown out. This process can form a rotating ring of matter (also called an accretion disk) around the black hole that emits powerful X-rays, visible light and sometimes radio waves. Those jets are one way astronomers can indirectly infer the presence of a black hole.
The MIT team was particularly interested in systems where the companion star has about one solar mass and exhibits cyclical eruptions in the form of X-ray flashes. According to the authors, most scientists believe that a hot plasma located close to the black hole, called the X-ray corona, plays a role in these cycles, but questions remain about how the X-ray corona is formed in the first place, as well as how it evolves during an eruption.
Aurore Simonnet/NASA Goddard Space Flight Center
The emitted X-rays can sometimes reflect off the accretion disk, creating ‘echoes’ of the original emission. And detecting those echoes provides an excellent opportunity to monitor how the black hole evolves as it feeds. In particular, it is possible to estimate the time delay between when a telescope detects light from the corona and when it picks up the X-ray echoes, and to monitor how that delay shifts as the system works through a burst cycle.
Astronomers had previously detected X-ray echoes (or reverberations) from two binary systems in the Milky Way galaxy. To look for more, the MIT team developed an automated search tool called the “Reverberation Machine” and used it to analyze data collected by NASA’s Neutron star Interior Composition Explorer (NICER) aboard the ISS. The Reverberation Machine identified 26 candidate black hole binary systems, of which 10 (including the previously detected systems) emitted detectable X-ray echoes.
All eight new binary systems of black holes that emit echoes ranged from five to 15 solar masses, and all of the companion stars were about the size of our sun. “As far as we can tell, the fact that we only see detections in about half of the black holes is due to their higher quality of data, not because they are particularly unique,” Kara told Ars.
What does this new data tell astronomers about how a binary black hole evolves during an outburst? The MIT team was able to construct a fairly universal picture. The system typically starts in a relatively quiescent state. As material falls faster on the accretion disk, so does the brightness of the X-rays, dominated by “hard” X-rays. This so-called “hard state” produces the corona and a beam of particles that are emitted into space at nearly the speed of light. During this period, the team found that the time delays between emission and echo were short and fast, only a few milliseconds.
After a few weeks, the cycle of eruptions ends – as the black hole nearly finished its stellar meal – with one last dramatic flash before entering a “soft” state of lower energy and finally returning to rest. The MIT team was intrigued to find that during this transition, the delays got longer for all 10 systems, implying an increase in the distance between the corona and the accretion disk. They suggested that this could be the result of the corona expanding during the latest high-energy burst.
“We’re just beginning to be able to use these light echoes to reconstruct the environments closest to the black hole,” Kara said. “Now we have shown that these echoes are often observed, and we are able to investigate connections between the disk, beam and corona of a black hole in a new way.”
