Material falling into a black hole throws X-rays into space, and astronomers decided to use the echoes of this radiation to map the dynamic behavior and surroundings of the black hole itself.
Most black holes are too small to determine their immediate surroundings, but you can explore these mysterious objects by observing the behavior of matter as it approaches and falls into them.
As the material spirals towards the black hole, it heats up and emits X-rays, which are reflected as it interacts with nearby gas. These regions of space are highly distorted and deformed due to the extreme nature and overwhelmingly strong gravity of the black hole.
Researchers are using the European Space Agency's XMM-Newton X-ray Observatory to track these light echoes and map the vicinity of the black hole in the core of the active galaxy. Their results are published in the journal Nature Astronomy.
The galaxy IRAS 13224-3809 has a black hole, and it is one of the most volatile sources of X-rays in the sky, undergoing very large and rapid fluctuations in brightness - 50 times in a few hours.
"Everyone is familiar with how voices echo when talking in a room compared to a cathedral - it has to do with the geometry and materials of the rooms, which makes the sound behave differently," said Dr. William Allston of the Cambridge Institute of Astronomy, lead author of the new research. "Likewise, you can watch an X-ray echo propagate in the vicinity of a black hole to map the geometry of the region and the state of matter before it disappears into a singularity. It's a bit like a cosmic echo."
Because the dynamics of the incident gas is closely related to the properties of the consuming black hole, Allston and his colleagues were also able to determine the mass and rotation of the galaxy's central black hole by observing the properties of matter as it absorbs.
The material forms a disc when it enters a black hole. Above this disk is a region of hot electrons with a temperature of about a billion degrees, called the corona. Scientists expected to see reverberation echoes, which they used to map the geometry of the region, but also noticed something unexpected: the crown itself quickly changed in size within a few days.
"As the size of the crown changes, so does the light echo - as if the ceiling of a cathedral was moving up and down, changing the sound of the echo of the voice," Alston said. "By tracking the light echo, we were able to track the change in the size of the corona, and we get much better values for the mass and rotation of the black hole than those obtained if the corona did not change in size. We know that the mass of a black hole cannot fluctuate, so any changes in the echo must be related to the gas environment."
The study used the longest observation of an accreting black hole made by XMM-Newton, collected in 16 spacecraft orbits in 2011 and 2016 for a total of 2 million seconds - slightly over 23 days. This, combined with the strong and transient volatility of the black hole itself, allowed Allston and his team to comprehensively simulate echoes throughout the day.
The region of interest is inaccessible to observatories such as the Event Horizon Telescope, which captured the first ever photograph of a black hole in the center of nearby massive galaxy M87.
"Our approach allows us to explore the next few hundred supermassive black holes that are actively engulfing matter, and this number will increase significantly with the launch of ESA's Athena satellite."
Characterizing the environment close to black holes is one of the main scientific objectives of the Athena mission, which is scheduled to launch in the early 2030s and will reveal the secrets of a hot and energetic universe.
Measuring the mass, rotational speed, and accretion of a large sample of black holes is key to understanding gravity throughout the cosmos. In addition, since supermassive black holes are closely related to the properties of the host galaxy, these studies are also key to deepening our knowledge of the formation and evolution of galaxies over time.
“The large dataset provided by XMM-Newton was important to this result,” said Norbert Chartel, ESA's XMM-Newton Project Scientist. "Reverb mapping is a technique that promises to reveal a lot about both black holes and the Universe in the coming years. I hope that XMM-Newton will conduct similar campaigns in the coming years to observe several more active galaxies, so this method will fully developed when Athena starts up."