As material spirals towards a black hole, it is heated up and emits X-rays that, in turn, echo and reverberate as they interact with nearby gas. These regions of space are highly distorted and warped due to the extreme nature and crushingly strong gravity of the black hole. Now, a team of astronomers and astrophysicists has used ESA’s XMM-Newton X-ray observatory to track these light echoes and map the surroundings of the black hole in the center of the highly variable active galaxy IRAS 13224-3809.
These illustrations show the surroundings of a black hole feeding on ambient gas as mapped using ESA’s XMM-Newton X-ray observatory. As the material falls into the black hole, it spirals around to form a flattened disk, as shown here, heating up as it does so. At the very center of the disk, close to the black hole, a region of very hot electrons — with temperatures of around a billion degrees — known as the corona produced high-energy X-rays that stream out into space. Alston et al used the reverberating echoes of this radiation, as observed by XMM-Newton, to map the surroundings of a black hole. They focussed on the black hole at the core of the active galaxy IRAS 13224-3809, which is one of the most variable X-ray sources in the sky, undergoing very large and rapid fluctuations in brightness of a factor of 50 in mere hours. By tracking the X-ray echoes, it was possible to trace the dynamic behaviour of the corona itself, where the intense X-ray emission originates from. The corona is shown here as the bright region hovering over the black hole, changing in size and brightness. The researchers found that the corona of the black hole within IRAS 13224-3809 changed in size incredibly quickly, over a matter of days. Image credit: ESA.
IRAS 13224-3809, also known as LEDA 88835 and 2MASX J13251937-3824524, is located approximately one billion light-years away in the constellation of Centaurus.
The galaxy hosts a relatively low-mass (about one million solar masses) supermassive black hole in its center.
It is one of the most variable X-ray sources in the sky, undergoing very large and rapid fluctuations in brightness of a factor of 50 in mere hours.
“Everyone is familiar with how the echo of their voice sounds different when speaking in a classroom compared to a cathedral — this is simply due to the geometry and materials of the rooms, which causes sound to behave and bounce around differently,” said Dr. William Alston, an astrophysicist at the University of Cambridge.
“In a similar manner, we can watch how echoes of X-ray radiation propagate in the vicinity of a black hole in order to map out the geometry of a region and the state of a clump of matter before it disappears into the singularity. It’s a bit like cosmic echo-location.”
As the dynamics of infalling gas are strongly linked to the properties of the consuming black hole, Dr. William and colleagues were also able to determine the mass and spin of the supermassive black hole in IRAS 13224-3809 by observing the properties of matter as it spiralled inwards.
The inspiralling material forms a disk as it falls into the black hole. Above this disk lies a region of very hot electrons — with temperatures of around a billion degrees — called the corona.
While the researchers expected to see the reverberation echoes they used to map the region’s geometry, they also spotted something unexpected: the corona itself changed in size incredibly quickly, over a matter of days.
“As the corona’s size changes, so does the light echo — a bit like if the cathedral ceiling is moving up and down, changing how the echo of your voice sounds,” Dr. William said.
“By tracking the light echoes, we were able to track this changing corona, and — what’s even more exciting — get much better values for the black hole’s mass and spin than we could have determined if the corona was not changing in size.”
“We know the black hole’s mass cannot be fluctuating, so any changes in the echo must be down to the gaseous environment.”
The scientists used the longest observation of an accreting black hole ever taken with XMM-Newton, collected over 16 spacecraft orbits in 2011 and 2016 and totaling 2 million seconds — just over 23 days.
This, combined with the strong and short-term variability of the black hole itself, allowed the team to model the echoes comprehensively over day-long timescales.
Their results appear in the journal Nature Astronomy.
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W.N. Alston et al. A dynamic black hole corona in an active galaxy through X-ray reverberation mapping. Nat Astron, published online January 20, 2020; doi: 10.1038/s41550-019-1002-x
This article is based on text provided by the European Space Agency.
