Material falling into a black hole casts X-rays out into space – and now, for the first
time, ESA’s XMM-Newton X-ray observatory has used the reverberating echoes of
this radiation to map the dynamic behaviour and surroundings of a black hole itself.
Most black holes are too small on the sky for us to resolve their immediate environment, but
we can still explore these mysterious objects by watching how matter behaves as it nears, and
falls into, them.
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.
For the first time, researchers have used XMM-Newton to track these light echoes and map the
surroundings of the black hole at the core of an active galaxy. Named IRAS 13224–3809, the
black hole’s host galaxy 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”, explains William Alston
of the University of Cambridge, and lead author of the new study.
“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, William and colleagues were also able to determine the mass and spin of the galaxy’s
central black hole by observing the properties of matter as it spiralled inwards.
The inspiralling material forms a disc as it falls into the black hole. Above this disc lies a region
of very hot electrons – with temperatures of around a billion degrees – called the corona.
While the scientists 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,” adds William.
“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 study used the longest observation of an accreting black hole ever taken with XMM-
Newton, collected over 16 spacecraft orbits in 2011 and 2016 and totalling 2 million seconds –
just over 23 days. This, combined with the strong and short-term variability of the black hole
itself, allowed William and collaborators to model the echoes comprehensively over day-long
timescales.
The region explored in this study is not accessible to observatories such as the Event Horizon
Telescope, which managed to take the first ever picture of gas in the immediate vicinity of a
black hole – the one sitting at the centre of the nearby massive galaxy M87. The result, based
on observations performed with radio telescopes across the world in 2017 and published last
year, immediately became a global sensation.
“The Event Horizon Telescope image was obtained using a method known as interferometry –
a wonderful technique that can only work on the very few nearest supermassive black holes to
Earth, such as those in M87 and in our home galaxy, the Milky Way, because their apparent
size on the sky is large enough for this method to work,” says co-author Michael Parker, who is
an ESA research fellow at the European Space Astronomy Centre near Madrid, Spain.
“By contrast, our approach is able to probe the nearest few hundred supermassive black holes
that are actively consuming matter – and this number will increase significantly with the
launch of ESA’s Athena satellite.”
Characterising the environments closely surrounding black holes is a core science goal for
ESA’s Athena mission, which is scheduled for launch in the early 2030s and will unveil the
secrets of the hot and energetic Universe.
Measuring the mass, spin and accretion rates of a large sample of black holes is key to
understanding gravity throughout the cosmos. Additionally, since supermassive black holes are
strongly linked to their host galaxy’s properties, these studies are also key to furthering our
knowledge of how galaxies form and evolve over time.
“The large dataset provided by XMM-Newton was essential for this result,” says Norbert
Schartel, ESA XMM-Newton Project Scientist.
“Reverberation mapping is an exciting technique that promises to reveal much about both
black holes and the wider Universe in coming years. I hope that XMM-Newton will perform
similar observing campaigns for several more active galaxies in coming years, so that the
method is fully established when Athena launches.”
Notes for editors
“A dynamic black hole corona in an active galaxy through X-ray reverberation mapping” by W.
N. Alston et al. is published in the journal Nature Astronomy.
The study uses data gathered by XMM-Newton’s European Photon Imaging Camera (EPIC).
For more information, please contact:
William Alston
Institute of Astronomy, University of Cambridge, UK
Email: wna@ast.cam.ac.uk
Michael Parker, European Space Agency
European Space Astronomy Centre
Villanueva de la CanĚada, Madrid, Spain
Email: Michael.Parker@esa.int
Norbert Schartel
XMM-Newton project scientist
European Space Agency
Email: norbert.schartel@esa.int