XMM-Newton maps black hole surroundings

 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 Cañada, Madrid, Spain
Email: Michael.Parker@esa.int

 Norbert Schartel
 XMM-Newton project scientist
 European Space Agency
 Email: norbert.schartel@esa.int