One of the galaxies studied in the paper. The active galactic nucleus is the bright central spot.
Credit: David W. Hogg, Michael R. Blanton and the Sloan Digital Sky Survey Collaboration.
BRISTOL: A unique method to study the physics near supermassive black holes has been used by astrophysicists in the U.S. to produce some surprising results.
Supermassive black holes are gigantic black holes believed to sit at the centre of most, if not all, galaxies. While they are thought to be some of the most important objects in the night sky, our understanding of them is poor.
The most obvious reason for this lack of understanding is that light cannot escape from the black hole itself, meaning direct observation is impossible. But supermassive black holes have swathes of matter surrounding them, known as accretion disks, that emit tremendous amounts of light, so physicists can use them to study the physics of black holes.
However, a new study published in The Astrophysical Journal suggests that our current theories regarding accretion disks may require revision.
X-ray fingerprints
David Ballantyne from the Georgia Institute of Technology and collaborators examined the X-ray light emitted from these accretion disks.
"We know that the X-rays must come from the innermost regions of the accretion disk, close to the central supermassive black hole. This region of the disk is therefore really interesting to study," said Ballantyne.
Gas at the accretion disk's surface ionises because it is bombarded by X-rays and ionisation leaves a characteristic fingerprint which can be measured and analysed.
Using this fingerprint, Ballantyne and his co-workers found that, contrary to theory, the relationship between ionisation and rate of accretion is linear as opposed to cubic.
Filling in the puzzle
Knowing this ionisation parameter, scientists can now constrain a number of other properties of active galactic nuclei such as black hole spin and the local gas density and structure.
Jonathan Trump of the University of California, who was not involved in the study, highlighted the significance of these findings: "There's still a lot that we don't understand about the extreme physics near supermassive black holes, and this work definitely helps to fill in the puzzle."
The discrepancy between the new result and established theory could be because our knowledge of the disk density or viscosity is wrong or we may need to change how we think energy flows within the disk. Whatever the reason, it is clear that further tests are required, as Trump explained, "It's a very small and carefully selected sample, and we need to test these ideas with a larger set of carefully measured active galactic nuclei before the result is robust."
Ballantyne agreed. "More measurements of the ionisation state of different accreting black holes are needed. A good place to start would be a complete and comprehensive re-analysis of data that exist in the archives," he said.
High school physics
Groundbreaking science such as this is usually solely the realm of experienced scientists. But Ballantyne was happy to enlist a keen student from South Cobb High School near Atlanta.
"John Rusin just emailed me out of the blue one morning asking if there was anything he could get involved with."
Helping with data acquisition, Rusin made a big impact. "He did tremendously well. Given the right project, I'm certainly open to the idea of collaborating with high school students again," said Ballantyne.
