Dr Veledina explains that the accretion disks are formed by the matter that is rotating and falling onto a massive body of compact matter. As such, they are inevitable part of the energy extraction mechanism by black holes, neutron stars and white dwarfs. The matter that is being swallowed by, for example, the black hole, forms itself in a disk as it gets closer to the compact object.
Depending on the properties of the central object, the dynamics of the disk may vary substantially. For instance, one of the interesting questions is how the disk interacts with the spinning black hole and how its dynamics changes when the matter rotation is out-of-plane from the black hole rotation plane.
„The early work by Shakura & Sunyaev (1973) laid the basis for our understanding of these engines and we had a pleasure to have both of the authors of this seminal paper at the workshop!“, explains Veledina enthusiastically.
Alright, but let’s face it: we all want to go back to the fondue analogue, don’t we?! …so… Just as cheese and wine mix to create a delicious fondue, gas and dust swirl around white dwarfs, neutron stars, and black holes, creating spectacular accretion disks.
White Dwarfs: The Mild Emmental
White dwarfs are the mellow, steady remnants of stars (such as our Sun), much like Emmental cheese. Their size is similar to the Earth and their accretion disks are relatively calm, with occasional outbursts, similar to the gentle bubbles in a perfectly heated fondue.
Neutron Stars: The Tangy Gruyère
„Neutron stars are stellar remnants so dense that a spoon of its matter has the weigh of a Mount Everest, with magnetic fields that are thousands to Millions times higher than the record steady magnetic field observed on the Earth“, explains Veledina. Neutron stars are denser and more energetic than White Dwarfs, so akin to the sharp flavour of Gruyère cheese: The intense magnetic fields and rapid spin of neutron stars make their accretion disks more dynamic and exciting, much like the zesty kick of Gruyère in your fondue mix.
Black Holes: The Robust Vacherin
Stellar-mass and supermassive black holes are left behind by most massive stars and those residing in the centres of galaxies, respectively. With their immense gravitational pull, black holes are the bold Vacherin cheese in our cosmic fondue. Their accretion disks are the most extreme, producing powerful jets and high-energy radiation, comparable to the rich, hearty flavour that Vacherin adds to the fondue pot.
Interdisciplinary Meetings: The Perfect Blend
Like the perfect fondue, scientific discoveries are best enjoyed in a warm, welcoming environment. ISSI in Bern, at the fore-front of the Swiss Alps, offers a neutral and beautiful setting for scientists to come together, share ideas, and stir the pot of knowledge. Imagine discussing accretion disks while enjoying the scenic views of the Swiss Alps and the historic charm of Bern.
„I have been in Bern several times, but this was really the first time that I had a chance to enjoy the beauty of the town during summer period; I’ll be more than happy to come again“, says Veledina.
A Recipe for Success
Just as making a fondue requires the right mix of ingredients and careful stirring, advancing our understanding of the universe requires collaboration, open-mindedness, and a dash of creativity.
Veledina agrees: „This ISSI Workshop was very useful to revise our understanding and to hear different aspects of accretion disk studies (observations, simulations), and meet scientists working on different types of accretion disks, including early-career researchers, who are now pushing the frontiers of our understanding of disks. It is highly intriguing to try collecting different pieces of studies into one big puzzle.“
Accretion disks tell us a lot about the strong gravity laws! They provide fundamental insights into the interactions between matter and a strong magnetic field. They can be the most efficient energy extraction mechanisms—more efficient than extracting energy in nuclear reactions!
Alexandra Veledina and her colleagues compiled key knowledge of the past—or as they like to call it—first fifty years of accretion disk research to develop an up-to-date transdisciplinary understanding of what is known, and what is still to be studied as Veledina elaborates:
„Yes, undoubtedly, we have been able to achieve substantial advances with the advent of multi-wavelength observations (when telescopes across the world coordinate to look at one source, such as an accreting black hole), modern technologies such as high-perfomance computing replicating a lot of aspects of the disks and making them visually accessible, and new observing techniques such as the recently launched satellite Imaging X-ray Polarimetry Explorer, which is the first dedicated instrument that routinely measures X-ray polarisation). During the workshop, we have heard numerous talks on these topics.“
There is still more to discover for up-coming accretion disk enthusiasts, says Veledina: „There are many open questions! Perhaps, worth mentioning the origin of variability and oscillations of the disks, albeit this may be my somewhat biased answer…“.
So, let’s raise a toast (or rather a skewered piece of cheese-covered bread) to the cosmic fondue, to Accretion Disks, and to the joy of scientific discovery in the heart of Switzerland.🇨🇭🫕🌌
