by crawler » Sat Feb 04, 2023 10:13 am
Five years of flickering black holes
In our new work, we used data from NASA’s ATLAS telescope in Hawaii. It scans the entire sky every night (weather permitting), monitoring for asteroids approaching Earth from the outer darkness.
These whole-sky scans also happen to provide a nightly record of the glow of hungry black holes, deep in the background. Our team put together a five-year movie of each of those black holes, showing the day-to-day changes in brightness caused by the bubbling and boiling glowing maelstrom of the accretion disc.
The twinkling of these black holes can tell us something about accretion discs.
In 1998, astrophysicists Steven Balbus and John Hawley proposed a theory of “magneto-rotational instabilities” that describes how magnetic fields can cause turbulence in the discs. If that is the right idea, then the discs should sizzle in regular patterns. They would twinkle in random [SHOULD SAY NON-RANDOM] patterns that unfold as the discs orbit. Larger discs orbit more slowly with a slow twinkle, while tighter and faster orbits in smaller discs twinkle more rapidly.
But would the discs in the real world prove this simple, without any further complexities? (Whether “simple” is the right word for turbulence in an ultra-dense, out-of-control environment embedded in intense gravitational and magnetic fields where space itself is bent to breaking point is perhaps a separate question.)
Using statistical methods we measured how much the light emitted from our 5,000 discs flickered over time. The pattern of flickering in each one looked somewhat different.
But when we sorted them by size, brightness and colour, we began to see intriguing patterns. We were able to determine the orbital speed of each disc – and once you set your clock to run at the disc’s speed, all the flickering patterns started to look the same.
This universal behaviour is indeed predicted by the theory of “magneto-rotational instabilities”.
That was comforting! It means these mind-boggling maelstroms are “simple” after all.
And it opens new possibilities. We think the remaining subtle differences between accretion discs occur because we are looking at them from different orientations.
The next step is to examine these subtle differences more closely and see whether they hold clues to discern a black hole’s orientation. Eventually, our future measurements of black holes could be even more accurate.
This article is republished from The Conversation is the world's leading publisher of research-based news and analysis. A unique collaboration between academics and journalists. It was written by: Christian Wolf, Australian National University.
Hence the twinkling is not due to Earth's atmosphere.
https://jfuchs.hotell.kau.se/kurs/amek/prst/16_ackr.pdf
Accretion Disks: An Overview ….Author: Simon Kronberg…. Supervisor: Jürgen Fuchs…..FYGB08 - Analytisk Mekanik
Institutionen för Ingenjörsvetenskap och Fysik
Abstract: This report is intended as an overview and introduction to the formation and properties of accretion disks. The current theoretical framework surrounding the formation of stars is first presented followed by the specifics relating to disk formation and the structure of thin disks. This is superseded by a brief discussion of the evolution of accretion disks and finally a moderately detailed overview of angular momentum transport throughout the disk.
C. Magneto-Rotational Instabilities
Balbus & Hawley (1998) propose a model in which the presence of magnetic fields produces a coupling between different annuli within the disk. This interaction may be viewed as a weak spring, the schematic view of which is available in Fig. 4. A requirement for the instability of such a magnetised disk is that the orbital velocity decreases with radius,
i.e. dΩ/dr < 0.
For a Keplerian disk, this is clearly valid as Ω = ΩK, see Eq. (2) for the definition of which. Figure 4: Two masses in orbit connected by a weak spring. The spring exerts a tension force T resulting in a transfer of angular momentum from the inner mass mi to the outer mass mo. If the spring is weak, the transfer results in an instability as mi loses angular momentum, drops through more rapidly rotating inner orbits, and moves further ahead. The outer mass mo gains angular momentum, moves through slower outer orbits, and drops further behind. The spring tension increases and the process runs away, i.e. becomes unstable due to its accelerating pace (figure and caption (edited) from Balbus & Howley [10]). This is not a sufficient prerequisite, however. The disk also needs to be ionised since electrically neutral gas does not interact well with the magnetic field lines. The critical ionization degree is examined by Sano & Stone (2002) and they found it to be ne/ntot ∼ 10−12 [11].
[i][color=#000080]Five years of flickering black holes
In our new work, we used data from NASA’s ATLAS telescope in Hawaii. It scans the entire sky every night (weather permitting), monitoring for asteroids approaching Earth from the outer darkness.
These whole-sky scans also happen to provide a nightly record of the glow of hungry black holes, deep in the background. Our team put together a five-year movie of each of those black holes, showing the day-to-day changes in brightness caused by the bubbling and boiling glowing maelstrom of the accretion disc.
The twinkling of these black holes can tell us something about accretion discs.
In 1998, astrophysicists Steven Balbus and John Hawley proposed a theory of “magneto-rotational instabilities” that describes how magnetic fields can cause turbulence in the discs. If that is the right idea,[u] then the discs should sizzle in regular patterns.[/u] They would twinkle in [u]random[/u] [/i] [color=#BF0040][SHOULD SAY NON-RANDOM][/color][i] patterns that unfold as the discs orbit. Larger discs orbit more slowly with a slow twinkle, while tighter and faster orbits in smaller discs twinkle more rapidly.
But would the discs in the real world prove this simple, without any further complexities? (Whether “simple” is the right word for turbulence in an ultra-dense, out-of-control environment embedded in intense gravitational and magnetic fields where space itself is bent to breaking point is perhaps a separate question.)
Using statistical methods we measured how much the light emitted from our 5,000 discs flickered over time. The pattern of flickering in each one looked somewhat different.
But when we sorted them by size, brightness and colour, we began to see intriguing patterns. We were able to determine the orbital speed of each disc – and once you set your clock to run at the disc’s speed, all the flickering patterns started to look the same.
This universal behaviour is indeed predicted by the theory of “magneto-rotational instabilities”.
That was comforting! It means these mind-boggling maelstroms are “simple” after all.
And it opens new possibilities. We think the remaining subtle differences between accretion discs occur because we are looking at them from different orientations.
The next step is to examine these subtle differences more closely and see whether they hold clues to discern a black hole’s orientation. Eventually, our future measurements of black holes could be even more accurate.
This article is republished from The Conversation is the world's leading publisher of research-based news and analysis. A unique collaboration between academics and journalists. It was written by: Christian Wolf, Australian National University.[/color][/i]
Hence the twinkling is not due to Earth's atmosphere.
https://jfuchs.hotell.kau.se/kurs/amek/prst/16_ackr.pdf
Accretion Disks: An Overview ….Author: Simon Kronberg…. Supervisor: Jürgen Fuchs…..FYGB08 - Analytisk Mekanik
Institutionen för Ingenjörsvetenskap och Fysik
Abstract: This report is intended as an overview and introduction to the formation and properties of accretion disks. The current theoretical framework surrounding the formation of stars is first presented followed by the specifics relating to disk formation and the structure of thin disks. This is superseded by a brief discussion of the evolution of accretion disks and finally a moderately detailed overview of angular momentum transport throughout the disk.
[i][color=#BF0000]C. Magneto-Rotational Instabilities
Balbus & Hawley (1998) propose a model in which the presence of magnetic fields produces a coupling between different annuli within the disk. This interaction may be viewed as a weak spring, the schematic view of which is available in Fig. 4. A requirement for the instability of such a magnetised disk is that the orbital velocity decreases with radius,
i.e. dΩ/dr < 0.
For a Keplerian disk, this is clearly valid as Ω = ΩK, see Eq. (2) for the definition of which. Figure 4: Two masses in orbit connected by a weak spring. The spring exerts a tension force T resulting in a transfer of angular momentum from the inner mass mi to the outer mass mo. If the spring is weak, the transfer results in an instability as mi loses angular momentum, drops through more rapidly rotating inner orbits, and moves further ahead. The outer mass mo gains angular momentum, moves through slower outer orbits, and drops further behind. The spring tension increases and the process runs away, i.e. becomes unstable due to its accelerating pace (figure and caption (edited) from Balbus & Howley [10]). This is not a sufficient prerequisite, however. The disk also needs to be ionised since electrically neutral gas does not interact well with the magnetic field lines. The critical ionization degree is examined by Sano & Stone (2002) and they found it to be ne/ntot ∼ 10−12 [11].[/color][/i]