Even astronomers find black holes to be strange objects. Nothing can escape, not even light itself, because of how tightly space is bent around them due to their immense bulk.
However, certain black holes are observable despite their well-known darkness. Before making their one-way journey into the hole, the gas and stars that these galactic vacuums eat are drawn into a luminous disk that may shine brighter than whole galaxies.
These glowing black holes are much stranger. Nobody is quite clear why the brightness of the luminous discs varies from day to day.
To try to figure out why this twinkle happens, we built on NASA’s asteroid defense program by monitoring over 5,000 of the fastest-growing black holes in the sky for five years. We described our response in a study that was published in Nature Astronomy on February 2, 2023: This type of turbulence is fueled by strong magnetic and gravitational fields combined with friction.
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Giant Star-Eating Black Holes
We investigate supermassive black holes, which are as enormous as millions or billions of suns and are located at the heart of galaxies.
One of these giants, with a mass of almost 4 million suns, sits in the heart of our own galaxy, the Milky Way. The 200 billion or more stars that comprise the remainder of the galaxy, including our sun, largely orbit the central black hole in a happy manner.
But not every galaxy has it all that easy. Several stars may find themselves drawn uncomfortably near to their galaxy’s black hole as a result of galaxies pulling on one another through gravity. The stars have a terrible ending as they are broken up and eaten.
We can’t think of any other way for black holes to have grown to such a massive size, thus we are certain that this must have occurred in galaxies with black holes weighing as much as a billion suns. It could have also occurred in the Milky Way’s history.
Another, more gradual method of feeding for black holes is via suckling in gas clouds expelled by elderly stars called red giants.
Time of Black Hole Feeding
In this work, we examined in detail the feeding mechanism of the universe’s 5,000 black holes with the highest rates of growth.
We found the black holes with the greatest hunger in previous tests. A black hole the size of Earth is devouring material every second, according to research done in 2022. Another black hole that devours an entire sun every 48 hours was found in 2018.
However, there are many unanswered issues regarding their true eating habits. It is known that material entering the hole spirals into a brilliant “accretion disc” that may outshine whole galaxies. Quasars are black holes that are visible to the naked eye.
The majority of these black holes are really far away, making it impossible for us to observe any fine detail in the discs. Images of accretion discs surrounding neighboring black holes are available, but they don’t appear to be starving; instead, they appear to be inhaling cosmic gas.
Five Years of Black Holes That Flickered
We used data from NASA’s ATLAS telescope in Hawaii for our most recent study. Every night, weather permitting, it searches the whole sky for asteroids that could be headed toward Earth from space.
In addition, these full-sky images offer a nightly log of the luminous black holes in the distance. Our team created a five-year video of each of those black holes to illustrate how the accretion disc’s seething, boiling, blazing maelstrom affects brightness on a daily basis.
These black holes’ flashing can provide some insight on accretion disks.
Astrophysicists John Hawley and Steven Balbus introduced the concept of “magneto-rotational instabilities” in 1998 to explain how magnetic fields might induce turbulence in the discs. If that’s the correct thought process, then the discs ought should sizzle in predictable ways. As the discs orbit, their erratic patterns of twinkles would emerge. Smaller discs have tighter, quicker orbits that glitter more quickly than larger discs, which orbit more slowly.
Would discs in the real world, however, demonstrate similar simplicity in the absence of any additional complexity? (It may be debatable if “simple” describes turbulence in a very dense, uncontrollably distorted environment surrounded by powerful gravitational and magnetic forces where space itself is bent to the breaking point.)
We monitored the amount of flickering light radiated from our 5,000 discs over time using statistical techniques. Each one’s flashing pattern seemed somewhat differently.
However, we saw several interesting patterns when we arranged them according to size, brightness, and color. We were able to calculate the orbital speed of every disk, and all of the flickering patterns began to resemble one another when your clock was adjusted to match the disc’s speed.
In fact, the idea of “magneto-rotational instabilities” predicts this ubiquitous behavior. That gave me comfort. It implies that these seemingly incomprehensible whirlwinds are actually “simple”.
It also creates new opportunities. We believe that the remaining slight discrepancies between the accretion discs are the result of our seeing them from different angles.
The next step is to investigate these minute variations in further detail to see whether they provide any hints as to how to determine the direction of a black hole. Our measurements of black holes in the future could eventually be much more precise.
Christian Wolf teaches astronomy and astrophysics as an associate professor at the Australian National University. He is a member of the Astronomical Society of Australia (ASA) and receives support from the Australian Research Council (ARC).