Black holes come in a vast array of sizes. There are the stellar-mass black holes with a few times the mass of the sun — and then there are the supermassive black holes with billions of times the mass of our star that dominate the hearts of all large galaxies. However, new research suggests the "burps" that black holes emit when feeding on unfortunate stars seem to be remarkably similar — no matter their size.
For many years, scientists have looked at black holes and theorized that despite these objects' immense differences in size and mass, the underlying physics should be the same. That has been incredibly difficult to test, however. This is because supermassive black holes at the heart of galaxies evolve over thousands, or sometimes even millions, of years.
A way around this hurdle, though, is to study tidal disruption events, or TDEs. During these destructive events, the feeding process is compressed into a timescale of a few years, making it observable by human beings during our brief lives.
TDEs occur when a star's orbit brings it too close to a black hole's immense gravitational influence. That proximity causes immense tidal forces within the star. These forces essentially squash the star horizontally while also stretching it vertically, a process called spaghettification.
The plasma pasta then wraps around the black hole like spaghetti around a fork, but just like eating good pasta can be a messy affair, black holes spill a massive amount of stellar food. This material is then channeled to the poles of black holes, from where it is blasted out as near-light-speed jets that can extend for thousands of light-years. This is sometimes referred to as a "burp." Yet the mechanism that powers these jets remains somewhat mysterious, especially when there is a delay between black holes snacking and launching jets.
"Why do some supermassive black holes blast out radio jets right after shredding a star, while others just sit there looking completely dormant, only to suddenly fire up their jets months or even years later?" research author Andrew Mummery of the Institute for Advanced Study (IAS) School of Natural Sciences said in a statement. "We really wanted to figure out this massive puzzle."
After all, black hole jets are responsible for shaping entire galaxies.
Mummery and study co-author Adelle J. Goodwin were in a bar in Madrid, Spain, during a conference when they realized that the rules that apply to stellar-mass black hole jets also must apply to supermassive black hole jets.
To test this idea, the duo turned to telescope data across a wide range of electromagnetic radiation to study a sample of 20 TDEs. Of these, they found half could be studied to reliably model the feeding and outflow-blasting behavior of black holes.
What Mummery and Goodwin found was that jet-launching occurs in two distinct phases: the first, perhaps unsurprisingly, occurs when the supermassive black hole is feeding at its most extreme rate. The second phase occurs much later, potentially years after the star has been ripped apart. That is when the supermassive black hole's feeding has dropped to 2% of what is known as the Eddington limit.
The Eddington limit occurs just before radiation from an accreting source becomes so intense that the pressure it generates pushes away further matter that could be consumed. The 2% threshold is significant because that is the point at which stellar-mass black holes launch their own jets. That means no matter the scale of the black hole, a few times the mass of the sun or a billion, jet formation is the same across black holes.
This is an exciting prospect for astronomers, because it means that when looking at future TDEs, they will be able to roughly predict when jets will be launched.
"We hope that our work will pave the way for even more profound discoveries about our universe," Mummery said.
The team's research was published on Thursday (Sept. 17) in the journal Nature Astronomy
