A black hole may be one of the universe’s most powerful objects, but scientists say it is far from an efficient eater. Now, new study, published in the journal Nature Astronomy, two researchers are helping scientists understand when those cosmic “burps” happen, offering a new way to watch some of the universe’s most extreme environments. Catching a black hole in the act Adelle Goodwin, an astrophysicist at Curtin University in Western Australia and co-author of the study, said these jets can become enormous, with the potential to affect the galaxies in which the black holes reside. “I think one of the things that has puzzled astronomers for a really long time is that occasionally we see these supermassive black holes at the centre of galaxies, and they can power these really large, what we call jets or outflows, and these can be bigger than the galaxies themselves, so there’s all this gas that’s coming, being channeled by the black hole.” The difficulty is timing. These structures can evolve over tens of thousands of years, making it rare for astronomers to observe the precise moment a jet is launched. “Being able to catch these specific black holes that we were looking at in the act of burping or launching these jets and outflows allows us to really understand that physics, and learn how they can influence their host galaxies,” Goodwin said. A cosmic alarm clock The researchers studied radio observations of black holes after stars were torn apart, looking for patterns in when jets and outflows emerged. Andrew Mummery, an astrophysicist at the Institute for Advanced Study and study co-author, said knowing the trigger could fundamentally change how researchers plan observations. “Understanding the conditions of which things happen is great because you can plan a bit better,” Mummery said. “You can time your resources, your telescope time around watching this event in real time.” The study found that an crucial phase of jet activity occurs after the feeding rate of the black hole has dropped dramatically to about two per cent of its peak rate. Mummery said the period before that threshold appears relatively quiet. “So it seems quite quiet, it’s not launching a jet ... so at that time it’s just feeding, it seems happy to just take all this matter from the disc.” Then comes what the researchers describe as a critical trigger. “And (when) you reach this magic number of two per cent ... we know around all black holes of all sizes, there’s a trigger,” Mummery added. Same physics, vastly different black holes One of the most striking aspects of the finding is that the pattern appears to apply to black holes of dramatically different sizes. “What we’ve been able to show with this study is that these really large black holes that are millions to billions of times the mass of our sun, they launch jets and outflows at the same kind of feeding times or accretion rates as these small black holes,” Goodwin said. That suggests the underlying physics may be shared across an enormous range of black-hole sizes. The bigger mystery: how do they grow? The finding could also help answer a much larger question: how did supermassive black holes become so massive? Goodwin said scientists understand the basic origins of smaller black holes, but their supermassive counterparts present a much bigger puzzle. “We fundamentally don’t understand how you go from a small black hole to a big black hole in the lifespan of the universe and the time available that the universe has been around,” she said. For Mummery, the jets also raise another intriguing possibility: learning how quickly a black hole is spinning. “It’s this big and it’s spinning this fast ... it’s just a fun thing to learn.”
