Originally predicted by Albert Einstein’s theory of general relativity, black holes represent the most extreme object in the known universe. These objects are so gravitationally powerful that nothing (not even light) can escape their surface. Of course, it is a popular misconception that black holes behave like cosmic vacuum cleaners, voraciously absorbing any matter around them. In reality, only things that happen beyond the event horizon are swallowed. And from time to time, a black hole will gobble up both stars and other objects in its range, releasing huge amounts of light and radiation in the process.
Precisely in October 2018, astronomers witnessed one of those events when observing a black hole in a galaxy located 665 million light years away from Earth that engulfed a small star when it approached the black hole.
Now, nearly three years after the cosmic event, the same black hole is once again lighting up the skies, and the curious thing is that nothing new has been swallowed, scientists say.
“This took us completely by surprise: no one had seen anything like this before ,” says Yvette Cendes, a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of a new study looking at the phenomenon.
Where does this shine come from?
The team, publishing their results in The Astrophysical Journal, concludes that the black hole is ejecting material traveling at half the speed of light, but they aren’t sure why the outflow has been delayed by several years.
The team detected the unusual outburst while reviewing tidal disruption events, when invading stars are spaghetti-slapped by black holes, that have occurred in recent years.
The black hole had mysteriously reanimated
When AT2018hyz was first discovered, radio telescopes didn’t detect any signs of material emission in the first few months, but last June, researchers decided to double-check several ‘spaghetti’ star events (or TDEs) in recent years that they had not previously shown any broadcasts. To his surprise, AT2018hyz was lighting up the skies again.
“AT2018hyz was radio silent for the first three years, and has now brightened dramatically to become one of the brightest radio TDEs ever observed,” the authors explain.
The team concluded that this was because the black hole ejected residual material from the star at relativistic speeds (a fraction of the speed of light).
“This is the first time we have witnessed such a long delay between feeding and departure,” say the experts. ” The next step is to explore if this really happens more often and we just haven’t been looking at TDEs late enough in their evolution.”
The scientists hope their results will increase understanding of black hole feeding behavior, comparable to “belching” after a meal. This, in turn, could provide insight into how they grow and evolve over time and their role in galactic evolution.
Referencia: Y. Cendes, E. Berger, K. D. Alexander, S. Gomez, A. Hajela, R. Chornock, T. Laskar, R. Margutti, B. Metzger, M. F. Bietenholz, D. Brethauer, M. H. Wieringa. A Mildly Relativistic Outflow Launched Two Years after Disruption in Tidal Disruption Event AT2018hyz. The Astrophysical Journal, 2022; 938 (1): 28 DOI: 10.3847/1538-4357/ac88d0
