One of the first things that people learn about black holes is that they absorb everything which comes close to them, but this isn’t exactly accurate. It is true that once anything passes the event horizon of a black hole it can never escape, but there is a significant area around the black hole where its gravitational effects are still extremely strong but things can still escape. In fact, black holes regularly give off dramatic jets of matter, which are typically thrown out when material falls into the black hole and a small amount is ejected outward at great speeds.
But astronomers recently discovered a totally mysterious phenomenon, where a black hole is ejecting material years after it ripped apart a star. The black hole AT2019dsg is located 665 million light-years away and was observed tearing apart the star in 2018, then for unknown reasons, it became extremely active again in 2021. “This caught us completely by surprise — no one has ever seen anything like this before,” said lead author Yvette Cendes, a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA).
The black hole is throwing out material at a tremendous speed of half the speed of light. This happened years after the star was spaghettified by the black hole, in what is called a tidal disruption event (TDE), and there is no obvious explanation for this delay.
“We have been studying TDEs with radio telescopes for more than a decade, and we sometimes find they shine in radio waves as they spew out material while the star is first being consumed by the black hole,” said co-author Edo Berger. “But in AT2018hyz there was radio silence for the first three years, and now it’s dramatically lit up to become one of the most radio luminous TDEs ever observed.”
The particularly strange thing is that the researchers had observed this spaghettification event and found it was “unremarkable.” Yet for some reason, this outflow is both very delayed and much faster than typical outflows.
“This is the first time that we have witnessed such a long delay between the feeding and the outflow,” Berger says. “The next step is to explore whether this actually happens more regularly and we have simply not been looking at TDEs late enough in their evolution.”
The research is published in The Astrophysical Journal.
