Star-bursting by a black hole rarely occurs outside the center of the galaxy

AT2024tvd, an “extra-nuclear” tidal disruption event, exhibits two extremely fast radio peaks and hints at a wandering black hole; Hebrew University researchers co-discover and analyze

A team of astronomers, led by Dr. Itay Sefardi and Prof. Raffaella Margotti from the University of California, Berkeley, and with the participation of an international team of researchers, including Prof. Assaf Harash from the Rakeh Institute for Physics at the Hebrew University of Jerusalem, has discovered a supermassive black hole (TDE) star-destroying event, the first outside the center of a galaxy to emit intense radio waves. The event, designated AT 2024tvd, revealed radio signals evolving at the fastest rate ever measured from an event of this type.

"This is a truly extraordinary phenomenon," says Dr. Itay Sefardi, the study's lead author. "We've never seen such strong radio emission from an event in which a black hole rips apart a star far from the center of the galaxy — and at such a rapid rate. This changes the way we understand how black holes work."

Typically, TDE events occur when a star gets too close to a massive black hole at the center of a galaxy and is torn apart by its immense gravity. But in the unusual case of AT 2024tvd, the black hole is located about 2,600 light-years (about 0.8 kiloparsecs) from the center of the galaxy—evidence that supermassive black holes can hide in unexpected places.

The radio discovery was made possible by unprecedented quality observations from several radio telescopes, including the Very Large Array (VLA), the ALMA telescope in Chile, the ATA, the SMA, and AMI-LA, a radio telescope in England, whose observations were led by the Hebrew University team led by Prof. Assaf Harash. The data collected shed new light on how massive black holes work – even when they are hidden from our eyes.

The event produced two separate radio bursts that developed at an extremely fast rate – faster than any TDE recorded to date – the findings show that material was being ejected from the black hole’s surroundings with enormous force – not immediately after the star was destroyed by the black hole, but months later, suggesting mysterious processes that we still don’t fully understand. The team’s in-depth analysis suggested that the radio signals could be due to two bursts of material from the black hole’s surroundings – with a delay of months after the star was destroyed. This is a direct hint that black holes can be involved in ejecting material from their surroundings at a variable rate over time, sometimes “waking up again” after months of relative quiet.

In tracking the source, an international team used a series of radio and millimeter-wave telescopes, including the VLA and ALMA. The picture that emerged from the observations is particularly surprising: instead of a single burst that slowly rises and falls, two separate radio “jumps” were recorded at different times—as if the material being ejected did so in two stages. Each of the bursts rose and fell extremely quickly, much faster than is known from similar events.

Why is this exciting? First, because it is a “natural laboratory” for studying how matter behaves when it is torn apart and thrown at enormous speeds around a black hole. Second, because the event occurred far from the center of the galaxy, it may indicate a “wandering” black hole—for example, one that was kicked out of place after a collision between galaxies. If so, there may be a population of black holes in nature that are not sitting in their “classical” place, and we are just starting to discover them.

Dr. Itay Sephardi, the research leader, is a graduate of Prof. Assaf Harash's research group from the Hebrew University of Jerusalem. Prof. Harash noted that "this is one of the most fascinating discoveries I have ever had the opportunity to take part in. The fact that the discovery was made by my former research student, Dr. Itay Sephardi, makes it even more exciting. This is another scientific achievement that places Israel at the forefront of global research."

The research was conducted in collaboration with scientists from several universities in the US, Europe, and Israel, including Prof. Paz Benyamini from the Open University, and was published in the scientific journal  The Astrophysical Journal Letters Link.

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