There are decades of radio astronomy data in the archives of the National Radio Astronomy Observatory (NRAO), and there are still new discoveries lurking within it. Astronomers have spotted the telltale signature jet from a black hole devouring a star several decades ago in archival data collected by the Very Large Array (VLA) telescope in New Mexico. According to a new paper published in The Astrophysical Journal, it’s only the second such candidate event discovered in the radio regime; the first was discovered in 2020. The discovery was presented virtually yesterday at a meeting of the American Astronomical Society.
As we’ve reported previously, it’s a popular misconception that black holes behave like cosmic vacuum cleaners, ravenously sucking up any matter in their surroundings. In reality, only stuff that passes beyond the event horizon—including light—is swallowed up and can’t escape, although black holes are also messy eaters. That means that part of an object’s matter is actually ejected in a powerful jet.
If that object is a star, the process of being shredded (or “spaghettified”) by the powerful gravitational forces of a black hole occurs outside the event horizon, and part of the star’s original mass is ejected violently outward. This in turn can form a rotating ring of matter (aka an accretion disk) around the black hole that emits powerful X-rays and visible light—and sometimes radio waves. Those jets are one-way astronomers can indirectly infer the presence of a black hole. They’re known as “tidal disruption events” (TDEs).
For instance, astronomers in 2018 announced the first direct image of the aftermath of a star being shredded by a black hole 20 million times more massive than our Sun. The encounter took place in a pair of colliding galaxies called Arp 299 about 150 million light years from Earth. The astronomers used a combination of radio and infrared telescopes, including the Very Long Baseline Array (VLBA), to follow the formation and expansion of the jet of matter. The matter was ejected in the wake of a star being shredded by a supermassive black hole at the center of one of the colliding galaxies.
Astronomers found another TDE in 2020 (dubbed AT 2019qiz), which provided the first direct evidence that outflowing gas during disruption and accretion produces the powerful optical and radio emissions previously observed.
However, these powerful bursts of light are often shrouded behind a curtain of interstellar dust and debris, making it difficult for astronomers to spot or study them in greater detail using optical or X-ray telescopes. “This study demonstrates the power of radio surveys to discover TDEs,” said co-author Vikram Ravi of Caltech, by picking up powerful jets shining in the radio-frequency regime. The events are potentially powerful tools for studying the inner workings of black holes.
The new TDE candidate has been dubbed J1533+2727. Two of Ravi’s high school interns first spotted it when he was a postdoc at Harvard University. Poring over the archives, they noticed that the image of a bright radio object taken in the mid-1990s had faded dramatically by 2017. They found images of the same object in the archives of the Green Bank 300-foot telescope, which showed the object had been even brighter in 1986/1987.
Bolstered further by new observations taken by the VLA, Ravi and his team concluded that the object was a TDE—the result of a supermassive black hole at the center of a galaxy some 500 million light years away that devoured a star and expelled a powerful radio jet traveling near the speed of light. The discovery is being published jointly with astronomers at the University of Toronto, who independently also found the object.
It’s the closest example of this type of TDE candidate yet found, suggesting that such radio-bright events might be more common than astronomers previously believed. Ravi’s group has already spotted another possible radio-bright TDE using the VLA, although it may also be a flare from an active supermassive black hole.