There are decades of radio astronomy data in the archives of the National Radio Astronomical Observatory (NRAO), and there are still new discoveries lurking there. Astronomers spotted the telltale signature jet of a star-devouring black hole decades ago in archival data collected by the Very Large Array Telescope (VLA) in New Mexico. According to a new article published in The Astrophysical Journal, this is only the second such candidate event discovered in the radio regime; the first one was discovered in 2020. The discovery was presented virtually yesterday at a meeting of the American Astronomical Society.
As we reported earlier, it is a popular misconception that black holes behave like cosmic vacuum cleaners, greedily sucking up any material in their environment. In reality, only things that pass the event horizon, including light, are engulfed and cannot escape, although black holes are mess eaters as well. This 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 “spaghetted”) by the powerful gravitational forces of a black hole occurs outside the event horizon, and some of the original mass of the star is violently ejected outwards. This in turn can form a rotating ring of matter (aka a accretion disc) around the black hole which emits powerful X-rays and visible light, and sometimes radio waves. These jets are a way for astronomers to indirectly infer the presence of a black hole. They are known as “tidal disturbance events“(TDEs).
For example, astronomers announced in 2018 the first direct image consequences of a star torn apart 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. Astronomers have used a combination of radio and infrared telescopes, including the Very Long Baseline Array (VLBA), to track the formation and expansion of the material jet. The material was ejected in the wake of a jagged star by a supermassive black hole in the center of one of the colliding galaxies.
Astronomers have found another TDE in 2020 (dubbed AT 2019qiz), which provided the first direct evidence that the gas exiting during disruption and accretion produces the powerful optical and radio emissions seen previously.
However, these powerful bursts of light are often hidden behind a curtain of interstellar dust and debris, making it difficult for astronomers to spot or study them in more detail using optical or x-ray telescopes. ” This study demonstrates the power of radio polls to uncover TDEs, “said co-author Vikram Ravi of Caltech, picking up powerful jets glowing in the radio frequency regime. Events are potentially powerful tools for studying the inner workings of black holes.
The new TDE candidate was named J1533 + 2727. Two of Ravi’s high school interns first spotted him while he was a postdoctoral fellow at Harvard University. Examining the archives, they noticed that the image of a luminous radio object taken in the mid-1990s had faded considerably by 2017. They found images of the same object in the archives of Green’s 300-foot telescope. Bank, which showed the object had been even brighter in 1986/1987.
Strengthened 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 published jointly with astronomers at the University of Toronto, who also independently found the object.
This is the closest example of this type of TDE candidate ever found, suggesting that such radio-luminous events may be more frequent than astronomers previously thought. Ravi’s group has already spotted another possible radio-luminous TDE using the VLA, although it could also be an eruption from an active supermassive black hole.
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