New research suggests that the flickering light emissions from the centers of galaxies are not always due to star-devouring black holes – but sometimes to internal disk changes
Lightning may not strike twice, but black holes do. An international team of researchers led by astronomers from Tel Aviv University observed a flash of light created when a star falls on a black hole and is destroyed. However, this flash occurred about two years after a nearly identical flash called AT 2022dbl from the same location. This is the first confirmed case of a star surviving its encounter with a supermassive black hole, and returning for another encounter. This discovery undermines the accepted assumptions about star-ripping events by black holes, and suggests that many of these flashes of light in the universe may actually be just the beginning of a much longer-term and more complex astronomical drama.
The study was led by Dr. Lydia Macrigiani (formerly a postdoctoral fellow at Tel Aviv University and currently a researcher at Lancaster University in England) and supervised by Prof. Yair Harkavy, a faculty member in the Department of Astrophysics at Tel Aviv University, and Director of the Wise Observatory in Mitzpe Ramon. The study also involved Prof. Ehud Necker, Head of the Department of Astrophysics at Tel Aviv University, and students Sarah Pars and Yael Degani from Harkavy's research group, along with many international researchers. The study was published in the July issue of the Astrophysical Journal Letters.
The team of researchers explains that at the center of every large galaxy lies a black hole with a mass of millions to billions of times the mass of the Sun. There is also such a supermassive black hole at the center of our Milky Way, which earned its discoverers the 2020 Nobel Prize in Physics. But beyond knowing that they are there, we do not know how these monsters are formed and how they affect the galaxies in which they are located. One of the challenges in understanding these black holes is that they are, well, black. A black hole is a region of space where gravity is so strong that not even light can escape. The black hole at the center of the Milky Way was discovered through the motion of stars around it. But in other, more distant galaxies, we do not have the ability to see this motion.
Fortunately, or unfortunately, depending on your perspective, once every 10,000 to 100,000 years, a star will get too close to the supermassive black hole at the center of its galaxy and be torn apart. Half of it will be “swallowed” by the black hole and half will be blown out. As matter falls toward a black hole, it spins, much like water draining from a bathtub. But around a black hole, the speed of matter’s rotation approaches the speed of light, the matter heats up and radiates a powerful light that can be seen over vast distances. Thus, such an unlucky star “illuminates” the black hole for a few weeks or months, providing a fleeting opportunity to study its properties.
But these flashes of light did not behave as expected. The light was fainter than expected and at a lower temperature than expected. After about a decade of trying to understand why this was so, AT 2022dbl may have provided the explanation. The return of the same flash of light, in the same place, about two years later, suggests that at least the first flash resulted from only a partial disintegration of the star, with most of it surviving and returning for another (nearly identical) transit. In other words, these flashes describe more of a "snack" by the black hole than a "meal."
"Now the question is whether we will see a third flash of light after another two years, in early 2026," says Prof. Harkaby. "If we see a third flash, it means that the second one was also just a partial disintegration of the star, and then perhaps all the flashes of this type, which we have been trying to explain for 10 years as complete disintegrations of stars, are actually not what we thought." If we do not see a third flash, then it is possible that the second flash was the complete disintegration of the star. This means that the partial disintegration of a star and its complete disintegration by a black hole look almost exactly the same, a possibility that was predicted even before this discovery by Prof. Zvi Piren's research group at the Hebrew University. "Either way," adds Harkaby, "we will have to rewrite our interpretation of those flashes, and what they can teach us about the monsters that reside in the centers of galaxies."
More of the topic in Hayadan:
