The significant fading of a light source nearly 870 million light-years from Earth supports a precise model created by a group of astrophysicists, including Professor Eric Coughlin from Syracuse University.
The significant fading of a light source roughly 860 million light-years away from Earth supports a detailed model built by a team of astrophysicists, which includes Eric Coughlin, a professor at Syracuse University.
Advanced telescopes such as NASA’s Hubble, James Webb, and Chandra X-ray Observatory offer scientists a chance to explore the mysteries of black holes in the far reaches of space. Although black holes are known for absorbing all light, researchers can observe them thanks to tidal disruption events (TDEs). During a TDE, a star is torn apart by a supermassive black hole, resulting in a “luminous accretion flare” which provides an extraordinary brightness. These accretion events can shine thousands of billions of times brighter than the Sun, helping astrophysicists study supermassive black holes (SMBHs) even at vast cosmic distances.
TDEs happen when a black hole’s powerful gravitational force violently breaks apart a star. As this star is disintegrated, its remnants form a stream of debris that spirals back toward the black hole, creating a very hot, bright disk of material known as an accretion disc. Researchers can observe these events to directly study TDEs and compare their findings with theoretical models to connect observations to the properties of the disrupted stars and their black holes.
A collaborative effort from physicists at Syracuse University, MIT, and the Space Telescope Science Institute employed intricate modeling to forecast the brightness changes of AT2018fyk, which is classified as a repeating partial TDE. In this case, the dense core of the star survived its encounter with the SMBH, allowing it to orbit the black hole and be torn apart multiple times. Their model accurately predicted that AT2018fyk would dim in August 2023, a prediction confirmed when the light source dimmed last summer, demonstrating a new approach to studying the physics of black holes. Their research was published in The Astrophysical Journal Letters.
An Intense Energy Source
With comprehensive extragalactic surveys, researchers are observing more fluctuating light sources than ever. These surveys scan vast areas in search of sudden increases or decreases in brightness, signaling changes in the sources. Unlike standard household telescopes that only detect visible light, instruments like Chandra can capture X-rays, which are emitted from superheated materials reaching millions of degrees.
Although both visible light and X-rays belong to the electromagnetic spectrum, X-rays possess shorter wavelengths and greater energy. When a gas in an accretion disc gets extremely heated, the closest material to the black hole emits X-rays instead of visible light. This phenomenon is comparable to how a stove turns “red hot.” The hot gas radiates energy in wavelengths that are invisible to the human eye. These are the same X-rays used in medical imaging, capable of penetrating soft tissues, leading NASA X-ray telescopes to be tailored to detect this high-energy radiation.
A Repeated Event
In January 2023, physicists, including Eric Coughlin from Syracuse University, Dheeraj R. “DJ” Pasham from MIT, and Thomas Wevers from the Space Telescope Science Institute published a paper in The Astrophysical Journal Letters proposing a detailed model for a repeating partial TDE. Their work marked the first time a star’s unusual orbital return near a supermassive black hole was mapped, shedding light on one of the universe’s most extreme phenomena.
The team focused their research on a TDE known as AT2018fyk (where AT stands for “Astrophysical Transient”). This event suggested that a star was captured by a SMBH through a process called “Hills capture.” Initially part of a binary system (two stars orbiting each other), one star was believed to be captured by the black hole’s gravitational field while the other star was expelled at speeds near 1000 km/s from the galaxy’s center.
Once tied to the SMBH, the star powering AT2018fyk’s emissions has repeatedly lost its outer layers whenever it comes close to the black hole. The ejected outer layers form the bright accretion disk, which scientists can study using X-ray and Ultraviolet/Optical telescopes that observe distant galaxies’ light.
While TDEs usually occur only once, due to the intense gravitational pull of the SMBH obliterating the star which leads to a fade after the accretion flare, AT2018fyk provided a unique chance to understand a repeated partial TDE.
The research team used a combination of telescopes for their observations: NASA’s Swift and Chandra, along with the European mission XMM-Newton. First recorded in 2018, AT2018fyk is located about 870 million light-years from Earth, meaning the event occurred roughly 870 million years ago in “real time” due to the light travel duration.
The researchers utilized their modeling techniques to predict that the light source would abruptly fade around August 2023 and then brighten again in 2025 as the freshly stripped material gathers around the black hole.
Model Confirmation
Validating their model, the team observed a drop in X-ray flux over the course of two months, beginning on August 14, 2023. This sudden change signals the second emission shutoff.
“The observed emission shutdown indicates that our model and its assumptions are indeed reliable, suggesting we are observing a star being slowly consumed by a distant, massive black hole,” explains Coughlin. “Last year, we utilized constraints from the initial outburst, fading, and rebrightening to forecast that AT2018fyk should exhibit a sudden dimming in August 2023, if the star survived the second encounter that led to the second brightening.”
The occurrence of this predicted shutoff infers several aspects about both the star and the black hole:
- the star withstood its second encounter with the black hole;
- the rate at which stripped debris returns to the black hole is closely linked to the brightness of AT2018fyk;
- and the star orbits the black hole approximately every 1300 days, or about 3.5 years.
This second cutoff suggests that another rebrightening is expected between May and August 2025, and if the star endures the second encounter, a third emission shutoff may occur between January and July 2027.
As for the likelihood of a rebrightening in 2025, Coughlin believes the observation of a second cutoff indicates that the star has lost more mass, which will return to the black hole and ignite a third brightening.
“The only uncertainty lies in the intensity of the emission,” he states. “The second rebrightened peak was significantly less bright than the first, and it remains uncertain whether the third event will also be dimmer. This is the only factor that might limit the possibility of detecting this third event.”
Coughlin emphasizes that this modeling approach opens up an exciting avenue for studying the extremely rare occurrences of repeating partial TDEs, thought to take place only once every million years in a galaxy. To date, scientists have identified only four to five such systems exhibiting this behavior.
“As advances in detection technology reveal more instances of these repeating partial TDEs, we expect this model to become an essential tool for scientists exploring these phenomena,” he concludes.
