Digital illustration of a star shedding stellar debris as it orbits a supermassive black hole. This artist’s impression represents the centre of a galaxy about 860 million light-years from Earth. Credit: NASA/ CXC/ M Weiss.

Dramatic dimming of a light source around 860 million light-years away from Earth confirms the accuracy of a detailed model developed by a team of astrophysicists.

The team, made up of scientists from Syracuse University, MIT and the Space Telescope Science Institute, used powerful telescopes like NASA’s Hubble, James Webb, and Chandra X-ray Observatory which provide a window into deep space to probe the physics of black holes.

While one might wonder how you can ‘see’ a black hole, which famously absorbs all light, this is made possible by tidal disruption events (TDEs) – where a star is destroyed by a supermassive black hole and can fuel a ‘luminous accretion flare’.

With luminosities thousands of billions of times brighter than the Sun, accretion events enable astrophysicists to study supermassive black holes (SMBHs) at cosmological distances.

A team of physicists from Syracuse University, MIT and the Space Telescope Science Institute used detailed modelling to predict the brightening and dimming of AT2018fyk (AT stands for Astrophysical Transient), which is a repeating partial TDE, meaning the high-density core of the star survived the gravitational interaction with the SMBH, allowing it to orbit the black hole and be shredded more than once.

The model predicted that AT2018fyk would ‘dim’ in August 2023, a forecast which was confirmed when the source went dark last summer, providing evidence that their model delivers a new way to probe the physics of black holes.  

X-ray and optical image of AT2018fyk. Credit: X-ray: NASA/ SAO/ Kavli Inst at MIT/ DR Pasham; Optical: NSF/ Legacy Survey/ SDSS.

Supermassive black hole

In January 2023, a team of physicists, including Eric Coughlin, a professor at Syracuse University’s Department of Physics, Dheeraj R ‘DJ’ Pasham, a research scientist at MIT, and Thomas Wevers, a Fellow at the Space Telescope Science Institute, published a paper in The Astrophysical Journal Letters that proposed a detailed model for a repeating partial TDE.

The results were the first to map a star’s surprising return orbit about a supermassive black hole – revealing new information about one of the cosmos’ most extreme environments.

AT2018fyk was originally part of a binary system (two stars that orbit one another under their mutual gravitational attraction), and the team hypothesised one of the stars was to have been captured by the gravitational field of the black hole and the other (non-captured) star was ejected from the centre of the galaxy at speeds comparable to ~ 1000 km/s.

Once bound to the SMBH, the star powering the emission from AT2018fyk has been repeatedly stripped of its outer envelope each time it passes through its point of closest approach with the black hole.

The stripped outer layers of the star form the bright accretion disk, which researchers can study using X-Ray and Ultraviolet /Optical telescopes that observe light from distant galaxies.

While TDEs are usually ‘once-and-done’ because the extreme gravitational field of the SMBH destroys the star, meaning that the SMBH fades back into darkness following the accretion flare, AT2018fyk offered the unique opportunity to probe a repeating partial TDE.

The team used detailed modelling to forecast that the light source would abruptly disappear around August 2023 and brighten again when the freshly stripped material accretes onto the black hole in 2025.

Confirming the accuracy of their model, the team reported an X-ray drop in flux over two months, starting on August 14, 2023. This sudden change can be interpreted as the second emission shutoff.

Third brightening

Coughlin said: “The observed emission shutoff shows that our model and assumptions are viable, and suggests that we are really seeing a star being slowly devoured by a distant and very massive black hole.

“In our paper last year, we used constraints from the initial outburst, dimming and rebrightening to predict that AT2018fyk should display a sudden and rapid dimming in August of 2023, if the star survived the second encounter that fuelled the second brightening.”

As for whether we can count on seeing a rebrightening in 2025, Coughlin says the detection of a second cutoff implies that the star has had more mass freshly stripped, which should return to the black hole to produce a third brightening.

He added: “The only uncertainty is in the peak of the emission. The second re-brightened peak was considerably dimmer than the first, and it is, unfortunately, possible that the third outburst will be dimmer still. This is the only thing that would limit the detectability of this third outburst.”

The results were published in The Astrophysical Journal Letters.