Supermassive black holes disrupt or destroy nearby stars, leading to tidal disturbance events (TDEs). Observations of polarized light from TDEs have now revealed key details about the processes involved.
The universe is a violent place where even a star’s life can be cut short. This happens when a star is in a “bad” neighborhood, especially near a supermassive black hole.
These black holes, millions or even billions of times more massive than our sun, are usually found in the midst of silent galaxies. As the star moves away from the black hole, it experiences an increasing gravitational pull from the supermassive black hole, which ultimately dominates the forces that maintain the integrity of the star. This results in the star being disrupted or destroyed, an event known as a tidal disruption event (TDE).
“Once a star is torn apart, its gas forms an accretion disk around the black hole. Bright bursts from the disk can be observed at nearly all wavelengths, especially with telescopes and satellites that detect X-rays, says postdoctoral researcher Yannis Lioudakis of the University of Turku and the Finnish Astronomical Center with ESO (FINCA).
Until recently, only a few researchers knew of TDE, as there weren’t many experiments able to detect it. However, in recent years scientists have developed the tools to monitor more TDE. Interestingly, but perhaps not surprisingly, these sightings have led to new mysteries that researchers are now looking into.
Observations from large-scale experiments with optical telescopes have revealed that a large number of TDEs do not produce X-rays, although bursts of visible light can clearly be detected. This result contradicts our basic understanding of the evolution of turbulent stellar matter in TDEs,” notes Liodakis.
During a tidal disruption event, a star gets close enough to a supermassive black hole that the black hole’s gravitational pull bends the star until it is destroyed (Image 1). Interstellar matter from the destroyed star forms an elliptical outflow around the black hole (image 2). Tidal shocks form around the black hole as gas collides on its way back after orbiting the black hole (image 3). Tidal shocks create bright bursts of polarized light that can be seen at both optical and ultraviolet wavelengths. Over time, the gas from the destroyed star forms an accretion disk around the black hole (image 4) from where it is slowly drawn into the black hole. Note: Image scale is not accurate. 1 credit
A study published in the journal knowledge An international team of astronomers led by the Finnish Astronomical Center with ESO suggests that polarized light from TDE may be the key to solving this puzzle.
Rather than the formation of a bright X-ray accretion disk around the black hole, the outburst visible in optical and ultraviolet light detected in many TDEs may have been caused by tidal shocks. These shocks form away from the black hole when gas from the destroyed star collides on its way again after orbiting the black hole. The glowing X-ray accretion disk would form later in these events.
“The polarization of light can provide unique insights into the processes behind astrophysical systems. The polarized light we measured from TDE can only be explained by these tidal shocks, says Lioudakis, lead author of the study.
Polarized light has helped researchers understand star destruction
The team received a general alert in late 2020 from the Gaia satellite about a passing nuclear event in a nearby galaxy called AT 2020mot. The researchers then observed AT 2020mot in a wide range of wavelengths, including optical polarization and spectroscopy observations carried out at the Scandinavian Optical Telescope (NOT), of the University of Turku. The observations made at NOT were particularly crucial in making this discovery possible. In addition, observations of polarization were made as part of an observational astronomy course for high school students.
“The Scandinavian Optical Telescope and the polarimeter we use in the study have been instrumental in our efforts to understand supermassive black holes and their environments,” says doctoral researcher Jenny Jormaninen of FINCA and the University of Turku who led observations and polarization analyzes with NOT.
The researchers found that the optical light from AT 2020mot was highly polarized and varied over time. Despite many attempts, no radio or X-ray telescopes have been able to detect radiation from the event before, during, or even months after the peak of the eruption.
“When we saw how polarized AT2020mot was, we immediately thought of a jet shooting out from a black hole, as we often observe around supermassive black holes that accumulate gas around them. “No planes were found,” says Elena Lindfors, a researcher at the University of Turku and Finca.
The team of astronomers found the data most consistent with a scenario where the interstellar gas stream collides with itself and forms bumps near the center and orbit of its orbit around the black hole. The shocks then amplify the magnetic field and arrange it into the stellar flux that will naturally result in highly polarized light. The level of visual bias was too high for most models to explain, and the fact that it changed over time made it even more difficult.
“All the models we looked at could not explain the observations except for the tidal shock model,” notes Kari Kollionen, who was an astronomer at FINCA at the time of the observations and now works at the Norwegian University of Science and Technology (NTNU).
The researchers will continue to monitor the polarized light from the TDEs and may soon discover more about what happens after a star crashes.
Reference: “Optical Polarization of Colliding Stellar Flux Shocks During a Tidal Disturbance Event” ed. by I.Liodakis, KII Koljonen, D. Blinov, E. Lindfors, KD Alexander, T. Hovatta, M. Berton, A. Hajela, J. Jormanainen, K. Kouroumpatzakis, N. Mandarakas, and K. Nilsson, May 11.
DOI: 10.1126 / science.abj9570
