Unravel the mystery of tidal disturbance events

Supermassive black holes disturb 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 so the life of a star can be cut short here. This happens when a star finds itself in a “bad” neighborhood, specifically near a massive cluster Black hole.

These black holes, which boast a mass millions or even billions of times greater than our sun, are usually found in the centers of quiet galaxies. As the star moves away from the black hole, it experiences an upward gravitational pull from the supermassive black hole, which eventually overcomes the forces that keep the star intact. This results in the star being disrupted or destroyed, an event known as a tidal disruption event (TDE).

“After the star is ruptured, its gas forms an accretion disk around the black hole. Bright bursts from the disk can be observed at nearly all wavelengths, particularly with telescopes and satellites that detect X-rays,” says postdoctoral researcher Yannis Lioudakis of the University of Turku and the Finnish Center for Astronomy. ESO (vinca).

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 observations have led to new mysteries that researchers are currently studying.

“Observations from large-scale experiments with optical telescopes have revealed that a large number of TDEs do not produce X-rays even though bursts of visible light can be clearly detected. This finding contradicts our basic understanding of the evolution of stellar matter disrupted in TDEs,” notes Liodakis.

Cartoon tidal disturbance event

In a tidal disruption event, a star moves 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 stream around the black hole (image 2). Tidal shocks form around the black hole as gas hits itself on its way back after orbiting the black hole (image 3). Tidal shocks create bright bursts of polarized light observable at optical and ultraviolet wavelengths. Over time, gas from the destroyed star forms an accretion disk around the black hole (image 4) as it is slowly pulled into the black hole. Note: Image size is not accurate. Credit: Jenny Gurmaninen

A study published in the journal Sciences An international team of astronomers led by the Finnish Center for Astronomy with ESO suggests that the polarized light coming from the TDE may be the key to solving this puzzle.

Instead of forming a bright X-ray accretion disk around the black hole, the outburst observed in the optical and ultraviolet light detected in many TDEs could originate from tidal shocks. These shocks form far from the black hole as gas from the destroyed star strikes itself on its way back after orbiting the black hole. The bright X-ray accretion disk would form later in these events.

“The polarization of light can provide unique information about fundamental processes in astrophysical systems. The polarized light we measured from the 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 public alert in late 2020 from the Gaia satellite of a transient nuclear event in a nearby galaxy identified as 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), owned by the University of Turku. The observations made at NOT were particularly helpful 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 the polarization observations and analysis with NOT.

The researchers found that the optical light coming from AT 2020mot was highly polarized and changed over time. Despite many attempts, neither radio nor 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 being released from a black hole, as we often observe around supermassive black holes that accumulate surrounding gas. However, no jet was found,” says Elena Lindfors, an academic research fellow at the University of Turku and Fenca.

The team of astronomers realized that the data closely matched a scenario where a stream of interstellar gas collides with itself and forms bumps near the center and front of its orbit around the black hole. The shocks then amplify the magnetic field and arrange it into the stellar stream that will naturally result in highly polarized light. The optical polarization level was too high to be explained by most models, and the fact that it was changing 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 coming from the TDEs and may soon discover more about what happens after a star crashes.

Reference: “Optical Polarization from Stellar Stream Shock Collision in a Tidal Disturbance Event” by I.A. Leodakis, KII Koljonen, D. Blinov, E. Lindfors, KD Alexander, T. Hovatta, M. Berton, A. Hajela, J. Jormanainen, K. Kouroumpatzakis, N. Mandarakas and K.
DOI: 10.1126/science.abj9570

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