Supermassive black holes disturb or destroy nearby stars, causing Tidal Distance Events (TDEs). Observations of polarized light from TDEs have now revealed important details about the processes involved.
The universe is such a violent place that the life of a star can be shortened here. This happens when a star finds itself in a “bad” environment, particularly near a massive cluster of black holes.
These black holes, which have a mass millions or even billions of times that of our sun, are usually found in the centers of lonely galaxies. As the star moves away from the black hole, it experiences the upward gravitational pull of the supermassive black hole, which eventually overcomes the force holding the star together. This causes the star to be disrupted or destroyed, an event known as a Tidal Disruption Event (TDE).
“After the star disintegrates, its gas forms an accretion disk around the black hole. The bright bursts from the disk can be observed at almost any wavelength, especially with telescopes and satellites that detect X-rays,” said postdoctoral researcher Yannis Lioudakis of the University of Turku and the Finnish Center for Astronomy. ESO (vinka).
Until recently, only a few researchers knew about TDE, because not many experiments were able to detect it. However, in recent years scientists have developed tools to monitor more and more EFTs. Interestingly, but perhaps not surprisingly, these observations have given rise to a new mystery that researchers are 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 disturbed evolution of stellar matter in TDEs,” notes Liodakis.
In a tidal disturbance event, a star moves close enough to the supermassive black hole that the black hole’s gravitational pull bends the star until it is crushed (Figure 1). The interstellar material from the destroyed star forms an elliptical stream around the black hole (figure 2). Tidal shocks form around the black hole as gas crashes into itself on its way back after orbiting the black hole (figure 3). Tidal shocks create bright bursts of polarized light that are observable at both optical and ultraviolet wavelengths. Over time, gas from the disintegrating star forms an accretion disk around the black hole (figure 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 Knowledge An international team of astronomers led by the Finnish Center for Astronomy with ESO suggests that polarized light coming from the TDE may be the key to solving this puzzle.
Rather than forming a bright X-ray accretion disk around a black hole, the bursts observed in optical and ultraviolet light detected in many TDEs could come from tidal shocks. These shocks form away from the black hole as gas from the destroyed star strikes itself on its way back after orbiting the black hole. A bright X-ray accretion disk would form later in this event.
“The polarization of light can provide unique information about fundamental processes in astrophysical systems. The polarized light we measure from the TDE can only be explained by these tidal shocks,” said Lioudakis, lead author of the study.
Polarized light has helped researchers understand star collapse
The team received a public warning in late 2020 from the Gaia satellite of a temporary nuclear event in a nearby galaxy identified as AT 2020mot. The researchers then observed AT 2020mot at various wavelengths including optical polarization and spectroscopic observations made at the Scandinavian Optical Telescope (NOT), belonging to the University of Turku. The observations made at NOT were of great help in making this discovery possible. In addition, polarization observations were carried out as part of a course on astronomical observations for high school students.
“The Scandinavian optical telescope and polarimeter we used in this study have been instrumental in our efforts to understand supermassive black holes and their environment,” said doctoral researcher Jenny Jormaninen of FINCA and the University of Turku who led the observations and analysis of polarization with NOT.
The researchers found that the optical light coming from AT 2020mot is highly polarized and changes over time. Despite many attempts, neither radio nor X-ray telescopes were able to detect radiation from events before, during, or even months after the peak of the eruption.
“When we see how AT2020mot is polarized, we immediately think of jets ejected from a black hole, as we often observe around supermassive black holes accumulating gas around them. However, no jets were found,” said Elena Lindfors, academic researcher at the University of Turku and Fenca.
The team of astronomers realized that the data fits perfectly with a scenario in which a stream of interstellar gas collides with itself and forms a bulge near the center and front of its orbit around the black hole. The shocks then strengthened the magnetic field and set it into star streams which would naturally produce highly polarized light. The level of optical polarization is too high for most models to describe, and the fact that it changes over time makes it even more difficult.
“All the models we observed cannot explain the observations, except for the tidal shock model,” notes Kari Kollionen, an astronomer at FINCA at the time of the observations and now working at the Norwegian University of Science and Technology (NTNU).
Researchers will continue to monitor polarized light coming from the TDE and may soon find out more about what happens after a star falls.
Reference: “Optical Polarization of Collision Stellar Flow Shocks in Tidal Disruption Events” by IA 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
2023-07-21 10:52:05
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