Newswise — A recent research reveals insights into the vivid emissions of energy that emerge upon the annihilation of a star by a supermassive black hole. The emissions are not necessarily generated in the immediate proximity of the black hole, but arise from tidal jolts resulting from the collision of gas from the shattered star as it orbits the black hole.
The cosmos is a tumultuous realm where even a star's existence can be abruptly terminated. This transpires when a star happens to occupy an unfavorable locale, particularly in the vicinity of a supermassive black hole.
Such black holes have a mass that ranges from millions to even billions of times that of the Sun, and are typically located at the core of inactive galaxies. As a star draws near to the black hole, it undergoes escalating gravitational attraction from the supermassive black hole, surpassing the counteracting forces that maintain the star's structure. Consequently, the star is shattered or disintegrated, which is commonly referred to as a Tidal Disruption Event (TDE).
"Following the star's disintegration, its gas coalesces into an accretion disk encircling the black hole. The disk's intense emissions can be detected across various wavelengths, particularly via optical telescopes and X-ray-detecting satellites," remarks Yannis Liodakis, a postdoctoral researcher at the University of Turku and the Finnish Centre for Astronomy with ESO (FINCA).
Merely a few Tidal Disruption Events (TDEs) were known to researchers until recently, since there were limited methods to detect them. Nonetheless, over the past few years, scientists have devised the requisite instruments to observe a greater number of TDEs. Interestingly, and perhaps not entirely unexpected, these observations have given rise to fresh enigmas that researchers are presently scrutinizing.
"Large-scale observations conducted with optical telescopes have disclosed that numerous Tidal Disruption Events (TDEs) do not generate X-rays, despite the visible light bursts being distinctly detectable. This revelation challenges our fundamental comprehension of the progression of the fragmented stellar substance in TDEs," highlights Liodakis.
An international team of astronomers, led by the Finnish Centre for Astronomy with ESO, has proposed in a study published in the journal Science that the solution to this mystery may lie in the polarized light emanating from Tidal Disruption Events (TDEs).
In contrast to the prior assumption of the development of an X-ray luminous accretion disk surrounding the black hole, the optical and ultraviolet light outbursts identified in multiple Tidal Disruption Events (TDEs) might originate from tidal shocks. These shocks occur at a significant distance from the black hole when the gas from the disrupted star collides with itself upon returning after revolving around the black hole. In these events, the X-ray bright accretion disk would develop at a much later stage.
"Polarization of light can furnish exclusive insights into the fundamental processes taking place in astrophysical systems. The polarized light we gauged from the Tidal Disruption Event (TDE) could solely be clarified by these tidal shocks," affirms Liodakis, who is the primary author of the study.
Polarised light helped researchers to understand the destruction of stars
The team was informed of a nuclear transient event in a neighboring galaxy, known as AT 2020mot, via a public alert from the Gaia satellite in late 2020. The researchers subsequently examined AT 2020mot using a broad range of wavelengths, including optical polarisation and spectroscopy observations conducted at the Nordic Optical Telescope (NOT), which is affiliated with the University of Turku. The observations made at the NOT played a crucial role in facilitating this discovery. Furthermore, high school students participated in the polarisation observations as part of an observational astronomy course.
"The Nordic Optical Telescope and the polarimeter utilized in this study have been essential to our endeavors in comprehending supermassive black holes and their surroundings," explains Doctoral Researcher Jenni Jormanainen from FINCA and the University of Turku, who supervised the polarisation observations and analysis conducted at the NOT.
The research team discovered that the optical light emitted from AT 2020mot exhibited substantial polarization and was fluctuating over time. Despite multiple attempts, none of the radio or X-ray telescopes could discern any radiation emanating from the event prior to, during, or even months following the outburst's climax.
"When we noticed the high level of polarization exhibited by AT2020mot, we immediately speculated that a jet might be emerging from the black hole, as we frequently observe around supermassive black holes that are accreting surrounding gas. Nonetheless, we were unable to detect any jet," explains Elina Lindfors, an Academy Research Fellow at the University of Turku and FINCA.
Upon analyzing the data, the team of astronomers concluded that the most probable explanation for the high level of optical polarization observed in AT2020mot was a scenario where the stream of stellar gas collides with itself, forming shocks near the pericenter and apocenter of its orbit around the black hole. These shocks subsequently amplify and order the magnetic field in the stellar stream, which naturally leads to highly polarized light. However, the level of optical polarization was so high that it could not be explained by most existing models, and the fact that it changed over time made it even more challenging to understand.
Karri Koljonen, who was formerly an astronomer at FINCA during the observations and is currently working at the Norwegian University of Science and Technology (NTNU), explains that the observations could not be explained by any models except the tidal shock model. The level of optical polarisation was too high for most models to account for, and the fact that it changed over time made it even more challenging.
Ongoing observations of polarised light from TDEs may provide further insight into the aftermath of a star being torn apart, and the researchers may uncover more discoveries in the near future.
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