Unraveling the mystery: Tidal shocks illuminate bright outbursts in star-black hole encounters

A groundbreaking study has shed light on the dazzling bursts of radiation that occur when a star meets its demise near a supermassive black hole. Contrary to previous assumptions that these outbursts originated in the immediate vicinity of the black hole, it appears that they are actually generated by tidal shocks. These shocks occur when the gas from the destroyed star collides with itself while orbiting the black hole.

In the vast and turbulent expanse of the universe, even stars can meet untimely ends, particularly when they find themselves in the vicinity of supermassive black holes. These cosmic behemoths possess a mass millions or even billions of times greater than that of the sun and are typically found at the centers of relatively tranquil galaxies. As a star draws nearer to a supermassive black hole, it experiences an increasingly powerful gravitational pull that eventually overwhelms the forces maintaining its structure. The star is then torn apart or destroyed, an event known as a tidal disruption event (TDE).

“After the star has been ripped apart, its gas forms an accretion disk around the black hole. The bright outbursts emanating from this disk can be detected across various wavelengths, particularly with optical telescopes and X-ray-detecting satellites,” explains Postdoctoral Researcher Yannis Liodakis from the University of Turku and the Finnish Center for Astronomy with ESO (FINCA).

Until recently, researchers had only identified a handful of TDEs due to the limited capabilities of available detection methods. However, in recent years, scientists have developed new tools that enable them to observe a greater number of these events. Interestingly, these expanded observations have brought forth fresh enigmas that researchers are currently investigating.

“Large-scale optical telescope experiments have revealed that numerous TDEs do not exhibit X-ray emissions, despite clearly detecting bursts of visible light. This discovery challenges our fundamental understanding of the evolution of disrupted stellar matter in TDEs,” Liodakis remarks.

An international team of astronomers led by the Finnish Center for Astronomy with ESO has published a study in the journal Science, proposing that polarized light emitted by TDEs might hold the key to unraveling this mystery.

Rather than attributing the observed outbursts of optical and ultraviolet light in many TDEs to the formation of an X-ray-bright accretion disk around the black hole, it seems that these outbursts actually arise from tidal shocks. These shocks manifest far away from the black hole as the gas from the destroyed star collides with itself during its return journey after encircling the black hole. The formation of the X-ray-bright accretion disk occurs much later in these events.

“The polarization of light can offer unique insights into the underlying processes within astrophysical systems. The polarized light we measured from the TDE can only be explained by these tidal shocks,” asserts Liodakis, the study’s lead author.

Polarized light helped researchers to understand the destruction of stars

In late 2020, the team of astronomers received a public alert from the Gaia satellite regarding a nuclear transient event in a neighboring galaxy named AT 2020mot. This event prompted the researchers to conduct extensive observations of AT 2020mot across various wavelengths, including optical polarization and spectroscopy. These observations were made possible by the Nordic Optical Telescope (NOT), which is owned by the University of Turku. Notably, the polarization observations were carried out as part of a high school student’s observational astronomy course.

“The Nordic Optical Telescope and the polarimeter we utilized in our study played a crucial role in our endeavors to comprehend supermassive black holes and their surroundings,” explains Doctoral Researcher Jenni Jormanainen from FINCA and the University of Turku, who led the polarization observations and analysis at the NOT.

The researchers made an intriguing discovery: the optical light emitted by AT 2020mot exhibited high levels of polarization that varied over time. Despite numerous attempts, radio and X-ray telescopes failed to detect any radiation from the event before, during, or even months after its peak.

“Upon observing the significant polarization of AT 2020mot, our initial thought was that a jet was emanating from the black hole, as we often observe in cases where supermassive black holes accrete surrounding gas. However, no jet was detected,” reveals Elina Lindfors, an Academy Research Fellow at the University of Turku and FINCA.

Upon further analysis, the team of astronomers realized that the data best aligned with a scenario where the stream of stellar gas collides with itself, creating shocks near the pericenter and apocenter of its orbit around the black hole. These shocks then enhance and organize the magnetic field within the stellar stream, resulting in highly polarized light. The level of optical polarization observed exceeded the explanations offered by most existing models, and the fact that it changed over time posed an additional challenge.

“Among the models we examined, only the tidal shock model could account for the observations,” highlights Karri Koljonen, 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 astronomers plan to continue observing the polarized light emitted by TDEs, which may lead to further insights into the aftermath of star disruption events.

Source: University of Turku

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