In the vast expanse of the Universe, the fate of stars can be cruelly altered when they venture too close to supermassive black holes, leading to Tidal Disruption Events (TDEs). Recent observations of polarized light emanating from these events have shed light on the intricate processes at play.
Supermassive black holes, possessing masses millions or billions of times greater than our Sun, typically lurk at the centers of serene galaxies. As a star ventures into the vicinity of one of these cosmic behemoths, it becomes ensnared in a relentless gravitational pull, overwhelming the forces that maintain its integrity. This results in the star being either disrupted or entirely destroyed, giving rise to a Tidal Disruption Event.
“After the star has been torn apart, its gas forms an accretion disk around the black hole. Bright outbursts from this disk can be observed across various wavelengths, especially through optical telescopes and X-ray detecting satellites,” explains Postdoctoral Researcher Yannis Liodakis from the University of Turku and the Finnish Centre for Astronomy with ESO (FINCA).
Until recently, the scarcity of tools capable of detecting TDEs limited researchers to studying only a few occurrences. However, advancements in scientific instruments now allow for the observation of more TDEs, leading to intriguing new mysteries that scientists are currently investigating.
“Observations from large-scale experiments with optical telescopes have revealed a puzzling fact: many TDEs do not exhibit X-rays despite clearly detectable bursts of visible light. This discovery challenges our fundamental understanding of how disrupted stellar matter evolves in TDEs,” notes Liodakis.
To unravel this enigma, an international team of astronomers, led by the Finnish Centre for Astronomy with ESO, conducted a study published in the journal Science. The team suggests that the key to understanding this phenomenon lies in the polarized light emitted during TDEs.
Rather than the expected formation of an X-ray bright accretion disk around the black hole, the observed outbursts of optical and ultraviolet light in many TDEs may actually result from tidal shocks. These shocks occur far away from the black hole as the gas from the destroyed star collides with itself upon circling the black hole. The formation of the X-ray bright accretion disk, it appears, occurs much later in these events.
“Polarization of light can provide unique insights into the underlying processes in astrophysical systems. The polarized light we measured from the TDE could only be explained by these tidal shocks,” states Liodakis, the lead author of the study.
The team’s observations were triggered by a public alert in late 2020 from the Gaia satellite, indicating a nuclear transient event labeled AT 2020mot in a nearby galaxy. Subsequent wide-ranging observations, including optical polarimetry and spectroscopy, conducted at the Nordic Optical Telescope (NOT) owned by the University of Turku, were instrumental in making this groundbreaking discovery. The polarimetry observations were part of an observational astronomy course for high school students.
The researchers found that the optical light emitted by AT 2020mot was highly polarized and exhibited fluctuations over time. Despite extensive efforts, no radiation from the event could be detected by radio or X-ray telescopes before, during, or months after the outburst’s peak.
Contemplating the data, the astronomers concluded that the most plausible scenario involves the stream of stellar gas colliding with itself, creating shocks near the pericenter and apocenter of its orbit around the black hole. These shocks amplify and organize the magnetic field in the stellar stream, resulting in highly polarized light. The exceptionally high level of optical polarization challenged most existing models, making the tidal shock model the most compelling explanation.
The researchers’ ongoing observations of polarized light from TDEs hold the promise of unveiling more secrets about the aftermath of stars falling prey to black holes.
Frequently Asked Questions (FAQs) about Tidal Disruption Events
What are Tidal Disruption Events (TDEs)?
Tidal Disruption Events (TDEs) are astronomical phenomena that occur when a star gets too close to a supermassive black hole. The intense gravitational pull from the black hole tears the star apart, leading to its disruption or destruction.
How do TDEs produce observable light?
After a star is torn apart during a TDE, its gas forms an accretion disk around the black hole. Bright outbursts of light are emitted from this accretion disk, and these outbursts can be observed across various wavelengths, especially through optical telescopes and X-ray detecting satellites.
What are some unique characteristics of TDEs?
One intriguing characteristic of TDEs is the polarized light they emit. Researchers have found that the polarized light observed in many TDEs can be attributed to tidal shocks, which occur as the gas from the destroyed star collides with itself on its way back after circling the black hole. This discovery has led to new mysteries and questions for scientists to explore.
How are researchers studying TDEs?
Advancements in scientific instruments and observational techniques have enabled researchers to study more TDEs than ever before. The team of astronomers in the study made use of the Nordic Optical Telescope (NOT) to observe AT 2020mot, a TDE event in a nearby galaxy. Polarization observations have played a crucial role in understanding the underlying processes during TDEs.
Why is the absence of X-rays in some TDEs significant?
One surprising finding from recent observations is that a significant number of TDEs do not produce X-rays, despite emitting visible light. This discovery challenges the existing understanding of how the disrupted stellar matter evolves during TDEs and has sparked further investigation into the nature of these events.
More about Tidal Disruption Events
- Science: Optical polarization from colliding stellar stream shocks in a tidal disruption event
- University of Turku
- Finnish Centre for Astronomy with ESO (FINCA)
- Nordic Optical Telescope (NOT)