The “sudden death” of the stars

The Universe is a violent place, where even the life of a star can be cut short. This happens when it is in a “bad” neighborhood, namely the neighborhood of a massive black hole. These black holes have millions, if not billions, of the mass of the Sun and are located at the center of galaxies.

As the star approaches the black hole, it begins to feel its ever-increasing gravity, until that gravity becomes stronger than the forces holding the star together. The star will then be destroyed and its gas will be swallowed, in part, by the black hole.

These sudden star deaths are called Tidal Disruption Events (TDE). After the star dies, its gas will form an accretion disk on its way to the black hole. This results in a flare that is usually seen in the optical and ultraviolet spectrum, as well as X-rays, but sometimes even in the radio part of the spectrum and in γ-rays.

Until recently, only a few TDEs were known. Not because they are rare, but because there were not many experiments capable of detecting them. In recent years that has changed, giving scientists the tools they need to begin to understand how matter behaves at distances very close to a black hole.

But that led to more questions than answers. Observations from experiments with optical telescopes have revealed that many of these TDEs do not produce X-rays. This contradicts what scientists previously believed about TDEs. The dominant model predicts the rapid formation of an accretion disk, particularly bright on X-rays, immediately after the destruction of the star.

The polarization of light may be the key to solving this mystery. An international team of astronomers including members of the Institute of Astrophysics (IA) of the Institute of Technology and Research (ITE) and the University of Crete, published a study in the journal Science which argues that for TDEs without X-rays, the formation of the accretion disk is not rapid.

Instead, tidal shock waves form as gas from the star flows around the black hole. These shock waves are brilliant in the optical and ultraviolet light that make up what our telescopes observe as TDE. The incremental disk is created later.

“The polarization of light can provide unique insights into the processes of these astrophysical systems,” says study lead author Yannis Liodakis, an astronomer at the Finnish Astronomy Center with ESO (FINCA) and a graduate of the University of Crete. “The polarized light we measured from TDE could only be explained if we saw these tidal shock waves.”

At the end of 2020, the team was informed by the Gaia satellite of a new TDE in a nearby galaxy called AT2020mot. AT2020mot was then observed in a wide range of wavelengths from radio to X-rays with several different telescopes.

Observations of polarization in the optical spectrum made at the Skinakas Observatory using the unique RoboPol polarimeter, which made this discovery possible, were particularly important.

“The RoboPol polarimeter at the Skinakas Observatory has really been the cornerstone of our studies to understand supermassive black holes,” said Nikos Mandarakas, a PhD student at AI and the University of Crete, who led the observations. and data analysis with RoboPol.

The scientists found that the optical light from AT2020mot was highly polarized and its polarization changed over time. Despite numerous attempts, none of the radio or x-ray telescopes were able to detect TDE before, during, or even months after its peak.

Combining all of this information and comparing the observations with theoretical models, the team of astronomers realized that the data best fit the scenario where stellar gas collides with itself as it orbits the black hole and forms shock waves at the center and the eccentricity of the trajectory.

The shock waves amplify and order the magnetic field in the star gas leading to highly polarized light. The level of optical bias was very high and the fact that it changed over time made it very difficult
explained by most models.

Scientists will continue to observe polarized light from TDEs and may soon discover more about what happens after a star dies.