Black holes always cause destruction

Astrophysics: When Black Holes Destroy Stars

The first TDE candidates were found in the data from the ROSAT X-ray satellite and the Galaxy Evolution Explorer ultraviolet space telescope. They made themselves felt as weeks to months of radiation bursts in the center of previously inconspicuous galaxies. The discoveries established an entirely new area of ​​research. However, the astronomers initially relied on old data and could not study the phenomenon in real time or at many wavelengths. To catch a TDE in action, the researchers either had to be very lucky or continuously observe huge regions of the sky.

Fortunately, extensive sky surveys have actually become possible in recent years thanks to advances in data processing and sensor technology. Today, a high-performance camera is capable of capturing a section of the sky more than a square degree with a single image. By repeatedly scanning large areas of the sky and digitally comparing the images, even weak and temporary phenomena can be identified. The new missions used for this are called Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), Palomar Transient Factory (PTF) and All-Sky Automated Survey for Supernovae (ASAS-SN). They were originally supposed to identify supernovae and asteroids, but they do a lot more. As they capture the light of millions of galaxies night after night, they also track down the rare star shattering.

Surprising details of a star annihilation

Shortly after Pan-STARRS started work, a team led by the astronomer Suvi Gezari discovered a TDE called PS1-10jh in 2011, which took place in a black hole with a mass of around two million solar masses in a galaxy 2.7 billion light-years away. Since this TDE was noticed shortly after the data was collected, the team was able to follow the further development in the visible and UV area live, so to speak, for the first time. The result amazed the researchers.

Careful spectral analysis showed that this TDE seemed to generate far too little heat. At around 30,000 degrees Celsius, it had only a fraction of the heat required by the current models for accretion discs. In addition, PS1-10jh did not fade within weeks due to cooling and scattering of its accretion disk, but instead kept a constant temperature for many months after its discovery. It was particularly strange that Pan-STARRS discovered signs of ionized helium in the afterglow - which is actually only allowed to happen at temperatures of more than 100,000 degrees. While the TDE apparently contained a lot of helium, it lacked hydrogen, the most abundant element in the universe and the main component of stars. The theorists were puzzled.

To explain the lack of hydrogen, Gezari's team assumed that the destroyed star had already lost its thick hydrogen mantle - perhaps during an earlier interaction with the black hole - and only retained its helium-rich core, which it then used to feed the observed accretion disk. But how is the paradoxical thermal behavior of PS1-10jh explained, i.e. its surprisingly low temperature and, in contrast, the excess helium that can only be ionized at much higher temperatures? Other theorists have therefore suspected that the astronomers had not even observed the accretion disk itself, but instead a gas curtain much further away from the black hole, which absorbed the intense radiation from the disk and emitted it again at lower temperatures. Such a veil would have the added benefit of informally explaining the apparent absence of hydrogen. At the right temperature and at a fairly high density, it conceals all references to this element.

The only catch is that a dense gas curtain would not be stable given the required distance to the black hole. The gas would either fall into the black hole or evaporate into space. The unclear origin of this material is still causing lively discussions. Broadly speaking, there are two possibilities, both of which have to do with the voracious black hole dynamics. While the remnants of a destroyed star whirl around it and form a growing accretion disk, shock waves emanating from the disk could prevent the immediate crash of material circling further out and thus create a reasonably stable veil. According to another explanation, shortly after its formation, the accretion disk rolls so much material inward that the black hole is virtually overwhelmed by it and part of it is thrown far beyond the disk again. As the puzzles surrounding PS1-10jh and other TDEs discovered soon after illustrate, star destruction is an unexpectedly complex phenomenon. But the biggest surprise was yet to come.