At the center of almost every galaxy, there is a supermassive black hole. Some are very active, constantly feeding on materials, while others are quiet and only occasionally snack on the poor unfortunate stars that end up too close to them.
The latter types have been at the center of a mystery. When stars are being destroyed and eaten by a black hole, they emit either visible light or X-rays, but never both, so astronomers wondered if they were observing two different dining styles. In a new study published in The Astrophysical Journal, researchers write that they believe this difference in “mood lighting”, as they call it, is the result of us observing two different moments during the black hole feast.
Captured stars are stretched into long strings, as the intense gravity pulls them apart in what it is technically known as a tidal disruption event. The material stretches all the way around the black hole and smacks back into itself. The team, led by Peter Jonker from SRON and Radboud University, found that previous events that emitted visible light were detectable in X-rays after a few years.
The team has two possible explanations for the different light emissions. In the first one, the researchers suggest that as the material collides with itself, it emits visible light and afterward, as it gets closer to the black hole, it starts glowing in X-rays. The second suggestion is that both visible light and X-rays are emitted at the same time, but the collision produces a cloud of material that absorbs the X-rays. So only after that cloud disperses do the X-rays become visible.
This work has some interesting consequences. The study of the X-ray emission from tidal disruption events can provide independent confirmation that black holes really do spin themselves. That’s because there is a connection between the X-rays and the rotation of the black hole.
Another important fact is that a recently launched X-ray observatory, eROSITA, will soon be able to put these hypotheses to the test. In the paper, the team states that up to 990 of these tidal disruption events might be seen every year by this new telescope.