When they devour a star, supermassive black holes emit powerful jets of material in two phases: a study reveals the physics behind these phenomena and allows us to predict when they will happen.
From the point of view of black holes, these are large meals in multiple “episodes”, with well-spaced bites. For the poor stars that end up in their jaws, they are instead a slow agony. We are talking about tidal destruction events, phenomena of destruction of stars that end up too close to the event horizon (the boundary of no return) of supermassive black holes.
Astrophysicists already knew that these sumptuous meals may not all happen at once. Now a group of scientists from Curtin University, Australia, has discovered that black hole eruptions also occur in multiple phases – specifically, at two distinct moments. The discovery was published on Nature Astronomy.
What do black hole burps mean?
During tidal destruction events (tidal disruption eventTDE), when a black hole feeds, it is unable to swallow all the matter arriving from the star, which it attracts and expels part of it in the form of violent jets of gas and winds that cross the entire galaxy, from the nucleus outwards. They are what the experts call outflowsand which common sense could compare to “burps” of matter: they are studied because they can contribute to the birth of new stars.
Two distinct phases
The authors of the study analyzed around twenty tidal destruction events with the help of radio telescopes, and discovered that supermassive black holes – the largest, with a mass millions or billions of times greater than that of the Sun – emit outflows in two distinct phases of their feeding cycle: therefore not randomly, but at precise moments.
The first “burp” occurs immediately, during a moment of very intense and rapid growth; the second occurs later, hundreds or thousands of days after the star’s destruction, when the flow of engulfed matter drops to about 2% of the maximum rate at which a black hole can swallow matter.
This critical level below which the second eruption is triggered is the same threshold that triggers outflows even in much smaller black holes, those with a mass equal to about 10-50 times that of the Sun. The fact that the critical limit is the same even in objects with such different mass scales means that the physical process underlying these emissions from black holes is the same.
The discovery will allow scientists to more precisely predict the timing of the release of matter from black holes: it means having the possibility of organizing appropriate observation windows with the best telescopes, and freeing up precious time for the observation of other phenomena.
