A black hole can attract gas around it. This matter generally does not fall directly in: it forms a disk, heats up and gradually loses energy before moving closer. Some of it may also return to space in the form of jets or other fast outflows. But the moment when these emissions appear remained difficult to predict for giant black holes.
The problem mainly comes down to time. Around the supermassive black holes at the centers of galaxies, feeding can continue for thousands of years. Astronomers therefore studied brief events: the destruction of a star passing too close to a black hole.
Artist’s view of a star stretched and torn apart by the tidal forces of a supermassive black hole.
Credit: ESO/M. Kornmesser
In this scenario, the black hole’s gravity tears the star apart. Some of its matter then forms a new disk around it. Its evolution unfolds over a few years, allowing astronomers to observe the changes. Adelle Goodwin and Andrew Mummery studied these events using observations in visible light, ultraviolet, X-rays and radio waves.
The researchers first compiled twenty stellar disruptions, then selected ten cases that had been observed sufficiently well. This allowed them to estimate the amount of matter received by the black hole over time and determine when matter emissions had appeared.
Two periods stand out. The first occurs very early, when the black hole receives a huge amount of matter. The second happens much later, sometimes hundreds or thousands of days after the star’s destruction. By then, its feeding rate has fallen sharply.
This is where the researchers found a common value. This second emission appears when the black hole receives about 2% of a reference level called the Eddington limit. This limit corresponds to the point at which the pressure produced by radiation can counterbalance the gravitational attraction exerted on the surrounding matter.
Yet this threshold of about 2% was already known for much smaller black holes, formed after the death of a star. Despite the immense difference in mass, the two families therefore seem to change behavior at the same relative feeding level. The same physical rule could thus govern the launching of these jets.
This regularity also gives astronomers a new tool. By tracking the decline in feeding after a star’s destruction, they can predict when a delayed jet is most likely to appear. Radio observations can then be scheduled for the time when they are most likely to detect this emission.