Astronomers have found signs of a universal mechanism that may determine when powerful jets are launched by black holes regardless of their mass. The key factor appears to be the rate at which they consume matter: in supermassive black holes, jet activity can sharply resume when the accretion rate drops to about 2% of the Eddington limit. The same threshold had previously been observed in much smaller stellar-mass black holes.

A torn-apart star reveals a black hole at work in fast-forward

Researchers from the Institute for Advanced Study and the Curtin University node of the International Centre for Radio Astronomy Research studied tidal disruption events — episodes in which a star passes too close to a supermassive black hole and is torn apart by its gravity.

This gives astronomers a rare opportunity to watch changes around a black hole almost in real time. Normally, processes around supermassive black holes unfold so slowly that noticeable changes can take thousands or even millions of years. After a star is destroyed, the black hole receives a large amount of matter, and the subsequent restructuring of the accretion flow plays out over just a few years.

At the same time, the matter does not necessarily all fall inward. Part of it can be expelled into space in powerful outflows that can travel enormous distances and affect the surrounding galaxy.

“When a black hole tears apart a star, it doesn’t neatly swallow all the material,” noted study co-author Adele Goodwin.

Jets appear in two distinct phases

For the study, the scientists compiled observations of 20 tidal disruption events across optical, ultraviolet, X-ray, and radio wavelengths. For ten of the events, the data were sufficient to reliably compare the accretion rate with the emergence of radio emission from jets.

The analysis revealed two phases in which a black hole is capable of launching jets.

The first occurs in the early period after the star is disrupted, when matter flows toward the black hole at a very high rate. However, in some objects, jets do not arise immediately. They may appear only after hundreds or even thousands of days.

It was the second phase that proved especially interesting. By this point, the accretion rate falls to about 2% of the Eddington limit. This is a conventional value that characterizes the maximum stable rate of matter consumption: above a certain level, radiation pressure becomes comparable to the gravitational pull that keeps matter bound to the black hole.

A similar threshold is already known for stellar-mass black holes in the Milky Way. Now observations of supermassive objects show that the same transition may also occur in black holes whose mass reaches millions of solar masses.

A universal rule for black holes on different scales

The scales of these objects differ by many orders of magnitude, yet their behavior at a certain accretion rate, judging by the study’s results, remains similar.

This is precisely what allows scientists to speak of a possible universal rule of jet formation: what matters is not the black hole’s absolute size, but the stage of the matter-accretion cycle it is in.

Such a conclusion may help explain why one supermassive black hole begins emitting a powerful radio jet almost immediately after a star is destroyed, while another remains relatively quiet and then unexpectedly becomes active months or years later.

Astronomers will be able to look for outbursts of activity in advance

The discovered threshold also has practical significance. If it becomes possible to predict with sufficient accuracy when the accretion rate will approach the critical level, astronomers will be able to point telescopes at such objects in advance.

This is especially important for radio astronomy: jets may be relatively short-lived, and time on major observatories is limited. The ability to identify the most promising period ahead of time will make it possible to allocate observing time more efficiently.

The study’s authors expect that their approach will also prove useful for future large-scale radio observatories, in particular the Square Kilometre Array project, which plans to begin collecting scientific data in 2028.

If the discovered relationship is confirmed in a larger number of objects, it will become further evidence that the physical processes around black holes obey common laws regardless of their mass — from objects of just a few dozen solar masses to supermassive giants at the centers of galaxies.