Astronomers identify a common threshold for black hole jet formation — ScienceDaily
An international group of astronomers has found evidence that black holes of different masses may produce powerful jets at the same critical accretion rate — the rate at which matter falls onto a black hole. As ScienceDaily reports, this rule may apply both to stellar-mass black holes, roughly ten times more massive than the Sun, and to supermassive black holes with masses millions of times greater than the Sun's.
Observations of destroyed stars
The study was published in the journal Nature Astronomy. It was conducted by Andrew Mummery of the Institute for Advanced Study and Adele Goodwin of the International Centre for Radio Astronomy Research at Curtin University in Western Australia.
The scientists used years of observations in the optical, ultraviolet, X-ray and radio ranges. The data were collected by telescopes in the Americas, Australia, India, South Africa and space.
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The team studied tidal disruption events, in which a star comes too close to a supermassive black hole and is torn apart by its gravity. Afterward, some of the stellar material moves toward the black hole, while a significant portion may be ejected into space by powerful outflows.
Two phases of jet formation
The researchers initially analyzed 20 tidal disruption events and later selected ten cases with sufficient data to determine the accretion rate and the time when radio outflows appeared. The analysis indicated two periods when jets may form: at an early stage with an extremely high accretion rate and much later — hundreds or thousands of days after the star's disruption.
In the late phase, the accretion rate falls to about 2% of the Eddington limit — the level at which radiation pressure balances gravitational attraction. This threshold has already been linked to jet formation in much smaller black holes in the Milky Way. If astronomers can predict the appearance of delayed jets, this may help them plan observations more effectively, including with the future Square Kilometre Array radio telescope project.
