Scientists Propose Explanation For Energetic Jets From Supermassive Black Holes Losing Energy

Black holes are objects in space whose gravitational pull is so strong that even light is unable to escape, but despite that, there is some amount of radiation that actually goes outward from black holes. Highly energetic jets traveling at near the speed of light, called active galactic nuclei (AGN) jets, take energy from that radiation and are among the most extreme outbursts that have been observed in the universe.

These jets are so called because they are observed beaming out of the centers of galaxies, where it is thought supermassive black holes reside. They usually form in opposing pairs but very little was hitherto known about the mechanisms that affect their movement and eventual dissipation, as well as the differences that exist among AGN jets in different galaxies.

“These jets are notoriously hard to explain. Why are they so stable in some galaxies and in others they just fall apart?” Alexander Tchekhovskoy, a member of the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) who has been studying the jets for 10 years, said in a statement Monday.

Tchekhovskoy co-led a new study, based on theories supported by 3D simulations, that proposes new explanations for how AGN jets lose their energy, about half of which is dissipated as X-rays or other stronger forms of radiation. According to the study, published in the Aug. 21 edition of Monthly Notices of the Royal Astronomical Society, there are two separate mechanisms at work, and that both are linked to the jets’ interaction with the ambient medium — the matter that surrounds the jets.

Rodolfo Barniol Duran, formerly a postdoctoral research associate at Purdue University who is now a faculty member at California State University, Sacramento, was another of the study’s leaders. He said in the statement: “We were finally able to simulate jets that start from the black hole and propagate to very large distances — where they bump into the ambient medium.”

Of the two effects that lead to energy dissipation from the space jets, one is called magnetic kink stability and the other triggers a series of shocks within other jets. Which of the two effects gets triggered depends on the density of the ambient medium, Tchekhovskoy explained. The longest of these jets extend for millions of light-years into space. The size and intensity of the jets also provide clues about the properties of their associated black holes.

“When we look at black holes, the first things we notice are the central streaks of these jets. You can make images of these streaks and measure their lengths, widths, and speeds to get information from the very center of the black hole. Black holes tend to eat in binges of tens and hundreds of millions of years. These jets are like the ‘burps’ of black holes – they are determined by the black holes’ diet and frequency of feeding,” Tchekhovskoy explained.

AGN jets, which are a form of plasma, gather their energy from the outflow of radiation that is caused as a backlash when the extreme friction and heating of gases falling into a black hole cause a massive rise in temperature and compression of magnetic fields.

“For a long time, we have speculated that shocks and instabilities trigger the spectacular light displays from jets. Now these ideas and models can be cast on a much firmer theoretical ground,” Dimitrios Giannios, assistant professor of physics and astronomy at Purdue and the third study leader, said in the statement.

The researchers now hope to use higher-resolution images of those parts of space where supermassive black holes are thought to exist to improve their models and theories. They hope to use data from instruments such as the Event Horizon Telescope (EHT).

“Seeing deeper into where the jets come from— we think the jets start at the black hole’s event horizon (a point of no return for matter entering the black hole) — would be really helpful to see in nature these ‘bounces’ in repeating shocks, for example. The EHT could resolve this structure and provide a nice test of our work,” Tchekhovskoy said.

The study, titled “Simulations of AGN jets: magnetic kink instability versus conical shocks,” is available online.