International Team Reveals How 'MAD' Accretion Forms Around Black Hole

An international team of astronomers has revealed for the first time the magnetic field transport processes in the accretion flow of a black hole and the formation of a "MAD"—a magnetically arrested disk—in the vicinity of a black hole.

The researchers made the discovery while conducting multi-wavelength observational studies of an outburst event of the black hole X-ray binary MAXI J1820+070, using Insight-HXMT, China's first X-ray astronomical satellite, as well as multiple telescopes.

Key to their discovery was the observation that the radio emission from the black hole jet and the optical emission from the outer region of the accretion flow lag behind the hard X-rays from the hot gas in the inner region of the accretion flow (i.e., the hot accretion flow) by about eight days and 17 days, respectively.

These findings were published in the journal Science on August 31.

The study was led by You Bei, an associate professor at Wuhan University in China, and co-authored by Xinwu Cao of Zhejiang University and Zhen Yan of the Shanghai Astronomical Observatory (SHAO).

"Our study for the first time reveals the process of magnetic field transport in the accretion flow and the process of MAD formation in the vicinity of the black hole," said Bei. "This represents the direct observational evidence for the existence of a magnetically arrested disk."

A black hole is an object with such strong gravity that not even light can escape. When a black hole accretes gas, it forms an accretion disk around it. The gas in the accretion disk spirals inward, getting hotter and hotter as it falls closer to the black hole. This heat is released as radiation, which can be observed by telescopes.

The magnetic field around a black hole can play a role in the accretion process. The magnetic field can help to transport angular momentum away from the gas in the accretion disk, which allows the gas to spiral inward more easily.

In a MAD, the magnetic field is so strong that it prevents the gas from falling into the black hole. Instead, the gas forms a disk around the black hole, but it does not accrete onto the black hole.

The researchers believe that the MAD in MAXI J1820+070 formed as a result of the weakening of the magnetic field in the outer region of the accretion disk. This weakening allowed the gas in the outer region to fall inward, which in turn strengthened the magnetic field in the inner region.

The discovery of the MAD in MAXI J1820+070 is a significant step forward in our understanding of black hole accretion. It provides direct observational evidence for the existence of MADs, which have been theorized for many years.

The study also has implications for our understanding of black hole jets. Jets are powerful beams of plasma that are ejected from some black holes. The researchers believe that the MAD in MAXI J1820+070 may be responsible for powering the black hole jet.

The discovery of the MAD in MAXI J1820+070 is a major breakthrough in astronomy. It provides new insights into the physics of black holes and their surroundings. The study also paves the way for further research into the formation and evolution of MADs.

Journal Information: Bei You et al, Observations of a black hole X-ray binary indicate formation of a magnetically arrested disk, Science (2023). DOI: 10.1126/science.abo4504. www.science.org/doi/10.1126/science.abo4504