An international research team has discovered the magnetic field transport mechanisms in a black hole’s accretion flow and the production of a “MAD”—a magnetically arrested disk—in its surrounding area for the first time.
An illustration of the black hole X-Ray binary MAXI J1820+070 with a magnetically arrested disk formed around the black hole. Image Credit: YOU Bei
The finding was made while doing multi-wavelength observations of an outburst event of the black hole X-Ray binary MAXI J1820+070 utilizing Insight-HXMT, China’s first X-Ray astronomical satellite, as well as several telescopes.
The fact that the hard X-Rays from the hot gas in the inner region of the accretion flow (i.e., the hot accretion flow) lag behind the radio emission from the black hole jet and the optical emission from the outer region of the accretion flow by about eight and 17 days, respectively, was crucial to their discovery.
Science published the findings on August 31st, 2023.
Professor Zhen Yan from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, Professor Xinwu Cao from Zhejiang University, and Associate Professor Bei You from Wuhan University served as the study’s leads.
“Accretion” is the process through which a black hole absorbs gas, and an accretion flow is the stream of gas that enters the black hole. Gravitational potential energy is successfully released by the viscous processes inside the accretion flow, with some of the energy being transformed into multi-wavelength radiation. By using ground- and space-based telescopes to look at this radiation, black holes can be “seen.”
The black hole is surrounded by “unseen” magnetic fields, though. The magnetic field is pulled inward when the black hole swallow gas. Previous concepts suggested that the magnetic field gradually becomes stronger in the inner area of the accretion flow as the accreting gas continues to draw in weak external magnetic fields.
The accretion flow’s outward magnetic force grows and offsets the black hole’s inward gravitational pull. The accreted matter is imprisoned by the magnetic field and unable to fall freely into the black hole in the inner area of the accretion flow near the black hole when the magnetic field reaches a particular strength. A magnetically arrested disk is this phenomenon.
The long-proposed MAD theory has satisfactorily described several empirical black hole accretion-related events. However, the development of MADs and their magnetic transport methods remained a mystery, and there was no direct observable proof of their existence.
There are many more stellar-mass black holes in the universe in addition to the supermassive black holes at the heart of almost every galaxy. Numerous binary star systems in the Milky Way contain stellar-mass black holes, according to astronomers. These black holes often weigh ten times as much as the Sun.
These black holes often exist in a quiescent state and generate incredibly faint electromagnetic radiation. They do, however, periodically go through an outburst phase where intense X-Rays are produced and can linger for several months or even years. These binary star systems are hence frequently referred to as black hole X-Ray binaries.
In this investigation, the X-Ray binary black hole MAXI J1820+070’s outburst was subjected to a multi-wavelength data analysis. They noticed that the hard X-Ray emission showed a high, which was followed eight days later by a peak in the radio emission. It has never happened before for there to be such a delay between the radio emission from the jet and the hard X-Rays from the hot accretion flow.
These data imply that the hot gas carries the weak magnetic field in the accretion disk’s outer area into the inner region, and the radial extent of the hot accretion flow rapidly grows as the accretion rate lowers.
The magnetic field increases as the radial area of the hot accretion flow increases. This causes the magnetic field surrounding the black hole to rapidly increase, resulting in the creation of a MAD roughly eight days after the peak of the hard X-Ray emission.
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. This represents the direct observational evidence for the existence of a magnetically arrested disk.
Bei You, Study Co-Corresponding Author and Professor, Chinese Academy of Sciences
Furthermore, the researchers discovered an unprecedented delay (approximately 17 days) between the optical emission from the accretion flow's outer area and the hard X-Rays from the hot accretion flow.
It was observed through numerical simulations of the black hole X-Ray binary outburst that as the outburst nears its finish, the irradiation of hard X-Rays causes more accreting material from the far outer area to descend towards the black hole owing to instability. This causes an optical flare in the accretion flow’s outer area, with the peak happening roughly 17 days after the peak of the hot accretion flow’s hard X-Rays.
Due to the universality of black hole accretion physics, where accretion processes for black holes of different mass scales follow the same physical laws, this research will advance the understanding of scientific questions related to large-scale magnetic field formation, jet powering, and acceleration mechanisms for accreting black holes of different mass scales.
Xinwu Cao, Study Co-Corresponding Author and Professor, Chinese Academy of Sciences
According to Prof. Zhen Yan, co-corresponding author of the work, similar events to those discovered in MAXI J1820+070 are predicted to be detected in additional accreting black hole systems in the near future.
Journal Reference:
You, B., et al. (2023) Observations of a black hole x-ray binary indicate formation of a magnetically arrested disk. Science. doi:10.1126/science.abo4504