A black hole mystery solved? Study offers key to magnetism

Scientists make strides in understanding magnetic fields near black holes and 'magnetically arrested disk' (MAD) formation.
Sade Agard
An artist's illustration of a black hole accreting matter from a companion star and producing jets.
An artist's illustration of a black hole accreting matter from a companion star and producing jets.


An international team of scientists has made significant strides in understanding the magnetic field transport processes in black hole accretion flows and the formation of magnetically arrested disks (MADs) near black holes, according to a recent study published in Science on August 31.

As material is drawn toward a black hole, it forms a swirling disk that emits X-rays, optical light, and occasionally a visible jet of energy in the form of radio waves.

Theoretical predictions suggested that if this disk had a sufficiently strong magnetic field, it could push back against the gravitational pull of the black hole and temporarily halt the process of material falling into it.

Now, this latest study examined a brief event where a black hole — dubbed MAXI J1820+070 — was actively accreting material, using observations in X-rays, optical light, and radio waves.

Significantly, they observed time lags between the brightening in these different types of emissions and used models to explain that this phenomenon is due to the MAD phenomenon.

What are black hole X-ray binaries?

Apart from the supermassive black holes found at the centers of galaxies, numerous smaller stellar-mass black holes are scattered throughout the universe. 

These smaller black holes are often part of binary star systems within the Milky Way. Typically, they are quiet, emitting minimal electromagnetic radiation. 

However, they occasionally enter an outburst phase, during which they emit intense X-rays. Such binary systems are termed black hole X-ray binaries.

In this new study on the outburst of the black hole X-ray binary MAXI J1820+070, the researchers discovered radio emissions were about eight days slower, and optical emissions were roughly 17 days behind super-bright X-rays close to the black hole. According to them, these delays suggest the delays hinted at a significant phenomenon. 

A black hole mystery solved? Study offers key to magnetism
An illustration of the black hole X-ray binary MAXI J1820+070 with a magnetically arrested disk formed around the black hole.

That is, the weak magnetic field in the outer region of the accretion disk was carried inward by the hot gas, causing the radial extent of the hot accretion flow to expand. 

This expansion led to an intensified magnetic field near the black hole, forming a MAD approximately eight days after the peak of hard X-ray emissions.

"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 first author Assoc. Prof. YOU Bei in a press release.

"This represents the direct observational evidence for the existence of a magnetically arrested disk."

Prof. CAO Xinwu, who co-led the study, highlighted that black holes collect matter consistently across all sizes. 

For this reason, his team's research will help us better understand how magnetic fields form on a large scale and how black holes, regardless of their size, produce powerful jets of energy and speed up objects around them.

The team anticipates observing similar phenomena in other accreting black hole systems in the future, offering further insights into the workings of these cosmic enigmas. 

Understanding accretion and magnetic fields

Accretion is the process by which black holes capture surrounding gas, forming an accretion flow. 

Within this flow, viscous processes release gravitational potential energy, some of which transform into multi-wavelength radiation. This radiation is observable, allowing scientists to study black holes.

However, magnetic fields enveloping black holes remain unseen. As gas is drawn into the black hole, it also drags these magnetic fields inward. 

Previous theories proposed that as gas continuously introduced weak external magnetic fields, the field's strength would progressively increase toward the inner accretion flow. 

This would counteract the black hole's gravitational pull, leading to the formation of a magnetically arrested disk (MAD) when the magnetic field reached a certain strength.

While the MAD theory has been around for some time and explained various observations related to black hole accretion, direct observational evidence was lacking.

The complete study was published in Science on August 31 and can be found here.

Study abstract:

Accretion of material onto a black hole drags any magnetic fields present inwards, increasing their strength. Theory predicts that sufficiently strong magnetic fields can halt the accretion flow, producing a magnetically arrested disk (MAD). We analyzed archival multiwavelength observations of an outburst from the black hole x-ray binary MAXI J1820+070 in 2018. The radio and optical fluxes were delayed compared with the x-ray flux by about 8 and 17 days, respectively. We interpret this as evidence for the formation of a MAD. In this scenario, the magnetic field is amplified by an expanding corona, forming a MAD around the time of the radio peak. We propose that the optical delay is due to thermal viscous instability in the outer disk.

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