Deciphering the Magnetic Enigmas of Black Holes: The Formation of “MAD” Accretion in Proximity to a Black Hole

An artistic rendering of the black hole X-ray binary MAXI J1820+070 depicts a magnetically arrested disk (MAD) enveloping the black hole. Image Credit: You Bei

For the inaugural time, an international consortium of scientists has revealed the mechanisms of magnetic field transference in the matter falling towards a black hole—referred to as accretion flow—as well as the genesis of a magnetically arrested disk (MAD) in proximity to the black hole.

This groundbreaking insight emerged from a multi-spectral analysis of a burst event originating from the X-ray binary black hole identified as MAXI J1820+070. The researchers employed China’s first X-ray astronomy satellite, Insight-HXMT, in conjunction with various other telescopes for their investigations.

Central to this discovery was the finding that the black hole’s radio wave emission and the optical emission from its peripheral accretion flow trailed the hard X-ray emissions from the central, heated part of the accretion flow—specifically by intervals of eight and 17 days, respectively.

This research was disclosed in the journal Science, dated August 31. The undertaking was spearheaded by Associate Professor You Bei from Wuhan University, Professor Cao Xinwu from Zhejiang University, and Professor Yan Zhen from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences.

When a black hole ingests gaseous matter—a phenomenon termed “accretion”—the matter undergoes what is known as an accretion flow. Through this flow, gravitational energy is effectively released, partially transforming into multi-spectral radiation visible through terrestrial and orbital telescopes. Consequently, this allows the black hole to become perceptible.

Nevertheless, invisible magnetic fields surround the black hole. As the black hole absorbs gas, it concurrently pulls these magnetic fields inward. Prior theoretical models posited that as matter flows toward the black hole, carrying with it weaker, external magnetic fields, these fields intensify near the black hole’s central accretion flow. Here, a balance is struck between the inward gravitational pull and the outward magnetic force. Once the magnetic field reaches a particular threshold, it ensnares the inbound matter, preventing it from plummeting freely into the black hole. This is the essence of a magnetically arrested disk (MAD).

Although MAD theories have been extant for years, offering explanations for observed black hole phenomena, empirical evidence confirming MAD’s existence remained elusive, as did the specifics of MAD formation and magnetic field transportation mechanisms.

Besides the supermassive black holes that reside at the cores of almost every galaxy, the universe also contains numerous stellar-mass black holes, which typically have a mass approximately ten times that of the sun. Ordinarily, these black holes exist in a dormant state, emitting negligible electromagnetic radiation. Occasionally, however, they undergo outburst phases, lasting from several months to years, that result in the generation of bright X-rays. Hence, these configurations are commonly known as black hole X-ray binaries.

In their study, the researchers conducted a comprehensive multi-spectral analysis of the eruption from the black hole X-ray binary MAXI J1820+070. The hard X-ray emission presented a zenith, succeeded by a peak in radio emissions eight days later. This considerable time gap between the radio and hard X-ray emissions was unparalleled.

Subsequent observations demonstrated that the attenuated magnetic field in the accretion disk’s exterior was transferred to its core by the heated gas. This led to an expansion of the accretion flow’s radial reach, accompanied by an augmentation of the magnetic field, thereby culminating in the formation of a MAD around eight days post the hard X-ray emission peak.

Associate Professor You Bei, the principal author and co-corresponding author, stated, “Our research is the inaugural demonstration of magnetic field transference in accretion flows and MAD genesis near a black hole, providing the first concrete empirical evidence for the existence of a magnetically arrested disk.”

Furthermore, an unprecedented 17-day delay between optical and hard X-ray emissions was observed. Numerical simulations revealed that, as the eruption neared its termination, hard X-ray irradiation triggered additional matter from the distant outer regions to be drawn toward the black hole. This culminated in a flare in optical emissions around 17 days following the hard X-ray emission peak.

Professor Cao Xinwu, co-corresponding author, remarked, “Due to the general applicability of the physics governing black hole accretion, our findings offer fresh insights into grander scientific inquiries concerning the genesis of large-scale magnetic fields, the mechanisms powering jets, and the acceleration processes in accreting black holes across various mass scales.”

Professor Yan Zhen, also a co-corresponding author, noted that similar observations are anticipated in additional black hole accretion systems in the foreseeable future.

Reference: “Observations of a black hole x-ray binary indicate formation of a magnetically arrested disk” by Bei You, Xinwu Cao, Zhen Yan, Jean-Marie Hameury, Bozena Czerny, Yue Wu, Tianyu Xia, Marek Sikora, Shuang-Nan Zhang, Pu Du, and Piotr T. Zycki, published on 31 August 2023 in Science.
DOI: 10.1126/science.abo4504

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More about Magnetically Arrested Disk (MAD)

• Science Journal Publication
• Insight-HXMT Satellite
• Shanghai Astronomical Observatory
• Wuhan University
• Zhejiang University
• Introduction to Black Hole Accretion
• Magnetically Arrested Disk Theory
• Black Hole X-ray Binaries