Observations of a black hole x-ray binary indicate formation of a magnetically arrested disk
Bei You, Xinwu Cao, Zhen Yan, J. M. Hameury, B. Czerny, Yue Wu, Tianyu Xia, M. Sikora, Shuang‐Nan Zhang, Pu Du, P. T. Życki
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.