Observations of a black hole x-ray binary indicate formation of a magnetically arrested disk

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. Description Editor’s summary As material falls toward a black hole, it forms an accretion disk that emits x-rays and optical light and sometimes also a jet that is visible at radio wavelengths. Theory predicts that if the disk contains a sufficiently strong magnetic field, then it can resist the gravitational pull of the black hole and temporarily halt the accretion process. You et al. compared observations of a transient black hole accretion event at x-ray, optical, and radio wavelengths. They found time delays between brightening at the different wavelengths and used models to show that this behavior is produced by a magnetically arrested disk. —Keith T. Smith Multiwavelength observations show that accretion onto a black hole was halted by a magnetic field in the accretion disk.