  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 analyze archival multi-wavelength observations of an outburst
from the black hole X-ray binary MAXI J1820+070 in 2018. The radio and optical
fluxes are delayed by about 8 and 17 days respectively, compared to the X-ray
flux. 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. The optical delay is then due to thermal
viscous instability in the outer disk.
