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

TL;DR

This paper presents observational evidence for the formation of a magnetically arrested disk (MAD) around a black hole during an outburst, based on multi-wavelength delays indicating magnetic field amplification and disk dynamics.

Contribution

It provides the first observational evidence supporting the formation of a MAD in a black hole X-ray binary through analysis of multi-wavelength delays.

Findings

Radio and optical fluxes lag X-ray flux by 8 and 17 days respectively.

Magnetic fields are amplified by an expanding corona to form a MAD.

Optical delay attributed to thermal viscous instability in the outer disk.

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 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.