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.

Visible light transmittance of micro-pore optic plates for the wide-field x-ray telescope on the Einstein probe.

Micro-pore optics (MPO) has been employed in space x-ray telescopes for large field-of-view observations. For x-ray focal plane detectors with visible photon sensing capability, the optical blocking filter (OBF) on MPO devices is critical for preventing signal contamination by those photons. In this work, we designed a piece of equipment to measure the light transmission with high accuracy. The transmittance test results of the MPO plates meet the design requirements of less than 5×10-4. Based on the multilayer homogeneous film matrix method, we estimated possible combinations of film thicknesses (with alumina) that show a good agreement with the OBF design.

Insight-HXMT observations of jet-like corona in a black hole X-ray binary MAXI J1820+070.

A black hole X-ray binary produces hard X-ray radiation from its corona and disk when the accreting matter heats up. During an outburst, the disk and corona co-evolves with each other. However, such an evolution is still unclear in both its geometry and dynamics. Here we report the unusual decrease of the reflection fraction in MAXI J1820+070, which is the ratio of the coronal intensity illuminating the disk to the coronal intensity reaching the observer, as the corona is observed to contrast during the decay phase. We postulate a jet-like corona model, in which the corona can be understood as a standing shock where the material flowing through. In this dynamical scenario, the decrease of the reflection fraction is a signature of the corona's bulk velocity. Our findings suggest that as the corona is observed to get closer to the black hole, the coronal material might be outflowing faster.