Optical appearance of the Kerr–Bertotti–Robinson black hole with a magnetically driven synchrotron emissivity model

Abstract We investigate the optical appearance of a Kerr–Bertotti–Robinson (Kerr-BR) black hole illuminated by a geometrically and optically thin accretion disk. Instead of using a phenomenological power-law emissivity, we adopt a magnetically driven synchrotron emissivity proxy coupled to the local electromagnetic environment. With a backward ray-tracing framework, we examine the effects of the spin a , magnetic parameter B , and observer inclination $\theta _O$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miθ</mml:mi> mml:miO</mml:mi> </mml:msub> </mml:math> on the ray-classification maps, redshift distributions, and specific-intensity images. We show that the ISCO position is modified by both a and B , and that rapidly rotating prograde configurations can develop an additional model-dependent inner cutoff when the magnetically dominated approximation underlying the emissivity prescription ceases to be applicable. High-resolution one-dimensional intensity profiles further separate the direct image, the $n=1$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mrow mml:min</mml:mi> mml:mo=</mml:mo> mml:mn1</mml:mn> </mml:mrow> </mml:math> lensing-ring contribution, and the higher-order $n\ge 2$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:mrow mml:min</mml:mi> mml:mo≥</mml:mo> mml:mn2</mml:mn> </mml:mrow> </mml:math> photon-ring subimages, while quantifying the Doppler-induced brightness asymmetry. Retrograde disks exhibit a wider emission-depleted central region because of the outwardly shifted ISCO, making the higher-order lensed components more clearly distinguishable from the direct emission. These results show that the disk inner boundary and the magnetic-field-dependent emissivity can substantially influence the observable appearance of Kerr-BR black holes.