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

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

$$n=1$$ n = 1

lensing-ring contribution, and the higher-order

$$n\ge 2$$ n ≥ 2

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.