The extended inner shadow of Kerr-Taub-NUT black hole with thin disk flows

TL;DR

This study uses numerical ray-tracing to analyze the appearance of Kerr-Taub-NUT black holes with thin disk flows, revealing a novel 'extended inner shadow' feature that could help detect NUT charge in black holes.

Contribution

It introduces the concept of the 'extended inner shadow' in Kerr-Taub-NUT black holes and explores its observational signatures, extending previous Kerr black hole shadow studies.

Findings

Deformation of the critical curve into a 'D' shape with increasing spin.

Development of a 'duck-cap-like' morphology in the inner shadow at specific parameters.

Identification of the 'extended inner shadow' structure with unique photon pathways.

Abstract

In this paper, we apply numerical backward ray-tracing to study the observational appearance of Kerr-Taub-NUT (KTN) black holes illuminated by thin accretion disk flows. We obtained the inner shadow, redshift characteristics, and intensity distribution of thin-disk images of the KTN black hole, as observed by a common observer located at different positions. The results show that increasing the spin parameter progressively deforms the critical curve into a "D" shape while simultaneously shrinking and distorting the inner shadow. More importantly, for n = 0.3 at theta_o = 80 degrees, the inner shadow develops a novel "duck-cap-like" morphology with a sharply protruding lower-right edge beyond the critical curve. We term this feature the "extended inner shadow", a structure distinct from the Kerr case. Unlike the standard inner shadow, it consists partly of photons absorbed by the horizon and partly of photons that avoid both absorption and crossing the disk plane, thus receiving no emission. Such deviations from Kerr predictions could be tested by future high-precision astronomical observations, potentially offering new evidence for the existence of NUT charge (or the gravitomagnetic monopole) in black holes.