Modelling the Sgr A$^*$ and M87$^*$ shadows by using the Kerr-Taub-NUT metrics in the presence of a scalar field

Abstract

The recent unveiling of the images of Sgr A* and M87* has significantly advanced our understanding of gravitational physics. In this study, we derive a class of Kerr-Taub-NUT metrics in the presence of a scalar field (KTNS). Treating these metrics as models for supermassive objects, we constrain the parameters using shadow size estimates done by observations of M87* and Sgr A* from the Event Horizon Telescope (EHT). Comparing the obtained results with M87* data, we show an upper limit on the NUT charge $n$ such that the constraint on the shadow deviation from circularity ($ \Delta C $) will be fulfilled for $ n<0.5 $, and this allowed range changes with a variation in other parameters. Additionally, our findings reveal that fast-rotating KTNS metrics are better candidates for supermassive M87* than slowly rotating ones. We continue our study by estimating parameters using Keck and VLTI observations of Sgr A* and find that the constraint on the fraction deviation $ \delta $ is maintained within a certain range of the NUT charge such that the Keck bound is satisfied for $ n<0.41 $. In contrast, the VLTI bound can be fulfilled for $ n>0.34 $. Finally, we investigate weak gravitational lensing using the Gauss-Bonnet theorem and illustrate that all model parameters increase the deflection angle, causing light rays to deviate more significantly near fast-rotating KTNS objects.