Parameters estimation and strong gravitational lensing of nonsingular Kerr-Sen black holes

TL;DR

This paper investigates gravitational lensing and shadow properties of nonsingular Kerr-Sen black holes, comparing them with Kerr black holes, and discusses astrophysical implications and parameter constraints based on observational data.

Contribution

It introduces a detailed analysis of lensing coefficients and shadow features of nonsingular Kerr-Sen black holes, extending previous models and applying results to real supermassive black holes.

Findings

Light deflection coefficients increase with parameters $k$ and $b$.

Nonsingular Kerr-Sen black holes have smaller, more distorted shadows than Kerr black holes.

Shadow size constraints help estimate black hole parameters.

Abstract

The recent time witnessed a surge of interest in strong gravitational lensing by black holes is due to the Event Horizon Telescope (EHT) results, which suggest comparing the black hole lensing in both general relativity and heterotic string theory. That may help us to assess the phenomenological differences between these models. Motivated by this, we consider gravitational lensing by the nonsingular Kerr-Sen black holes, which encompass Kerr black holes as a particular case, to calculate the light deflection coefficients $p$ and $q$ in strong-field limits, while the former increases with increasing parameters $k$ and charge $b$, later decrease. We also find a decrease in the light deflection angle $\alpha_D$, angular position $\theta_{\infty}$ decreases more slowly and impact parameter for photon orbits $u_{m}$ more quickly, but angular separation $s$ increases more rapidly with parameters $b$ and $k$. We compare our results with those for Kerr black holes, and also the formalism is applied to discuss the astrophysical consequences in the case of the supermassive black holes NGC 4649, NGC 1332, Sgr A* and M87*. In turn, we also investigate the shadows of the nonsingular Kerr-Sen black holes and show that they are smaller and more distorted than the corresponding Kerr black holes and nonsingular Kerr black holes shadows. The inferred circularity deviation $\Delta C\leq 0.10$, for the M87* black hole shadow, put constraints on the nonsingular Kerr-Sen black hole parameters ($a, k$) and ($a, b$). The maximum shadow angular diameter for $b=0.30M$ and $k=0.30M$ are, respectively, $\theta_d=35.3461\,\mu$as and $\theta_d=35.3355\,\mu$as. We also estimate the parameters associated with nonsingular Kerr-Sen black holes using the shadow observables.