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

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 αD, angular position θ∞ decreases more slowly and impact parameter for photon orbits um 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 Δ C≤ 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, θd=35.3461 μas and θd=35.3355 μas. We also estimate the parameters associated with nonsingular Kerr-Sen black holes using the shadow observables.

Testing Horndeski Gravity from EHT Observational Results for Rotating Black Holes

“…While the first-order correction to the circular shadow shape occurs due to the spin in Kerr spacetime, in MoGs the distortion for a given spin may arise due to other hairs (Cunha et al 2015(Cunha et al , 2019Afrin et al 2021;Ghosh et al 2021;Khodadi et al 2021)-this prompts the use of shadow observables for the determination of black hole parameters (Hioki & Maeda 2009;Afrin et al 2021;Afrin & Ghosh 2022). It has been shown in multiple studies that observables like the shadow radius R s and distortion δ s (Hioki & Maeda 2009) demand some specific symmetry in the shadow shape, and thus they may not be efficient in some MoGs (Tsukamoto et al 2014;Abdujabbarov et al 2015;).…”

Strong gravitational lensing and shadow constraint from M87* of slowly rotating Kerr-like black hole

“…where the oblateness D has been defined in (55). Moreover, the angular diameter of the shadow is defined as [78]…”

Estimating the cosmological constant from shadows of Kerr--de Sitter black holes

“…Estimation of the black hole parameters has been under constant endeavour [63][64][65][66][67][68] and the shadows have been utilized to estimate the parameters in both GR [32,33,69] as well as in modified theories of gravity (MoGs) [14,70,71]. Interestingly though the black hole spin and various other hairs have been estimated, the shadow has not been utilized for the estimation of the cosmological constant Λ thus far.…”