Strong gravitational lensing around dilaton–axion black holes having special coupling with electromagnetic field

Recent observational evidence reveals the existence of black holes in the galaxy. Using strong field lensing effect, we find out the expressions of the radius of the photon sphere, the deflection angle, the separation between the first and the other images, the ratio between the flux of the first image as well as the flux coming from all the other images for dilaton–axion black holes and compare the results with Reissner–Nordstrom black holes. We also investigate the variations of primary and secondary image positions as well as their magnifications with respect to the source position for this kind of black holes.

Strong gravitational lensing by Kerr–Newman-Nut-Quintessence black hole

We investigate the strong gravitational lensing on equatorial plane as well as quasi-equatorial plane by the Kerr–Newman-Nut-Quintessence (KNNQ) black hole (BH) with the equation of state (EoS) parameter of the quintessence [Formula: see text] and the quintessence density [Formula: see text]. Our results show that the strong gravitational lensing in the KNNQ black hole space–time has some distinct behaviors from those in the backgrounds of the four dimension Kerr black hole. Also, we investigate the strong gravitational lensing on equatorial plane as well as quasi-equatorial plane by the KNNQ BH with the effects of Nut charge, spin parameter and quintessence parameter. First, we calculate the null geodesic equations using the Hamilton–Jacobi separation method. Then we investigate the equatorial lensing by KNNQ black hole. We obtain the deflection angle and deflection coefficients in the equatorial plane, which is affected by EoS parameter of the quintessence [Formula: see text], quintessence density [Formula: see text], Nut parameter [Formula: see text], spin parameter [Formula: see text] and quintessence parameter [Formula: see text] [Formula: see text]. Next, we discuss the lens equation and the observables in the equatorial plane. Finally, we investigate gravitational lensing by the KNNQ black hole in the quasi-equatorial plane. In this work, the quintessence density [Formula: see text], the EoS parameter of the quintessence [Formula: see text], Nut parameter [Formula: see text], spin parameter [Formula: see text] and quintessence parameter [Formula: see text] [Formula: see text] have significant effects on the strong gravitational lensing both in equatorial plane as well as quasi-equatorial plane.

Probing variation of the fine-structure constant in runaway dilaton models using Strong Gravitational Lensing and Type Ia Supernovae

In order to probe a possible time variation of the fine-structure constant ($$\alpha $$ α ), we propose a new method based on Strong Gravitational Lensing and Type Ia Supernovae observations. By considering a class of runaway dilaton models, where $$\frac{\Delta \alpha }{\alpha }= - \gamma \ln {(1+z)}$$ Δ α α = - γ ln ( 1 + z ) , we obtain constraints on $$\frac{\Delta \alpha }{\alpha }$$ Δ α α at the level $$\gamma \sim 10^{-2}$$ γ ∼ 10 - 2 ($$\gamma $$ γ captures the physical properties of the model). Since the data set covers the redshift range $$0.075 \le z \le 2.2649$$ 0.075 ≤ z ≤ 2.2649 , the constraints derived here provide independent bounds on a possible time variation of $$\alpha $$ α at low, intermediate and high redshifts.