Lensing Signatures of a Primary Hair Black Hole Solution

Abstract

We investigate the weak and strong field gravitational lensing of light by a primary hair black hole (BH) solution obtained within the framework of the gravitational decoupling (GD) approach. In this method, the BH solution is constructed by introducing geometric deformations to a known seed spacetime, which, in the present case, is the Schwarzschild geometry. The deformation functions are determined by imposing the dominant energy condition, leading to the emergence of two characteristic parameters, the primary hair $Q$ and the coupling constant $α$, both of which deform the Schwarzschild spacetime. To explore the lensing properties of this BH, we employ the Gibbons-Werner approach in the weak field regime and Bozza's formalism in the strong field regime. In the weak field limit, we derive the deflection angle up to second-order corrections and find that the coupling parameter $α$ contributes more significantly to the leading-order deflection than the primary hair parameter $Q$. The primary hair is also found to exhibit an effectively repulsive gravitational behavior. In the strong field regime, we compute the deflection angle of photons propagating near the photon sphere, where the deflection diverges logarithmically, allowing photons to complete multiple loops around the BH and form relativistic images. Using the supermassive BH Sgr A* as the astrophysical lens, we further investigate the strong lensing observables, namely the asymptotic angular position $\vartheta_{\infty}$, the angular separation $s$, and the relative magnitude $r_{\mathrm{mag}}$. We also constrain the primary hair parameter $Q$ for different values of the coupling constant $α$ using the shadow observations of Sgr A* reported by the Event Horizon Telescope (EHT) Collaboration.