Strong field lensing and shadows of $\boldsymbol{f(Q,\mathcal{B})}$ gravity black holes
We investigate the strong field lensing of light by black holes (BHs)emerging in $f(Q, \mathcal{B})$ theory of gravity, which is an extension of $f(Q)$ gravity theory by including a boundary term $\mathcal{B}$. Our analysis starts with the study of horizon structure for the BH spacetime in $f(Q,\mathcal{B})$ gravity. The strong field lensing analysis is done by employing the method developed by V.~Booza. We calculate the deflection angle for the static and spherically symmetric $f(Q,\mathcal{B})$ gravity BH spacetime and compare the results with the Schwarzschild BH. It is found that for all considerable scenarios of model parameters the $f(Q,\mathcal{B})$ gravity BH produces less deflection of light than the Schwarzschild BH. We extend our study to examine the outermost relativistic Einstein rings and three other observables, viz., angular position $\vartheta_{\infty}$, angular separation $s$ and relative magnification $r_\text{mag}$. We see that except $r_\text{mag}$, other observables show the same kind of behavioural change with respect to model parameters. We also study the shadow of the considered BH. Moreover, using the shadow data of BH Sgr A* from EHT collaborations, we constrain the model parameters of the $f(Q,\mathcal{B})$ theory for the BH spacetime. The constraining analysis reveals that for all considerable values of the model parameter $s_0$, the allowed range of the other model parameter $q_0$ is $-3\lesssim q_0 \lesssim -2$.