Magnetic field by current loop in the Janis-Newman-Winicour spacetime

We investigate the influence of a scalar field on particle dynamics and electromagnetic phenomena in the spacetime of a gravitational compact object described by the Fisher-Janis–Newman–Winicour (FJNW) solution. The motion of test particles is analyzed using the Hamilton–Jacobi formalism, with particular attention to the innermost stable circular orbit (ISCO). We further explore photon motion and black hole shadows, deriving analytic expressions for the photon sphere radius and critical impact parameter. Applying these results to Event Horizon Telescope (EHT) observations of M87 and Sgr A, we show that the predicted shadow sizes remain consistent with the measured values within the 1 σ confidence interval, thereby constraining the scalar parameter. In addition, we obtain the analytical solution of Maxwell’s equations for the vector potential produced by a stationary current loop in the FJNW background. The resulting magnetic field exhibits a uniform structure in the interior region, reminiscent of the Wald solution, while in the exterior region it behaves like a dipolar field. We find that the external magnetic field strength decreases with increasing scalar parameter, in contrast to the tidal charge scenario of relativistic stars.