Deciphering signatures of Kerr-Sen black holes in presence of plasma from the Event Horizon Telescope data

Abstract

The present work explores the role of the dilaton charge $r_2$ and the plasma environment in explaining the observed images of M87* and Sgr A*. Dilaton charges are associated with Kerr-Sen black holes, the stationary, axi-symmetric black hole solution in the Einstein-Maxwell-dilaton-axion (EMDA) gravity which arise in the low energy effective action of superstring theories. We investigate the impact of the background spacetime (here dilaton charge and spin) and the plasma environment in modifying the shape and size of the black hole shadow. The theoretically derived shadow is compared with the observed images of M87* and Sgr A* which enable us to constrain the background spacetime in presence of the plasma environment. { Our analysis reveals that the shadow of M87* favors the Kerr scenario and rules out $r_2>0.48$, while the shadow of Sgr A* exhibits a marginal preference towards the Kerr-Sen scenario (although GR is allowed within 1-$\sigma$) and rules out $r_2>1$. Thus, large values of dilaton charge are disfavored for M87* and Sgr A* and this result holds good irrespective of the inhomogeneous plasma environment. Moreover, the shadows of M87* and Sgr A* rule out very dense inhomogeneous plasma environments surrounding these objects but the plasma density is further constrained from the electron number density and accretion rate estimates. As a consequence, with the current level of precision of the shadow related data we cannot distinguish between the Kerr and mildly charged Kerr-Sen black holes. }