Bright ring features and polarization structures in Kerr-Sen black hole images illuminated by radiatively inefficient accretion flows

Abstract

Using general relativistic radiative transfer (GRRT) simulations, we investigate the bright ring features and polarization structures in images of the Kerr-Sen black hole associated with Sgr A*, as illuminated by 230 GHz thermal synchrotron emission from radiatively inefficient accretion flows (RIAF). Our findings reveal that an increase in the dilaton parameter leads to a shrinking of the bright ring, accompanied by enhancements in both its width and brightness. As the disk thickness grows, the bright ring's diameter and width both decrease. The brightness enhancement induced by the disk thickness is less prominent than that driven by the dilaton parameter. Comparing with the Event Horizon Telescope (EHT) observational data of SgrA*, we present the allowed ranges of black hole parameters, and find that effects of the disk thickness on the allowed parameter space are stronger than those of the observer's inclination. Furthermore, we analyze the coefficient $β_2$, associated with the two-fold rotational symmetry of the electric vector position angles (EVPA), to probe the polarization structure of the black hole images, and reveal that effects of the disk thickness on $β_2$ are much weaker than those from the dilaton parameter.