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

TL;DR

This study uses GRRT simulations to analyze how the Kerr-Sen black hole's parameters and accretion disk properties influence the bright ring and polarization features in Sgr A* images, with implications for EHT observations.

Contribution

It introduces detailed simulations of Kerr-Sen black hole images, revealing how dilaton parameter and disk thickness affect observable features and parameter constraints.

Findings

Increasing dilaton parameter shrinks the bright ring and enhances its brightness.

Growing disk thickness reduces the bright ring's diameter and width.

Effects of disk thickness on polarization are weaker than those of the dilaton parameter.

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 $\beta_2$, 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 $\beta_2$ are much weaker than those from the dilaton parameter.