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

TL;DR

The study addresses how Kerr-Sen black holes with a dilaton parameter $r_2$ affect horizon-scale images illuminated by a 230 GHz radiatively inefficient accretion flow. It employs general relativistic radiative transfer (GRRT) with a semi-analytic MAD-like RIAF model, extracting bright-ring features via the REx algorithm and characterizing polarization using the complex mode $\beta_2$ across varied $r_2$ and disk thickness $H$, then benchmarking against EHT observations of Sgr A*. The results show that increasing $r_2$ shrinks the bright-ring diameter $d$ while increasing its width $w$ and brightness; increasing disk thickness $H$ reduces $d$ and $w$ but enhances brightness modestly, and $H$ has a stronger impact on the allowed BH parameter space than the observer inclination. Polarization analysis reveals that $Re\,\beta_2$ grows and $Im\,\beta_2$ diminishes with $r_2$, with $|\beta_2|$ generally decreasing and $\angle\beta_2$ increasing, while $H$-driven changes are weaker; these trends help discriminate spacetime geometry and accretion states, with ngEHT offering improved constraints.

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 more strong than those of the observer's inclination. Furthermore, we analyze the coefficient $β_2$ 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.

Figures (10)

• Figure 1: Black hole images (in the top row) and the corresponding blurred images (in the bottom row) at an inclination angle of $30^\circ$ and for a disk thickness $H = 0.3$. The left, middle and right panels are correspond to the cases with $r_2=0.0, 0.5, 0.98$, respectively. Here we set $a=0.5$.
• Figure 2: Visualization of extracted ring features of blurred images for different dilaton parameters $r_2$ and disk thickness $H$. Parameter $d$ is the diameter of bright ring; $w$ is the ring width. Parameters $r_{in}$ and $r_{out}$ delimit the radial full width at half maximum (FWHM).
• Figure 3: Unwrapped ring profiles of the simulated images in Fig. \ref{['fig:2']}. The red horizontal lines indicate the measured values of the ring radius $d/2$ and its ring width $(d\pm w)/2$. The error ranges are marked by dashed lines. Blue vertical solid lines correspond the orientation angle $\eta$ , with their error intervals indicated by blue dashed lines. The purple cross marks the peak brightness in each panels.
• Figure 4: Distribution of the bright ring size on the $r_2-a$ plane for the blurred images of Kerr-Sen black holes with different inclination $i$ and disk thicknesses $H$. The left, middle, and right panels at each row correspond to disk thicknesses of 0.05, 0.1, and 0.3, respectively. The top, middle, and bottom rows correspond to inclination angles of $5^\circ$, $15^\circ$, and $30^\circ$, respectively.
• Figure 5: Distribution of the bright ring size on the $r_2-a$ plane for the unblurred images of Kerr-Sen black holes with different inclination $i$ and disk thicknesses $H$. Others are same as in Fig. \ref{['fig:3']}.