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

TL;DR

This work assesses signatures of Kerr–Sen black holes in EMDA gravity by analyzing how the dilaton charge $r_2$ and surrounding plasma influence the shadow, and then confronts M87* and Sgr A* shadow data from the EHT to constrain $r_2$ and plasma parameters. A Hamiltonian formalism for photon propagation in a pressureless, non-magnetized plasma is developed in Kerr–Sen spacetime, yielding separable geodesic equations and a generalized Carter constant that enable analytic shadow outlines for three plasma profiles. By computing the theoretical angular diameter $\Delta\Theta_{th}$ for given $M$, $D$, and $\theta_i$ and comparing to $\Delta\Theta_{obs}$, the authors derive bounds such as $r_2\lesssim0.48$ (M87*) and $r_2\lesssim1.1$ (Sgr A*) at $1\sigma$ for certain profiles, with plasma parameters $\alpha_i$ also constrained; they find that plasma densities are likely too low for strong dispersive effects, and thus the shadow primarily tests the spacetime geometry. Overall, the current shadow data do not decisively distinguish Kerr from mildly charged Kerr–Sen black holes, though large dilaton charges are disfavored, and the Kerr geometry remains the favored description given present precision. The results underscore that shadow measurements are a cleaner probe of the background spacetime than of the accretion environment, even in string-inspired gravity scenarios.

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-$σ$) 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. }

Figures (8)

• Figure 1: Variation of $F_1(r,\theta)$ with r (in units of $M$) at $a=0.1, 0.5(1-\frac{r_2}{2})\text{ and } 0.999-\frac{r_2}{2}$ for $\theta=\frac{\pi}{6}\text{ (Red) and }\frac{\pi}{2}$ (Blue) for different $r_2$
• Figure 6: Variation of shadow of Kerr Sen black hole in presence of plasma profile 2 at inclination angle $\theta_i=15^\circ$.
• Figure 7: Variation of shadow of Kerr Sen black hole in presence of plasma profile 2 at inclination angle $\theta_i=45^\circ$.
• Figure 8: Variation of shadow of Kerr Sen black hole in presence of plasma profile 2 at inclination angle $\theta_i=90^\circ$.
• Figure 13: The figures represent contour plots of $\chi^2$ (calculated using \ref{['chi-square']} corresponding to $a_{min}$) for various choices of $\alpha_2$ and $r_2$, corresponding to plasma profile 2 for M87*. In order to evaluate the $\chi^2$ we use $D=16.8$Mpc, $\theta_i=17^\circ$ and black hole mass (a) $M=6.2\times 10^9 M_{\circ}$ and (b) $M=6.5\times 10^9 M_{\odot}$. In the figures, the red color region in $\alpha_1- r_2$ plane corresponds to $0.5\leq\chi^2\leq1$, the region with $0.1\leq\chi^2\leq0.5$ is colored green and the blue region represents $0\leq\chi^2\leq0.1$.