Decoding Horizonless Spacetime: Plasma-Induced Features in a Rotating Wormhole Shadow

TL;DR

This paper addresses how a horizonless rotating wormhole shadows behave when immersed in cold, non-magnetized plasma, and whether plasma-induced refractive effects can mimic or distinguish Kerr black holes. The authors derive a separable Hamilton-Jacobi formulation in a Kerr-like wormhole background, obtain the photon effective potential and unstable spherical orbits, and compute shadow boundaries for three plasma profiles. They compare the wormhole shadows to Kerr under comparable plasma conditions and constrain the geometrical and plasma parameters using EHT bounds on $\Delta C$ and $\delta$ from M87$^*$ and SgrA$^*$, finding that $\Delta C$ is not very constraining, while the fractional diameter deviation $\delta$ yields tight caps on $q$, $a/M$, and plasma densities, with modest differences across plasma models. The results illuminate observable discriminants and guide future work toward magnetized plasmas and more realistic accretion geometries to test horizonless spacetimes with current and upcoming radio observations.

Abstract

We investigate the shadow properties in a recently proposed geometry of a rotating wormhole under realistic astrophysical conditions, particularly in the presence of a cold and non magnetized plasma environment surrounding the wormhole throat. Using the Hamilton Jacobi formalism, we derive the orbit equation under specific plasma density profiles, where we consider plasma as dispersive medium and disregard its influence on the background geometry. The electron density distribution is chosen to preserve a generalized Carter constant. We explore the shadow cast by this class of rotating wormhole in the presence of both homogeneous and non homogeneous plasma as seen by an asymptotic observer. The photon regions are visualized, and the influence of geometric parameters, plasma parameters, and the observer inclination angle with the rotation axis on the resulting shadow morphology is analyzed. We tried to implement constraints on the plasma and the geometrical parameters of the wormhole such as the spin parameter and the deviation (from Kerr) parameter in the back drop of recent observational bounds coming from the deviation from circularity of the shadow boundary ($ΔC$) and deviation of the average shadow radius from Schwarzschild ($δ$). The bound on $ΔC$ is satisfied by the theoretically allowed range of parameters thus not found very useful to put any constraint, we could impose stringent constraints on the parameters based on the observed value of $δ$. By comparing the optical characteristics of the image of these wormholes with those of Kerr black holes under analogous plasma conditions, we identify the features that could serve as discriminants for similar types of compact objects.

Figures (10)

• Figure 1: Representing the behavior of effective potential with $l(r)$ in various situations.
• Figure 2: Photon orbits shown with $X = \mathcal{R}\cos(\phi)$ and $Y = \mathcal{R}\sin(\phi)$. Left panel corresponds to homogeneous plasma, fixed $\xi$ and varying $\eta$. Photon orbits in longitudinal plasma environment is shown in middle panel. Right panel depicts photon orbits in radial plasma environment. Orange curves correspond to throat-crossing trajectories, the blue indicates unstable circular orbit, and green curves represent scattered photon paths.
• Figure 3: Shadow profile of rotating wormhole in the presence of homogeneous plasma.
• Figure 4: Wormhole shadows for $q = 1.0$ with different values of $a$ and $\rho$.
• Figure 5: Shadow profiles for different $a$ and inclination angles $\theta_o$.