Escape and Trapping of Low-Frequency Gravitationally Lensed Rays by Compact Objects within Plasma

TL;DR

This paper investigates how plasma density and gravitational effects influence the propagation, trapping, and visibility of light rays emitted by compact objects like neutron stars, revealing frequency-dependent cloaking phenomena.

Contribution

It introduces a detailed analysis of plasma effects on gravitational lensing near compact objects, identifying frequency bands where rays are trapped or escape, and explores implications for neutron star observations.

Findings

Plasma effects can dominate over gravitational lensing when h<2.

Visible surface of the CO shrinks with decreasing frequency, vanishing at a threshold.

Certain frequency ranges cause rays to curve back, cloaking the star from view.

Abstract

We consider the gravitational lensing of rays emitted by a compact object (CO) within a distribution of plasma with power-law density $\propto r^{-h}$. For the simplest case of a cloud of spherically symmetric cold non-magnetized plasma, the diverging effect of the plasma and the converging effect of gravitational lensing compete with one another. When $h<2$, the plasma effect dominates over the vacuum Schwarzschild curvature, potentially shifting the radius of the unstable circular photon orbit outside the surface of the CO. When this occurs, we define two relatively narrow radio-frequency bands in which plasma effects are particularly significant. Rays in the escape window have $\omega_{0} < \omega \leq \omega_{+}$ and are free to propagate to infinity from the CO surface. To a distant observer, the visible portion of the CO surface appears to shrink as the observed frequency is reduced, and vanishes entirely at $\omega_{0}$, in excess of the plasma frequency at the CO surface. We define the anomalous propagation window for frequencies $\omega_{-}< \omega \leq \omega_{0}$. Rays emitted from the CO surface within this frequency range are dominated by optical effects from the plasma and curve back to the surface of the CO, effectively cloaking the star from distant observers. We conclude with a study of neutron star (NS) compactness ratios for a variety of nuclear matter equations of state (EoS). For $h=1$, NSs generated from stiff EoS should display significant frequency dependence in the EW, and lower values of $h$ with softer EoS can also show these effects.