Time delay induced by plasma in strong lens systems

Abstract

ABSTRACT If the gravitational lens is surrounded by non-homogeneous plasma, in addition to the vacuum gravitational deflection, chromatic refraction occurs. Also, the speed of signal propagation decreases compared to a vacuum. In this article, we investigate analytically the time delay in the case of gravitational lensing in plasma, focusing on strong lens systems. We take into account the following contributions: geometric delay due to trajectory bending in the presence of both gravity and plasma; potential delay of the ray in the gravitational field of the lens; dispersion delay in the plasma due to decrease in the speed of light signal in the medium. We consider the singular isothermal sphere as a model of a gravitational lens and the arbitrary spherically symmetric distribution of surrounding plasma. For this scenario, plasma corrections for the time delay between two images are found in a compact analytical form convenient for estimates.